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The Java Interview Prep Handbook – 50 Questions Solved + Code Examples

If you're trying to get a job in big tech or you want to refine your skills in software development, a strong grasp of Java is indispensable.

Java is well-known for its robustness in Object-Oriented Programming (OOP), and it provides a comprehensive foundation essential for developers at every level.

This handbook offers a detailed pathway to help you excel in Java interviews. It focuses on delivering insights and techniques relevant to roles in esteemed big tech companies, ensuring you're well-prepared for the challenges ahead.

This guide serves as a comprehensive Java review tutorial, bridging the gap between foundational Java knowledge and the sophisticated expertise sought by industry leaders like Google. And it'll help you deepen your understanding and practical application of Java, preparing you for professional success in the tech industry.

Table of Contents

• What is Java?
• What's the difference between the JDK, JRE, and JVM?
• How does the 'public static void main(String[] args)' method work?
• What is bytecode in Java?
• Differentiate between overloading and overriding
• What is the Java ClassLoader?
• Can we override static methods in Java?
• How does the 'finally' block differ from the 'finalize' method in Java?
• What is the difference between an abstract class and an interface?
• Explain the concept of Java packages
• What are Java annotations?
• How does multi-threading work in Java?
• Use throw to raise an exception
• Use throws to declare exceptions
• What is the significance of the transient keyword?
• How do you ensure thread safety in Java?
• Explain the Singleton pattern
• What are Java Streams?
• What are the primary differences between ArrayList and LinkedList?
• How do HashSet, LinkedHashSet, and TreeSet differ?
• Differentiate between HashMap and ConcurrentHashMap
• Describe the contract between hashCode() and equals() methods
• What is Java reflection?
• How do you create a custom exception in Java?
• What is the difference between a checked and unchecked exception?
• What are generics? Why are they used?
• Explain the concept of Java Lambda Expressions
• What is the diamond problem in inheritance?
• Describe the difference between fail-fast and fail-safe iterators
• What is type erasure in Java generics?
• Describe the differences between StringBuilder and StringBuffer
• What is the volatile keyword in Java?
• Explain the Java memory model
• What is the purpose of the default keyword in interfaces?
• How does switch differ in Java 7 and Java 8?
• Explain the concept of Autoboxing and Unboxing
• Describe the @FunctionalInterface annotation
• How can you achieve immutability in Java?
• What is the decorator pattern?
• Explain the Java I/O streams
• How does the garbage collector work in Java?
• What are the benefits of using Java NIO?
• Explain the Observer pattern
• What is the purpose of Java's Optional?
• Explain Java's try-with-resources
• Explain the difference between C++ and Java
• What is polymorphism? Provide an example
• How can you avoid memory leaks in Java?
• Explain the purpose of Java's synchronized block
• Explain the concept of modules in Java

1. What is Java?

Java is a high-level, object-oriented programming language known for its platform independence. It allows developers to write code once and run it anywhere using the Java Virtual Machine (JVM).

2. What's the Difference between the JDK, JRE, and JVM?

• JDK (Java Development Kit): This is a software package that provides developers with the tools and utilities necessary to develop, compile, and run Java applications.
• JRE (Java Runtime Environment): A subset of the JDK, the JRE contains the essential components, including the JVM, to run Java applications but not to develop them.
• JVM (Java Virtual Machine): An abstract computing machine, the JVM enables Java bytecode to be executed, providing the platform independence Java is known for.

3. How Does the public static void main(String[] args) Method Work?

This method is the entry point for Java applications. The public modifier means it's accessible from other classes, static denotes it's a class-level method, and void indicates it doesn't return any value. The argument String[] args allows command-line arguments to be passed to the application.

4. What is bytecode in Java?

Bytecode is an intermediate, platform-independent code that Java source code is compiled into. It is executed by the JVM, enabling the "write once, run anywhere" capability.

5. Differentiate between overloading and overriding

• Overloading: This occurs when two or more methods in the same class share the same name but have different parameters. It's a compile-time concept.
• Overriding: In this case, a subclass provides a specific implementation for a method already defined in its superclass. It's a runtime concept.

6. What is the Java ClassLoader?

The Java ClassLoader is a part of the JRE that dynamically loads Java classes into the JVM during runtime. It plays a crucial role in Java's runtime environment by extending the core Java classes.

7. Can We Override Static Methods in Java?

No, we cannot override static methods. While a subclass can declare a method with the same name as a static method in its superclass, this is considered method hiding, not overriding.

8. How Does the finally Block Differ from the finalize Method in Java?

Understanding the distinction between the finally block and the finalize method in Java is crucial for effective resource management and exception handling in your programs.

Finally Block:

• Purpose and Usage: The finally block is a key component of Java's exception handling mechanism. It is used in conjunction with try-catch blocks.
• Execution Guarantee: Regardless of whether an exception is thrown or caught within the try or catch blocks, the code within the finally block is always executed. This ensures that it runs even if there’s a return statement in the try or catch block.
• Common Uses: It is typically utilized for cleaning up resources, such as closing file streams, database connections, or releasing any system resources that were acquired in the try block. This helps in preventing resource leaks.

Finalize Method:

• Definition: The finalize method is a protected method of the Object class in Java. It acts as a final resort for objects garbage collection.
• Garbage Collector Call: It is called by the garbage collector on an object when the garbage collector determines that there are no more references to the object. However, its execution is not guaranteed, and it's generally unpredictable when, or even if, the finalize method will be invoked.
• Resource Release: The finalize method is designed to allow an object to clean up its resources before it is collected by the garbage collector. For example, it might be used to ensure that an open file owned by an object is closed.
• Caution in Use: It's important to note that relying on finalize for resource cleanup is generally not recommended due to its unpredictability and potential impact on performance.

Access Modifiers in Java:

• Private: This modifier makes a member accessible only within its own class. Other classes cannot access private members of a different class.
• Default (no modifier): When no access modifier is specified, the member has package-level access. This means it is accessible to all classes within the same package.
• Protected: A protected member is accessible within its own package and also in subclasses. This is often used in inheritance.
• Public: Public members are accessible from any class in the Java program. It provides the widest level of access.

Understanding these distinctions and access levels is vital for effective Java programming, ensuring resource management, security, and encapsulation are handled appropriately in your software development endeavors.

9. What is the Difference between an Abstract Class and an Interface?

An abstract class in Java is used as a base for other classes. It can contain both abstract methods (without an implementation) and concrete methods (with an implementation).

Abstract classes can have member variables that can be inherited by subclasses. A class can extend only one abstract class due to Java's single inheritance property.

Example of an Abstract Class:

An interface in Java, on the other hand, is a completely "abstract class" that is used to group related methods with empty bodies.

From Java 8 onwards, interfaces can have default and static methods with a body. A class can implement any number of interfaces.

Example of an Interface:

Both abstract classes and interfaces are foundational concepts in Java, used for achieving abstraction and supporting design patterns like Strategy and Adapter. The use of these concepts depends on the specific requirements and design considerations of your software project.

10. Explain the Concept of Java Packages

Java packages are a way of organizing and structuring classes and interfaces in Java applications. They provide a means to group related code together. Packages help prevent naming conflicts, enhance code readability, and facilitate code reusability.

For example, consider a banking application. You might have packages like com.bank.accounts , com.bank.customers , and com.bank.transactions . These packages contain classes and interfaces specific to their respective functionalities.

In essence, Java packages are like directories or folders in a file system, organizing code and making it more manageable.

11. What are Java Annotations?

Java annotations are metadata that can be added to Java source code. They provide information about the code to the compiler or runtime environment. Annotations do not directly affect the program's functionality – instead, they convey instructions to tools or frameworks.

A common use of annotations is for marking classes or methods as belonging to a specific framework or for providing additional information to tools like code analyzers, build tools, or even custom code generators.

For example, the @Override annotation indicates that a method is intended to override a method from a superclass, helping catch coding errors during compilation. Another example is @Deprecated , which indicates that a method or class is no longer recommended for use.

12. How Does Multi-threading Work in Java?

Multi-threading in Java allows a program to execute multiple threads concurrently. Threads are lightweight processes within a program that can run independently. Java provides a rich set of APIs and built-in support for multi-threading.

Threads in Java are typically created by either extending the Thread class or implementing the Runnable interface. Once created, threads can be started using the start() method, causing them to run concurrently.

Java's multi-threading model ensures that threads share resources like memory and CPU time efficiently while providing mechanisms like synchronization and locks to control access to shared data.

Multi-threading is useful for tasks such as improving application responsiveness, utilizing multi-core processors, and handling concurrent operations, as often seen in server applications.

13. Use throw to Raise an Exception

In Java programming, the throw keyword is crucial for handling exceptions deliberately and responsively. This approach to exception management allows developers to enforce specific conditions in their code and maintain control over the program flow.

In this example, an IllegalArgumentException is thrown if the age parameter is less than 18. This method of raising an exception ensures that the program behaves predictably under defined conditions, enhancing both the security and reliability of the code.

14. Use throws to Declare Exceptions

The throws keyword in Java serves to declare that a method may cause an exception to be thrown. It signals to the method's caller that certain exceptions might arise, which should be either caught or further declared.

In this scenario, the readDocument method declares that it might throw a FileNotFoundException . This declaration requires the caller of this method to handle this exception, ensuring that appropriate measures are in place to deal with potential errors, and thus improving the robustness of the application.

Both throw and throws are integral to managing exceptions in Java. throw is used for actively raising an exception in the code, while throws declares possible exceptions that a method might produce, thereby mandating their handling by the caller. This distinction is essential for writing error-resistant and well-structured Java programs.

15. What is the Significance of the transient Keyword?

The transient keyword in Java is used to indicate that a field should not be serialized when an object of a class is converted to a byte stream (for example, when using Java Object Serialization).

This is significant when you have fields in a class that you do not want to include in the serialized form, perhaps because they are temporary, derived, or contain sensitive information.

16. How Do You Ensure Thread Safety in Java?

Thread safety in Java is achieved by synchronizing access to shared resources, ensuring that multiple threads can't simultaneously modify data in a way that leads to inconsistencies or errors.

You can ensure thread safety through synchronization mechanisms like synchronized blocks, using thread-safe data structures, or utilizing concurrent utilities from the java.util.concurrent package.

In the code above, we have a SharedCounter class with a synchronized increment method, ensuring that only one thread can increment the count variable at a time. This synchronization mechanism prevents data inconsistencies when multiple threads access and modify the shared count variable.

We create two threads ( thread1 and thread2 ) that concurrently increment the counter. By using synchronized methods or blocks, we guarantee thread safety, and the final count will be accurate, regardless of thread interleaving.

17. Explain the Singleton Pattern

The Singleton pattern is a design pattern that ensures a class has only one instance and provides a global point of access to that instance. It is achieved by making the constructor of the class private, creating a static method to provide a single point of access to the instance, and lazily initializing the instance when needed.

Implementation without Singleton:

Let's imagine a scenario where you want to establish a database connection. Without the Singleton pattern, every time you'd need a connection, you might end up creating a new one.

Now, imagine initializing this connection multiple times in different parts of your application:

For the above code, "Establishing a new database connection..." would be printed twice, implying two separate connections were created. This is redundant and can be resource-intensive.

Implementation with Singleton:

With the Singleton pattern, even if you attempt to get the connection multiple times, you'd be working with the same instance.

Initializing this connection multiple times:

For the above code, "Establishing a single database connection..." would be printed just once, even though we've called getInstance() twice.

18. What are Java Streams?

Java Streams are a powerful abstraction for processing sequences of elements, such as collections, arrays, or I/O channels, in a functional and declarative style. They provide methods for filtering, mapping, reducing, and performing various transformations on data.

Streams can significantly simplify code and improve readability when working with data collections.

19. What Are the Primary Differences between ArrayList and LinkedList?

ArrayList and LinkedList are both implementations of the List interface. The primary differences between them lie in their internal data structures.

ArrayList uses a dynamic array to store elements, offering fast random access but slower insertions and deletions. LinkedList uses a doubly-linked list, which provides efficient insertions and deletions but slower random access.

20. How do HashSet , LinkedHashSet , and TreeSet Differ?

• HashSet stores elements in an unordered manner, offering constant-time complexity for basic operations.
• LinkedHashSet maintains the order of insertion, providing ordered iteration of elements.
• TreeSet stores elements in a sorted order (natural or custom), offering log(n) time complexity for basic operations.

In this code, we add a large number of elements to each type of set ( HashSet , LinkedHashSet , and TreeSet ) and measure the time it takes to perform this operation. This demonstrates the performance characteristics of each set type.

Typically, you will observe that HashSet performs the fastest for adding elements since it doesn't maintain any specific order, followed by LinkedHashSet , and TreeSet , which maintains a sorted order.

This output demonstrates the time taken (in nanoseconds) to add one million elements to each of the three sets: HashSet , LinkedHashSet , and TreeSet . As you can see, HashSet is the fastest, followed by LinkedHashSet , and TreeSet is the slowest due to its need to maintain elements in sorted order.

21. Differentiate between HashMap and ConcurrentHashMap

HashMap is not thread-safe and is suitable for single-threaded applications. ConcurrentHashMap , on the other hand, is designed for concurrent access and supports multiple threads without external synchronization. It provides high concurrency and performance for read and write operations.

22. Describe the Contract between the hashCode() and equals() Methods

The contract between hashCode() and equals() methods states that if two objects are equal ( equals() returns true), their hash codes ( hashCode() ) must also be equal.

However, the reverse is not necessarily true: objects with equal hash codes may not be equal. Adhering to this contract is crucial when using objects as keys in hash-based collections like HashMap .

23. What is Java Reflection?

Java reflection is a feature that allows you to inspect and manipulate the metadata of classes, methods, fields, and other program elements at runtime. It enables you to perform tasks such as dynamically creating objects, invoking methods, and accessing fields, even for classes that were not known at compile time.

24. How Do You Create a Custom Exception in Java?

You can create a custom exception in Java by extending the Exception class or one of its subclasses. By doing so, you can define your exception with specific attributes and behaviors tailored to your application's needs.

25. What is the Difference between a Checked and Unchecked Exception?

Checked exceptions are exceptions that must be either caught using a try-catch block or declared in the method signature using the throws keyword.

Unchecked exceptions (usually subclasses of RuntimeException ) do not require such handling.

Checked exceptions are typically used for recoverable errors, while unchecked exceptions represent programming errors or runtime issues.

Here is a code example to illustrate checked and unchecked exceptions.

In this code, we attempt to read a file using FileReader, which may throw a checked exception called IOException .

To handle this exception, we enclose the file reading code in a try-catch block specifically catching IOException . This is an example of how you handle checked exceptions, which are typically used for recoverable errors like file not found or I/O issues.

Now, let's take a look at an example of an unchecked exception:

In this code, we attempt to divide an integer by zero, which leads to an unchecked exception called ArithmeticException . Unchecked exceptions do not require explicit handling using a try-catch block. However, it's good practice to catch and handle them when you anticipate such issues. These exceptions often represent programming errors or runtime issues.

26. What Are Generics? Why Are They Used?

Generics in Java are a powerful feature that allows you to create classes, interfaces, and methods that operate on types. They provide a way to define classes or methods with a placeholder for the data type that will be used when an instance of the class is created or when a method is called.

Generics are used to make your code more reusable, type-safe, and less error-prone by allowing you to write generic algorithms that work with different data types. They help eliminate the need for typecasting and enable compile-time type checking.

For example, consider the use of a generic class to create a List of integers:

Generics ensure that you can only add integers to the list and that you don't need to perform explicit typecasting when retrieving elements from the list.

27. Explain the Concept of Java Lambda Expressions

Lambda expressions in Java are a concise way to express instances of single-method interfaces (functional interfaces) using a more compact syntax. They facilitate functional programming by allowing you to treat functions as first-class citizens.

Lambda expressions consist of a parameter list, an arrow (->), and a body. They provide a way to define and use anonymous functions.

For example, consider a functional interface Runnable that represents a task to be executed. With a lambda expression, you can define and execute a runnable task as follows:

We will talk about a more practical example later down the post.

28. What is the Diamond Problem in Inheritance?

The diamond problem in inheritance is a common issue in object-oriented programming languages that support multiple inheritance. It occurs when a class inherits from two classes that have a common ancestor class, resulting in ambiguity about which superclass's method or attribute to use.

Java solves the diamond problem by not supporting multiple inheritance of classes (that is, a class cannot inherit from more than one class).

But Java allows multiple inheritance of interfaces, which doesn't lead to the diamond problem because interfaces only declare method signatures, and the implementing class must provide concrete implementations. In case of method conflicts, the implementing class must explicitly choose which method to use.

Here's a simplified example to illustrate the diamond problem (even though Java doesn't directly encounter it):

In Java, the diamond problem is avoided through interface implementation and explicit method choice when conflicts arise.

29. Describe the Difference between Fail-fast and Fail-safe Iterators

In Java, fail-fast and fail-safe are two strategies for handling concurrent modification of collections during iteration.

Fail-fast iterators throw a ConcurrentModificationException if a collection is modified while being iterated. Fail-safe iterators, on the other hand, do not throw exceptions and allow safe iteration even if the collection is modified concurrently.

Fail-Fast Iterator Example:

In this example, when we attempt to remove an element from the list while iterating, it leads to a ConcurrentModificationException , which is characteristic of fail-fast behavior. Fail-fast iterators immediately detect and throw an exception when they detect that the collection has been modified during iteration.

Fail-Safe Iterator Example:

In this example, a ConcurrentHashMap is used, which supports fail-safe iterators. Even if we modify the map concurrently while iterating, there is no ConcurrentModificationException thrown. Fail-safe iterators continue iterating over the original elements and do not reflect changes made after the iterator is created.

30. What is Type Erasure in Java Generics?

Type erasure is a process in Java where type parameters in generic classes or methods are replaced with their upper bound or Object during compilation. This erasure ensures backward compatibility with pre-generic Java code. But it means that the type information is not available at runtime, which can lead to issues in some cases.

31. Describe the Differences between StringBuilder and StringBuffer

Thread safety:.

StringBuffer is thread-safe. This means it is synchronized, so it ensures that only one thread can modify it at a time. This is crucial in a multithreaded environment where you have multiple threads modifying the same string buffer.

StringBuilder , on the other hand, is not thread-safe. It does not guarantee synchronization, making it unsuitable for use in scenarios where a string is accessed and modified by multiple threads concurrently. But this lack of synchronization typically leads to better performance under single-threaded conditions.

Performance:

Because StringBuffer operations are synchronized, they involve a certain overhead that can impact performance negatively when high-speed string manipulation is required.

StringBuilder is faster than StringBuffer because it avoids the overhead of synchronization. It's an excellent choice for string manipulation in a single-threaded environment.

Use Case Scenarios:

Use StringBuffer when you need to manipulate strings in a multithreaded environment. Its thread-safe nature makes it the appropriate choice in this scenario.

Use StringBuilder in single-threaded situations, such as local method scope or within a block synchronized externally, where thread safety is not a concern. Its performance benefits shine in these cases.

API Similarity:

Both StringBuilder and StringBuffer have almost identical APIs. They provide similar methods for manipulating strings, such as append() , insert() , delete() , reverse() , and so on.

This similarity means that switching from one to the other in your code is generally straightforward.

Memory Efficiency:

Both classes are more memory efficient compared to using String for concatenation. Since String is immutable in Java, concatenation with String creates multiple objects, whereas StringBuilder and StringBuffer modify the string in place.

Introduced Versions:

StringBuffer has been a part of Java since version 1.0, whereas StringBuilder was introduced later in Java 5. This introduction was primarily to offer a non-synchronized alternative to StringBuffer for improved performance in single-threaded applications.

You should make the choice between StringBuilder and StringBuffer based on the specific requirements of your application, particularly regarding thread safety and performance needs.

While StringBuffer provides safety in a multithreaded environment, StringBuilder offers speed and efficiency in single-threaded or externally synchronized scenarios.

32. What is the volatile Keyword in Java?

Basic Definition: The volatile keyword is used to modify the value of a variable by different threads. It ensures that the value of the volatile variable will always be read from the main memory and not from the thread's local cache.

Visibility Guarantee: In a multithreading environment, threads can cache variables. Without volatile, there's no guarantee that one thread's changes to a variable will be visible to another. The volatile keyword guarantees visibility of changes to variables across threads.

Happens-Before Relationship: volatile establishes a happens-before relationship in Java. This means that all the writes to the volatile variable are visible to subsequent reads of that variable, ensuring a consistent view of the variable across threads.

Usage Scenarios: volatile is used for variables that may be updated by multiple threads. It's often used for flags or status variables. For example, a volatile boolean running variable can be used to stop a thread.

Limitations: Volatile cannot be used with class or instance variables. It's only applicable to fields. It doesn't provide atomicity.

For instance, volatile int i; i++; is not an atomic operation. For atomicity, you might need to resort to AtomicInteger or synchronized methods or blocks. It's not a substitute for synchronization in every case, especially when multiple operations on the volatile variable need to be atomic.

Avoiding Common Misconceptions: A common misconception is that volatile makes the whole block of statements atomic, which is not true. It only ensures the visibility and ordering of the writes to the volatile variable.

Another misconception is that volatile variables are slow. But while they might have a slight overhead compared to non-volatile variables, they are generally faster than using synchronized methods or blocks. Performance Considerations: volatile can be a more lightweight alternative to synchronization in cases where only visibility concerns are present. It doesn't incur the locking overhead that synchronized methods or blocks do. Best Practices: Use volatile sparingly and only when necessary. Overusing it can lead to memory visibility issues that are harder to detect and debug. Always assess whether your use case requires atomicity, in which case other concurrent utilities or synchronization might be more appropriate.

volatile use case:

We will create a simple program where one thread modifies a volatile boolean flag, and another thread reads this flag. This flag will be used to control the execution of the second thread.

Code Example:

Key points in the comments:.

• Visibility of volatile variable: The most crucial aspect of using volatile here is ensuring that the update to the running variable in one thread (main thread) is immediately visible to another thread ( thread1 ). This is what allows thread1 to stop gracefully when running is set to false .
• Use in a Simple Flag Scenario: The example demonstrates a common scenario for using volatile , that is as a simple flag to control the execution flow in a multithreaded environment.
• Absence of Compound Operations: Note that we are not performing any compound operations (like incrementing) on the running variable. If we were, additional synchronization would be needed because volatile alone does not guarantee atomicity of compound actions.
• Choice of volatile Over Synchronization: The choice to use volatile over other synchronization mechanisms (like synchronized blocks or Locks ) is due to its lightweight nature when dealing with the visibility of a single variable. It avoids the overhead associated with acquiring and releasing locks.

33. Explain the Java Memory Model

The JMM defines how Java threads interact through memory. Essentially, it describes the relationship between variables and the actions of threads (reads and writes), ensuring consistency and predictability in concurrent programming.

Happens-Before Relationship:

At the heart of the JMM is the 'happens-before' relationship. This principle ensures memory visibility, guaranteeing that if one action happens-before another, then the first is visible to and affects the second.

For example, changes to a variable made by one thread are guaranteed to be visible to other threads only if a happens-before relationship is established.

Memory Visibility:

Without the JMM, threads might cache variables, and changes made by one thread might not be visible to others. The JMM ensures that changes made to a shared variable by one thread will eventually be visible to other threads.

Synchronization:

The JMM utilizes synchronization to establish happens-before relationships. When a variable is accessed within synchronized blocks, any write operation in one synchronized block is visible to any subsequent read operation in another synchronized block.

Additionally, the JMM governs the behavior of volatile variables, ensuring visibility of updates to these variables across threads without synchronization.

Thread Interleaving and Atomicity:

The JMM defines how operations can interleave when executed by multiple threads. This can lead to complex states if not managed correctly.

Atomicity refers to operations that are indivisible and uninterrupted. In Java, operations on most primitive types (except long and double ) are atomic. However, compound operations (like incrementing a variable) are not automatically atomic.

Reordering:

The JMM allows compilers to reorder instructions for performance optimization as long as happens-before guarantees are maintained. However, this can lead to subtle bugs if not properly understood.

Use of Volatile Keyword:

The volatile keyword plays a significant role in the JMM. It ensures that any write to a volatile variable establishes a happens-before relationship with subsequent reads of that variable, thus ensuring memory visibility without the overhead of synchronization.

Locking Mechanisms:

Locks in Java (implicit via synchronized blocks/methods or explicit via ReentrantLock or others) also adhere to the JMM, ensuring that memory visibility is maintained across threads entering and exiting locks.

Safe Publication:

The JMM also addresses the concept of safe publication, ensuring that objects are fully constructed and visible to other threads after their creation.

High-Level Implications:

Understanding the JMM is critical for writing correct and efficient multi-threaded Java applications. It helps developers reason about how shared memory is handled, especially in complex applications where multiple threads interact and modify shared data.

Best Practices:

• Always use the appropriate synchronization mechanism to ensure memory visibility and atomicity.
• Be cautious about memory visibility issues; even simple operations can lead to visibility problems in a multi-threaded context.
• Understand the cost of synchronization and use volatile variables where appropriate.

34. What is the Purpose of the default Keyword in Interfaces?

The default keyword in Java interfaces, introduced in Java 8, marks a significant evolution in the Java language, especially in how interfaces are used and implemented. It serves several key purposes:

Adding Method Implementations in Interfaces:

Prior to Java 8, interfaces in Java could only contain method signatures (abstract methods) without any implementation.

The default keyword allows you to provide a default implementation for a method within an interface. This feature bridges a gap between full abstraction (interfaces) and concrete implementations (classes).

Enhancing Interface Evolution:

One of the primary motivations for introducing the default keyword was to enhance the evolution of interfaces.

Before Java 8, adding a new method to an interface meant breaking all its existing implementations. With default methods, you can add new methods to interfaces with default implementations without breaking the existing implementations.

This is particularly useful for library designers, ensuring backward compatibility when interfaces need to be expanded.

Facilitating Functional Programming:

\The introduction of default methods played a crucial role in enabling functional programming features in Java, such as Lambda expressions. It allowed for richer interfaces (like java.util.stream.Stream ) which are fundamental to functional-style operations in Java.

Multiple Inheritance of Behavior:

While Java does not allow multiple inheritance of state (that is, you cannot inherit from multiple classes), the default keyword enables multiple inheritance of behavior.

A class can implement multiple interfaces, and each interface can provide a default implementation of methods, which the class inherits.

Reducing Boilerplate Code:

default methods can be used to reduce the amount of boilerplate code by providing a general implementation that can be shared across multiple implementing classes, while still allowing individual classes to override the default implementation if a more specific behavior is required.

Example Usage:

In this example, any class implementing the Vehicle interface must provide an implementation for cleanVehicle , but it's optional for startEngine . The default implementation of startEngine can be used as is, or overridden by the implementing class.

Best Practices and Considerations:

• Use Sparingly: Default methods should be used judiciously. They are best suited for gradually evolving interfaces or for methods that have a common implementation across most implementing classes.
• Design With Care: When designing interfaces with default methods, consider how they might be used or overridden. It's important to document the expected behavior and interactions between default methods and other abstract methods in the interface.
• Overriding Default Methods: Just like any inherited method, default methods can be overridden in the implementing class. This should be done to provide a specific behavior different from the default implementation.

35. How Does switch Differ in Java 7 and Java 8?

Limited Case Types: In Java 7, the switch statement supports limited types for the case labels, namely byte , short , char , int , and their corresponding Wrapper classes, along with enum types and, as of Java 7, String .

Traditional Structure: The structure of the switch statement in Java 7 follows the conventional C-style format, with a series of case statements and an optional default case. Each case falls through to the next unless it ends with a break statement or other control flow statements like return .

No Lambda Expressions: Java 7 does not support lambda expressions, and thus, they cannot be used within a switch statement or case labels.

Lambda Expressions: While the basic syntax and supported types for the switch statement itself did not change in Java 8, the introduction of lambda expressions in this version brought a new paradigm in handling conditional logic.

This doesn’t directly change how switch works, but it offers alternative patterns for achieving similar outcomes, especially when used in conjunction with functional interfaces.

Functional Programming Approach: Java 8 promotes a more functional programming style, encouraging the use of streams, lambda expressions, and method references. This can lead to alternatives for traditional switch statements, like using Map of lambdas for conditional logic, which can be more readable and concise.

Enhanced Readability and Maintainability: Although not a direct change to the switch statement, the use of lambda expressions and functional programming practices in Java 8 can lead to more readable and maintainable code structures that might otherwise use complex switch or nested if-else statements.

Practical Considerations:

• When to Use switch in Java 8: Despite the advancements in Java 8, the switch statement remains a viable and efficient method for controlling complex conditional logic. It is particularly useful when dealing with a known set of possible values, such as enum constants or strings.
• Combining switch with Lambdas: While you cannot use lambdas directly in a switch statement, Java 8 allows for more elegant ways to handle complex conditional logic that might traditionally have been a use case for switch . For example, using a Map with lambdas or method references can sometimes replace a complex switch statement.
• Performance Considerations: The performance of a switch statement is generally better than a series of if-else statements, especially when dealing with a large number of cases, due to its internal implementation using jump tables or binary search.

36. Explain the Concept of Autoboxing and Unboxing

What is autoboxing.

Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. For example, converting an int to an Integer , a double to a Double , and so on.

When to use autoboxing

This feature is commonly used when working with collections, like ArrayList or HashMap , which can only store objects and not primitive types.

It simplifies the code by allowing direct assignment of a primitive value to a variable of the corresponding wrapper class.

Behind the Scenes:

When autoboxing, the compiler essentially uses the valueOf method of the respective wrapper class to convert the primitive to its wrapper type.

For example, Integer.valueOf(int) is used for converting int to Integer .

Performance Considerations:

• While convenient, autoboxing can introduce performance overhead, especially in scenarios with extensive boxing and unboxing in tight loops, due to the creation of additional objects.

What is unboxing?

Unboxing is the reverse process, where the Java compiler automatically converts an object of a wrapper type to its corresponding primitive type.

When to use unboxing

It is often used when performing arithmetic operations or comparisons on objects of wrapper classes, where primitive types are required.

During unboxing, the compiler uses the corresponding wrapper class's method to extract the primitive value. For instance, it uses Integer.intValue() to get the int from an Integer .

Null Pointer Exception:

A crucial point to consider is that unboxing a null object reference will throw a NullPointerException . This is a common bug in code that relies heavily on autoboxing and unboxing.

• Be Aware of Implicit Conversions: It's important to be aware that these conversions are happening, as they can sometimes lead to unexpected behavior, especially with regards to NullPointerExceptions during unboxing of null references.
• Consider Performance: In performance-sensitive applications, prefer using primitives to avoid the overhead of autoboxing and unboxing.
• Null Safety: Always check for null before unboxing, to avoid potential NullPointerExceptions .
• Readability vs Efficiency: While autoboxing and unboxing significantly improve code readability and reduce boilerplate, be mindful of their impact on performance and choose wisely based on the application's context.

37. Describe the @FunctionalInterface Annotation

The @FunctionalInterface annotation in Java is a key feature that dovetails with the language's embrace of functional programming concepts, particularly since Java 8. It serves a specific purpose in defining and enforcing certain coding patterns, making it a vital tool for developers focusing on functional-style programming.

Definition and Purpose

@FunctionalInterface is an annotation that marks an interface as a functional interface.

A functional interface in Java is an interface that contains exactly one abstract method. This restriction makes it eligible to be used in lambda expressions and method references, which are core components of Java's functional programming capabilities.

Enforcing Single Abstract Method

The primary role of @FunctionalInterface is to signal the compiler to enforce the rule of a single abstract method. If the annotated interface does not adhere to this rule, the compiler throws an error, ensuring the interface's contract is not accidentally broken by adding additional abstract methods.

Usage and Implications:

• Lambda Expressions: Functional interfaces provide target types for lambda expressions and method references. For example, Java's standard java.util.function package contains several functional interfaces like Function<T,R> , Predicate<T> , Consumer<T> , which are widely used in stream operations and other functional programming scenarios.
• Optional but Recommended: While the @FunctionalInterface annotation is not mandatory for an interface to be considered a functional interface by the Java compiler, using it is considered best practice. It makes the developer's intention clear and ensures the contract of the functional interface is not inadvertently broken.
• Existing Interfaces: Many existing interfaces from earlier versions of Java naturally fit the definition of a functional interface. For example, java.lang.Runnable and java.util.concurrent.Callable are both functional interfaces as they have only one abstract method.

In this example, SimpleFunction is a functional interface with one abstract method execute() . The @FunctionalInterface annotation ensures that no additional abstract methods are inadvertently added.

• Clarity and Documentation: Use @FunctionalInterface to communicate your intention clearly both to the compiler and to other developers. It serves as a form of documentation.
• Design with Care: When designing a functional interface, consider its general utility and how it fits into the broader application architecture, especially if it's intended to be used across different parts of the application.
• Avoid Overuse: While functional programming in Java can lead to more elegant and concise code, be cautious of overusing lambdas and functional interfaces, as they can make the code harder to read and debug if used excessively or inappropriately.
• Compatibility with Older Java Versions: Be aware that @FunctionalInterface is a Java 8 feature. If you're working on applications that need to be compatible with earlier Java versions, you won’t be able to use this feature.

38. How Can You Achieve Immutability in Java?

Achieving immutability in Java is a fundamental practice, particularly useful for creating robust, thread-safe applications.

An immutable object is one whose state cannot be modified after it is created. Here's a detailed and precise explanation of how to achieve immutability in Java:

Core Principles of Immutability:

• No Setters: Immutable objects do not expose any methods to modify their state after construction. This typically means not providing any setter methods.
• Final Class: The class should be declared as final to prevent subclassing. Subclasses could add mutable state, undermining the immutability of the parent class.
• Final Fields: All fields should be final , ensuring they are assigned only once, typically within the constructor, and cannot be re-assigned.
• Private Fields: Fields should be private to prevent external modification and to encapsulate the data.
• No Direct Access to Mutable Objects:
• If your class has fields that are references to mutable objects (like arrays or collections), ensure these fields are not directly exposed or modified:
• Do not provide methods that modify mutable objects.
• Do not share references to the mutable objects. Provide copies of mutable objects when needed.

How to Create an Immutable Class:

• Defensive Copies: When dealing with mutable objects passed to the constructor or returned by methods, create defensive copies. This practice prevents external code from modifying the internal state of the immutable object.
• Immutable Collections: Utilize immutable collections (like those provided in Java 9 and later) to simplify the creation of classes with immutable collection fields.
• Performance Considerations: Be mindful of the performance implications of creating defensive copies, especially in performance-critical applications.
• Use in Multi-threaded Environments: Immutable objects are inherently thread-safe, making them ideal for use in multi-threaded environments.
• String and Wrapper Types: Leverage the immutability of String and wrapper types (Integer, Long, and so on) as part of your immutable objects.
• Design Strategy: Consider immutability as a design strategy, especially for objects representing values that are not expected to change, such as configuration data, constants, or natural data types.

Advantages of Immutability:

• Simplicity and Clarity: Immutable objects are easier to understand and use. There's no need to track changes in state, reducing cognitive load.
• Thread Safety: Immutability eliminates issues related to concurrency and synchronization, as immutable objects can be freely shared between threads without synchronization.
• Caching and Reuse: Immutable objects can be cached and reused, as they are guaranteed not to change, reducing the overhead of object creation.
• Hashcode Caching: Immutable objects are great candidates for caching their hashcode, which can be beneficial in collections like HashMaps and HashSets .

39. What is the Decorator Pattern?

The Decorator Pattern is a structural design pattern used in object-oriented programming, and it's particularly useful for extending the functionality of objects at runtime. It is a robust alternative to subclassing, providing a more flexible approach to add responsibilities to objects without modifying their underlying classes.

Purpose of decorator pattern

The Decorator Pattern allows you to attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

The pattern involves a set of decorator classes that are used to wrap concrete components. Each decorator class has a reference to a component object and adds its own behavior either before or after delegating the task to the component object.

How to implement the decorator pattern

It typically involves an abstract decorator class that implements or extends the same interface or superclass as the objects it will dynamically add functionality to. Concrete decorators then extend the abstract decorator.

Key Components:

• Component: An interface or abstract class defining the operations that can be altered by decorators.
• Concrete Component: A class implementing or extending the Component, defining an object to which additional responsibilities can be attached.
• Decorator: An abstract class that extends or implements the Component interface and has a reference to a Component.
• Concrete Decorator: A class that extends the Decorator and adds functionalities to the Component it decorates.

Decorator example in Java:

Usage and advantages:.

• Flexibility: The Decorator Pattern provides a more flexible way to add responsibilities to objects compared to subclassing. New functionalities can be added at runtime.
• Avoid Class Explosion: It helps in avoiding an extensive hierarchy of subclasses when you need multiple combinations of functionalities.
• Single Responsibility Principle: Decorators allow functionalities to be divided into simple classes with single responsibilities.

Considerations:

• Complexity: Overuse of the decorator pattern can lead to complexity, making the code harder to understand and maintain.
• Instantiation Management: Managing the instantiation of decorated objects can be challenging, especially when dealing with multiple layers of decoration.

The Decorator Pattern is a powerful tool in a software developer's toolkit, offering a dynamic and flexible solution for extending object functionality. Understanding and applying this pattern can greatly enhance the design of software, particularly in situations where adding responsibilities to objects at runtime is necessary.

This pattern is highly valued in software development, as it showcases an ability to effectively manage and extend object functionalities without altering existing codebases, aligning with principles of maintainability and scalability.

40. Explain Java I/O Streams

Java I/O (Input/Output) streams are a fundamental part of the Java I/O API, providing a robust framework for handling input and output operations in Java. Understanding these streams is crucial for efficient data handling in Java applications.

Overview of Java I/O Streams

I/O streams in Java are used to read data from an input source and to write data to an output destination. The Java I/O API is rich and provides various classes to handle different types of data, like bytes, characters, objects, etc.

Stream Types:

Java I/O streams are broadly categorized into two types:

• Byte Streams: Handle I/O of raw binary data.
• Character Streams: Handle I/O of character data, automatically handling character encoding and decoding.

Byte Streams:

• Classes: InputStream and OutputStream are abstract classes at the hierarchy's root for byte streams.
• Usage: They are used for reading and writing binary data, such as image or video files.
• Example Classes: FileInputStream , FileOutputStream , BufferedInputStream , BufferedOutputStream , etc.

Character Streams:

• Classes: Reader and Writer are abstract classes for character streams.
• Usage: Suitable for handling textual data, ensuring correct interpretation of characters according to the default character encoding.
• Example Classes: FileReader , FileWriter , BufferedReader , BufferedWriter , etc.

Key Features of Java I/O Streams:

• Stream Hierarchy: Java uses a hierarchy of classes to manage different types of I/O operations, allowing for flexibility and reusability of code.
• Decorators: Java I/O uses decorators, where one stream wraps another and adds additional capabilities, like buffering, data conversion, and so on.
• Buffering: Buffering is a common practice in I/O streams to enhance I/O efficiency, allowing for the temporary storage of data in memory before it's written to or read from the actual I/O source.
• Exception Handling: I/O operations in Java are prone to errors like file not found, access denied, etc. Hence, most I/O operations throw IOException , which must be properly handled using try-catch blocks or thrown further.
• Use Buffered Streams: Always use buffered streams ( BufferedInputStream , BufferedOutputStream , BufferedReader , BufferedWriter ) for efficient I/O operations, as they reduce the number of actual I/O operations by buffering chunks of data.
• Close Streams: Ensure streams are closed after their operation is complete to free up system resources. This is typically done in a finally block or using try-with-resources introduced in Java 7.
• Error Handling: Implement robust error handling. I/O operations are susceptible to many issues, so proper exception handling is crucial.
• Character Encoding: Be mindful of character encoding while using character streams. Incorrect handling of encoding can lead to data corruption.

Practical Example:

In this example, BufferedReader and BufferedWriter are used for reading from and writing to a text file, demonstrating the use of character streams with buffering for efficiency.

Java I/O streams form the backbone of data handling in Java applications. Understanding the distinction between byte and character streams, along with the proper use of buffering and exception handling, is essential for writing efficient, robust, and maintainable Java code.

This knowledge is vital for Java developers and is often a subject of interest in technical interviews, showcasing one's capability to handle data proficiently in Java applications.

41. How Does the Garbage Collector Work in Java?

In Java, garbage collection (GC) is a critical process of automatically freeing memory by reclaiming space from objects that are no longer in use, ensuring efficient memory management.

Understanding how the garbage collector works in Java is essential for writing high-performance applications and is a key area of knowledge in professional Java development.

Overview of Garbage Collection in Java

The primary function of garbage collection in Java is to identify and discard objects that are no longer needed by a program. This prevents memory leaks and optimizes memory usage.

Automatic Memory Management

Unlike languages where memory management is manual (like C/C++), Java provides automatic memory management through its garbage collector, which runs in the background.

How the Garbage Collector Works

Object creation and heap storage:.

In Java, objects are created in a heap memory area. This heap is divided into several parts – Young Generation, Old Generation (or Tenured Generation), and Permanent Generation (replaced by Metaspace in Java 8).

• Young Generation: Newly created objects reside in the Young Generation, which is further divided into three parts: one Eden space and two Survivor spaces (S0 and S1). Most objects die young. When the Eden space fills up, a minor GC is triggered, moving surviving objects to one of the Survivor spaces (S0 or S1) and clearing Eden.
• Aging of Objects: As objects survive more garbage collection cycles, they age. After surviving certain cycles, they are moved to the Old Generation.
• Old Generation: The Old Generation stores long-living objects. A more comprehensive form of GC, known as major GC, occurs here, which is generally more time-consuming.
• Metaspace (Java 8 and above): Metaspace stores metadata of classes. Unlike the PermGen (Permanent Generation) space in earlier Java versions, Metaspace uses native memory, and its size is not fixed but can be configured.

Types of Garbage Collectors in Java:

• Serial GC: Suitable for single-threaded environments. It freezes all application threads during garbage collection.
• Parallel GC: Also known as Throughput Collector, it uses multiple threads for young generation garbage collection but stops all application threads during major GC.
• Concurrent Mark Sweep (CMS) GC: Minimizes pauses by doing most of its work concurrently with application threads but requires more CPU resources.
• G1 Garbage Collector: Designed for large heap memory areas, it divides the heap into regions and prioritizes GC on regions with the most garbage first.

Garbage Collection Processes

The process starts by marking all reachable objects. Reachable objects are those that are accessible directly or indirectly through references from root objects (like local variables, static fields, etc.).

Unreachable objects (those not marked as reachable) are considered for deletion .

To prevent fragmentation and optimize memory usage, some garbage collectors perform compaction , moving surviving objects closer together.

• Avoid Memory Leaks: Despite automatic garbage collection, memory leaks can still occur (for example, through static references). It's crucial to be mindful of object references and their lifecycles.
• GC Tuning: For high-performance applications, GC tuning can be essential. Understanding different garbage collector types and their configuration parameters allows for optimal tuning according to application needs.
• Monitoring and Profiling: Regular monitoring of garbage collection and memory usage is important, especially for applications with high throughput or large heaps.

Garbage collection in Java is a sophisticated system designed to efficiently manage memory in the Java Virtual Machine (JVM). An in-depth understanding of how garbage collection works, its types, and its impact on application performance is essential for Java developers, particularly those working on large-scale, high-performance applications.

This knowledge not only helps in writing efficient and robust applications but also is a valuable skill in troubleshooting and performance tuning, aspects highly regarded in the field of software development.

42. What Are the Benefits of Using Java NIO?

Java NIO (New Input/Output), introduced in JDK 1.4, marks a substantial advancement in Java's approach to I/O operations. It was developed to address the constraints of traditional I/O methods, leading to improved scalability and efficiency.

This makes Java NIO particularly advantageous in scenarios demanding high throughput and concurrent access.

Let’s discuss the key benefits of using Java NIO in detail.

1. Channels and Buffers: Enhanced Data Handling

• Channels : These are bi-directional conduits allowing both reading and writing operations. Unlike traditional unidirectional streams, channels simplify I/O patterns, especially for network sockets, by enabling two-way communication within a single channel.
• Buffers : Acting as fixed-size data containers, buffers allow batch processing of data. This is more efficient compared to the byte-by-byte processing in traditional I/O, as it enables handling data in larger, more manageable blocks.

2. Non-blocking and Asynchronous I/O

Java NIO supports non-blocking and asynchronous I/O operations, a stark contrast to the blocking nature of traditional I/O where a thread remains idle until an operation completes.

This feature of NIO means a thread can initiate an I/O operation and continue performing other tasks without waiting for the I/O process to finish. This capability significantly enhances the scalability and responsiveness of applications, making them more efficient in handling multiple concurrent I/O requests.

3. Practical Applications

Java NIO is particularly effective in environments that require high-performance and low latency, such as:

• Web and Application Servers : Managing high-volume network traffic efficiently.
• Real-time Systems : Like trading platforms where quick data processing is critical.
• Big Data Applications : Benefiting from efficient handling of large datasets.
• File-based Database Systems : Where efficient file I/O operations are crucial.

4. Channels: The Foundation of NIO’s Architecture

Channels serve as the backbone of NIO, providing a more unified and simplified interface for various I/O operations. They come in different types, each catering to specific needs:

• FileChannel : For file operations.
• SocketChannel and ServerSocketChannel : For TCP network communications.
• DatagramChannel : For UDP operations.
• Pipes : For inter-thread communication. Particularly in network operations, the ability of channels to operate in a non-blocking mode allows a single thread to handle multiple connections, enhancing the application’s scalability.

5. Buffers: Central to NIO’s Data Transfer

Buffers in NIO are essential for data transfer, acting as temporary storage for data during I/O operations. Their key operations include:

• Put and Get : For writing and reading data.
• Flip : To switch modes between reading and writing.
• Clear and Compact : Preparing the buffer for new data. Different buffer types (like ByteBuffer, CharBuffer, IntBuffer) cater to various data primitives, enhancing the flexibility and efficiency of data handling. Notably, direct buffers, which are allocated outside of the JVM heap, can provide faster I/O operations, though they come with higher allocation and deallocation costs.

6. Selectors: Streamlining Scalable I/O Operations

Selectors are a unique NIO feature enabling a single thread to monitor multiple channels for readiness, thus efficiently managing numerous I/O operations. This reduces the need for multiple threads, cutting down on resource usage and context switching, which is particularly advantageous in high-performance environments.

7. Improved Performance and Scalability

The amalgamation of channels, buffers, and selectors provides a substantial performance boost. The non-blocking nature of NIO minimizes idle thread time, and managing multiple channels with a single thread significantly improves the scalability. This is pivotal in server environments dealing with numerous simultaneous connections.

Java NIO offers a robust, scalable, and efficient framework for handling I/O operations, addressing many of the limitations of traditional I/O. Its design is particularly advantageous for high-throughput and concurrent-processing systems.

While the complexity of NIO might be higher compared to traditional I/O, the performance and scalability benefits it provides make it an indispensable tool for developers working on large-scale, I/O-intensive Java applications.

43. Explain the Observer Pattern

The Observer pattern is a design pattern where an object, known as the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

It's particularly useful in the scenario where a single object needs to notify an array of objects about a change in its state. In the context of a newsletter system, the Observer pattern can be effectively used to notify subscribers whenever a new post is available.

How to Implement the Observer Pattern for a Newsletter System

Let's break down the implementation using the Observer pattern in the context of a newsletter system:

• Subject (Newsletter) : This is the entity being observed. It will notify all attached observers when a new post is available.
• Observer (Subscriber) : These are the observers who wish to be notified about new posts in the newsletter.
• Client : This will use both the Subject and Observers.

Step 1: Create the Subject Class (Newsletter)

Step 2: create the observer abstract class (subscriber), step 3: create concrete observer classes.

EmailSubscriber.java

SMSSubscriber.java

Step 4: Use the Newsletter and Concrete Subscriber Objects

Step 5: output verification.

When running NewsletterSystemDemo , the output will be something like:

This output indicates that both the email and SMS subscribers are notified whenever the newsletter has a new post.

The Observer pattern provides a clean and straightforward way to implement a subscription mechanism in a newsletter system, ensuring that all subscribers are automatically updated with the latest posts.

This pattern enhances modularity and separation of concerns, making the system easier to understand, maintain, and extend.

44. Explain the Purpose of the this Keyword.

The this keyword in Java serves a very specific and useful purpose. It refers to the current instance of the class in which it is used. This is particularly valuable in scenarios where you need to distinguish between class fields (instance variables) and parameters or variables within a method that have the same name. Let's break it down:

Reference to Instance Variables: When a class’s field is shadowed by a method or constructor parameter, this can be used for referencing the class's field. For instance, in a setter method, this helps differentiate between the instance variable and the parameter passed to the method.

Calling One Constructor from Another: In a class with overloaded constructors, this can be used to call one constructor from another, avoiding code duplication.

Returning the Current Instance: Methods can return this to return the current class instance. This is often used in method chaining.

Passing the Current Instance to Another Method: this can be passed as an argument in the method call or constructor call. This is common in event handling.

Disambiguation: It eliminates ambiguity when instance variables and parameters or local variables share the same name.

45. Explain Java's try-with-resources.

Java's try-with-resources, introduced in Java 7, is a mechanism that ensures more efficient handling of resources, like files or sockets, in Java. Its primary purpose is to simplify the cleanup of resources which must be closed after their operations are completed.

Key Characteristics:

Automatic Resource Management: In try-with-resources, resources declared within the try clause are automatically closed at the end of the statement, even if exceptions are thrown. This reduces boilerplate code significantly as compared to traditional try-catch-finally blocks.

Syntax: The resources that implement java.lang.AutoCloseable or java.io.Closeable are declared and initialized within parentheses just after the try keyword.

• Here, the BufferedReader instance is automatically closed when the try block exits, regardless of whether it exits normally or due to an exception.
• Exception Handling: Any exception thrown by the automatic closure of resources is suppressed if an exception is thrown in the try block. These suppressed exceptions can be retrieved using Throwable.getSuppressed() method.
• Improved Readability and Reliability: This structure enhances code readability and reliability. It reduces the risk of resource leaks, as the closing of resources is handled automatically.
• Use in Custom Resources: Custom classes can also utilize this mechanism by implementing the AutoCloseable interface and overriding the close method.

Practical Implications:

In real-world applications, try-with-resources ensures that resources like file streams, database connections, or network sockets are closed properly, preventing resource leaks which could lead to performance issues and other bugs. It is especially valuable in large-scale applications where resource management is critical for efficiency and reliability.

46. Explain the Difference between C++ and Java.

When distinguishing between C++ and Java, it's important to understand that both are powerful programming languages with their unique characteristics and use cases.

They share some similarities, as both are object-oriented and have similar syntax (being influenced by C), but there are key differences that set them apart.

Language Nature and Design Philosophy:

C++ is a multi-paradigm language that supports both procedural and object-oriented programming. It's often chosen for system-level programming due to its efficiency and fine-grained control over memory management.

Java , on the other hand, is primarily object-oriented and designed with a simpler approach to avoid common programming errors (like pointer errors in C++). Java's design principle "Write Once, Run Anywhere" (WORA) emphasizes portability, which is achieved through the Java Virtual Machine (JVM).

Memory Management:

In C++ , memory management is manual. Programmers have direct control over memory allocation and deallocation using operators like new and delete .

Java abstracts away the complexity of direct memory management through its Automatic Garbage Collection, which periodically frees memory that's no longer in use, reducing the likelihood of memory leaks but at the cost of less control and potential overhead.

Platform Dependency and Portability:

C++ is platform-dependent. A C++ program needs to be compiled for each specific platform it's intended to run on, which can lead to more work when targeting multiple platforms.

Java is platform-independent at the source level. Java programs are compiled into bytecode, which can run on any device equipped with a JVM, making it highly portable.

Runtime and Performance:

C++ generally offers higher performance than Java. It compiles directly to machine code, which the CPU executes, resulting in faster execution suitable for performance-critical applications.

Java may have slower performance due to the added abstraction layer of the JVM. But improvements in Just-In-Time (JIT) compilers within the JVM have significantly narrowed this performance gap.

Pointers and Memory Safety:

C++ supports both pointers and references, allowing for powerful, albeit potentially risky, memory manipulation.

Java has references but does not support pointers (at least not in the traditional sense), reducing the risk of memory access errors, thereby increasing program safety.

Exception Handling:

C++ supports exception handling but does not enforce error handling (uncaught exceptions can lead to undefined behavior).

Java has a robust exception handling mechanism, requiring checked exceptions to be caught or declared in the method signature, promoting better error management practices.

Multi-Threading:

C++ has more complex approaches to multi-threading and requires careful management to ensure thread safety.

Java provides built-in support for multi-threading with synchronized methods and blocks, making concurrent programming more manageable.

Standard Template Library (STL) vs. Java Standard Library:

C++ 's STL is a powerful library that offers containers, algorithms, iterators, and so on for efficient data manipulation.

Java 's Standard Library provides a rich set of APIs, including collections, streams, networking, and so on with a focus on ease of use.

Legacy and Use Cases:

C++ is often chosen for system/software development, game development, and applications where hardware access and performance are critical.

Java is widely used in enterprise environments, web services, and Android app development due to its portability and robust libraries.

Both C++ and Java have their strengths and are chosen based on the requirements of the project.

C++ is preferred for scenarios where performance and memory control are crucial, while Java is ideal for applications where portability and ease of use are more important.

Understanding these differences is key in selecting the right language for a particular task or project, and adapting to the strengths of each can lead to more efficient and effective programming practices.

47. What is Polymorphism? Provide an Example.

Polymorphism, a fundamental concept in object-oriented programming, allows objects to be treated as instances of their parent class or interface. It’s a Greek word meaning “many shapes” and in programming, it refers to the ability of a single function or method to work in different ways based on the object it is acting upon.

There are two primary types of polymorphism: compile-time (or static) polymorphism and runtime (or dynamic) polymorphism.

Compile-Time Polymorphism : This is achieved through method overloading and operator overloading. It’s called compile-time polymorphism because the decision about which method to call is made by the compiler.

Method Overloading involves having multiple methods in the same scope, with the same name but different parameters.

In this example, the operate method is overloaded with different parameter types, allowing it to behave differently based on the type of arguments passed.

Runtime Polymorphism : This is mostly achieved through method overriding, which is a feature of inheritance in object-oriented programming. In runtime polymorphism, the method to be executed is determined at runtime.

Method Overriding involves defining a method in a subclass that has the same name, return type, and parameters as a method in its superclass.

In this example, the speak method in the subclass Dog overrides the speak method in its superclass Animal . When the speak method is called on an object of type Dog , the overridden method in the Dog class is executed, demonstrating runtime polymorphism.

Why Polymorphism is Important

• Flexibility and Extensibility : Polymorphism allows for flexible and extensible code. You can create a more generalized code that works on the superclass type, and it automatically adapts to the specific subclass types.
• Code Reusability : It enables the reuse of code through inheritance and the ability to override or overload methods.
• Loose Coupling : By using polymorphic behavior, components can be designed loosely coupled, which means a change in one part of the system causes minimal or no effect on other parts of the system.
• Simplifies Code Maintenance : With polymorphism, developers can write more maintainable and manageable code, as changes to a superclass are inherited by all subclasses, reducing the need for changes across multiple classes.

Polymorphism is a cornerstone in the world of object-oriented programming, enabling more dynamic and flexible code. It allows objects to interact in a more abstract manner, focusing on the shared behavior rather than the specific types.

Understanding and effectively using polymorphism can lead to more robust and maintainable code, a crucial aspect for any software developer looking to excel in their field.

48. How Can You Avoid Memory Leaks in Java?

Avoiding memory leaks in Java, despite its automated garbage collection mechanism, requires a deep understanding of how memory allocation and release work in Java, alongside meticulous coding practices and effective use of analysis tools.

Let’s delve into some advanced and specific strategies for preventing memory leaks in Java applications:

Understand Object Lifecycle and Scope:

• Scope Management : Ensure objects are scoped as narrowly as possible. For instance, use local variables within methods rather than class-level variables if the data does not need to persist beyond the method’s execution context.
• Reference Management : Be cautious with static references. Static fields can keep objects alive for the lifetime of the class, potentially leading to memory leaks.

Efficient Use of Collections:

• WeakHashMap : For cache implementations, consider using WeakHashMap . It uses weak references for keys, which allows keys (and their associated values) to be garbage-collected when no longer in use.
• Data Structure Choice : Be mindful of the choice of data structure. For example, use ArrayList over LinkedList for large lists of data where frequent access is required, as LinkedList can consume more memory due to the storage of additional node references.

Leveraging WeakReferences and SoftReferences :

• SoftReferences for Caches : Use SoftReference for memory-sensitive caches. The garbage collector will only remove soft-referenced objects if it needs memory, making them more persistent than weak references.
• WeakReferences for Listeners : Utilize WeakReference for listener patterns where listeners might not be explicitly removed.

Managing Resources and I/O:

• AutoCloseable and Try-with-Resources : For resources like streams, files, and connections, use try-with-resources for automatic closure. Ensure that objects implementing AutoCloseable are closed properly to release resources.

Inner Classes Handling:

• Static Inner Classes : Prefer static inner classes over non-static to avoid the implicit reference to the outer class instance, which can prevent the outer instance from being garbage-collected.

Profiling and Leak Detection:

• Heap Dump Analysis : Regularly analyze heap dumps in tools like Eclipse Memory Analyzer (MAT) to detect large objects and potential memory leaks.
• Java Flight Recorder : Use Java Flight Recorder for runtime analysis and monitoring, which can help identify memory leaks.

ThreadLocal Variables Management:

• Explicit Removal : Always remove ThreadLocal variables after use, particularly in thread-pooled environments like servlet containers or application servers.

ClassLoader Leaks:

• ClassLoader Lifecycle : In environments with dynamic class loading/unloading (for example, web servers), ensure that class loaders are garbage collected when not needed. This involves ensuring that classes loaded by these class loaders are no longer referenced.

Garbage Collection Tuning:

• GC Analysis : Analyze GC logs to understand the garbage collection behavior and identify potential memory leaks.
• GC Algorithm Choice : Choose an appropriate garbage collection algorithm based on application needs, which can be tuned with JVM options for optimal performance.

String Interning:

• Selective Interning : Be cautious with the String.intern() method. Unnecessary interning of strings can lead to a bloated String pool.

Static Analysis Tools:

Utilize tools like SonarQube, FindBugs, or PMD to statically analyze code for patterns that could lead to memory leaks.

Developer Training and Code Reviews:

Regularly train developers on best practices in memory management and conduct thorough code reviews with a focus on potential memory leak patterns.

Memory leak prevention in Java is a sophisticated practice that involves a thorough understanding of Java memory management, careful coding, diligent use of analysis tools, and regular monitoring.

By adopting these advanced practices, developers can significantly mitigate the risk of memory leaks, leading to more robust, efficient, and scalable Java applications.

49. Explain the Purpose of Java's Synchronized Block

The purpose of Java's synchronized block is to ensure thread safety in concurrent programming by controlling access to a shared resource among multiple threads.

In a multithreaded environment, where multiple threads operate on the same object, there's a risk of data inconsistency if the threads simultaneously modify the object. A synchronized block in Java is used to lock an object for exclusive access by a single thread.

Thread Safety and Data Consistency:

When different threads access and modify shared data, it can lead to unpredictable data states and inconsistencies. The synchronized block ensures that only one thread can execute a particular block of code at a time, thus maintaining data integrity.

Lock Mechanism:

In Java, each object has an intrinsic lock or monitor lock. When a thread enters a synchronized block, it acquires the lock on the specified object. Other threads attempting to enter the synchronized block on the same object are blocked until the thread inside the synchronized block exits, thereby releasing the lock.

Syntax and Usage:

The synchronized block is defined within a method, and you must specify the object that provides the lock:

The lockObject is a reference to the object whose lock the synchronized block acquires. It can be this to lock the current object, a class object for class-level locks, or any other object.

Advantages Over Synchronized Methods:

Compared to synchronized methods, synchronized blocks provide finer control over the scope and duration of the lock.

While a synchronized method locks the entire method, a synchronized block can lock only the part of the method that needs synchronization, potentially improving performance.

Avoiding Deadlocks:

Take care to avoid deadlocks, a situation where two or more threads are blocked forever, each waiting for the other's lock. This usually occurs when multiple synchronized blocks are locking objects in an inconsistent order.

Synchronized blocks also solve memory visibility problems. Changes made by one thread in a synchronized block are visible to other threads entering subsequent synchronized blocks on the same object.

Best Practices

• Minimize Lock Contention : Keep the synchronized sections as short as possible to minimize lock contention and avoid performance bottlenecks.
• Consistent Locking Order : Always acquire locks in a consistent order to prevent deadlocks.
• Avoid Locking on Public Objects : Locking on public objects can lead to accidental and uncontrolled access to the lock, increasing the deadlock risk. Prefer private objects as lock targets.
• Complement with Other Concurrency Tools : In some cases, using higher-level concurrency tools like ReentrantLock , Semaphore , or concurrent collections from java.util.concurrent package might be more appropriate.

Java's synchronized block is a critical tool for achieving thread safety in concurrent applications. Its proper use ensures data integrity and consistency by controlling access to shared resources. But, it requires careful consideration to avoid common pitfalls like deadlocks and performance issues due to excessive lock contention.

Understanding and applying these concepts is essential for developers working in a multithreaded environment to create robust and efficient Java applications.

50. Explain the Concept of Modules in Java

Modules in Java, introduced in Java 9 with the Java Platform Module System (JPMS), represent a fundamental shift in organizing Java applications and their dependencies.

Understanding modules is essential for modern Java development, as they offer improved encapsulation, reliable configuration, and scalable system architectures.

What are Java modules?

A module in Java is a self-contained unit of code and data, with well-defined interfaces for communicating with other modules. Each module explicitly declares its dependencies on other modules.

Modules enable better encapsulation by allowing a module to expose only those parts of its API which should be accessible to other modules, while keeping the rest of its codebase hidden. This reduces the risk of unintended usage of internal APIs.

Key Components of modules:

module-info.java : Each module must have a module-info.java file at its root, which declares the module's name, its required dependencies, and the packages it exports.

• Here, com.example.myapp is the module name, java.sql is a required module, and com.example.myapp.api is the exported package.
• Exports and Requires: The exports keyword specifies which packages are accessible to other modules, while requires lists the modules on which the current module depends.
• Improved Application Structure: Modules encourage a cleaner, more organized code structure, helping in maintaining large codebases and improving code quality.
• Reduced Memory Footprint: By only loading the required modules, applications can reduce their memory footprint and start-up time, enhancing performance.
• Enhanced Security and Maintenance: Modules reduce the surface area for potential security vulnerabilities. They also simplify dependency management, making it easier to update and maintain libraries without affecting the entire system.

Consider a scenario where you are developing a large-scale application with various functionalities like user management, data processing, and reporting. By organizing these functionalities into separate modules (like usermodule , dataprocessmodule , reportmodule ), you can maintain them independently, avoiding the complexities of a monolithic application structure.

Modules in Java are a powerful feature for building scalable, maintainable, and efficient applications. They offer clear boundaries and contracts between different parts of a system, facilitating better design and architecture.

For developers and teams aiming to build robust Java applications, understanding and leveraging modules is not just a technical skill but a strategic approach to software development.

This modular architecture aligns with modern development practices, enabling Java applications to be more scalable and easier to manage in the long term.

As we wrap up this roundup of Java interview questions, I want to take a moment to thank the freeCodeCamp team. This platform is a fantastic resource for people learning to code, and it's great to have such a supportive community in the tech world.

I also want to thank the editorial team for their help in making this guide possible. Working together has been a great experience, and it's been rewarding to combine our efforts to help others learn Java.

It's important to reflect on the journey we've undertaken together. Java's robustness in Object-Oriented Programming (OOP) is a critical asset for developers at all levels, especially those aspiring to join top-tier tech firms. This handbook has aimed to provide a clear pathway to mastering Java interviews, focusing on the insights and techniques that matter most in the competitive landscape of big tech.

From the fundamentals to the more complex aspects of Java, I've sought to bridge the gap between basic Java knowledge and the sophisticated expertise that industry leaders like Google value. This resource is crafted not just for those new to Java, but also for those revisiting key concepts, offering a comprehensive understanding of the language in a practical context.

As you continue to explore the depths of Java, remember that mastering this language is not just about enhancing coding skills, but also about expanding your professional horizons. Java's significant role in IoT and its presence in billions of devices worldwide make it a language that can truly shape your career.

In closing, I hope this handbook has provided you with valuable insights and a strong foundation for your future endeavors in Java programming and beyond. Whether you're preparing for a big tech interview or simply looking to refine your software development skills, this guide is a stepping stone towards achieving those goals.

If you're keen on furthering your Java knowledge, here's a guide to help you conquer Java and launch your coding career . It's perfect for those interested in AI and machine learning, focusing on effective use of data structures in coding. This comprehensive program covers essential data structures, algorithms, and includes mentorship and career support.

Additionally, for more practice in data structures, you can explore these resources:

• Java Data Structures Mastery - Ace the Coding Interview : A free eBook to advance your Java skills, focusing on data structures for enhancing interview and professional skills.
• Foundations of Java Data Structures - Your Coding Catalyst : Another free eBook, diving into Java essentials, object-oriented programming, and AI applications.

Visit LunarTech's website for these resources and more information on the bootcamp .

Connect with Me:

• Follow me on LinkedIn for a ton of Free Resources in CS, ML and AI
• Visit my Personal Website
• Subscribe to my The Data Science and AI Newsletter

About the Author

I'm Vahe Aslanyan, deeply engaged in the intersecting worlds of computer science, data science, and AI. I invite you to explore my portfolio at vaheaslanyan.com, where I showcase my journey in these fields. My work focuses on blending full-stack development with AI product optimization, all fueled by a passion for innovative problem-solving.

I've had the privilege of contributing to the launch of a well-regarded data science bootcamp and collaborating with some of the best minds in the industry. My goal has always been to raise the bar in tech education, making it accessible and standard for everyone.

As we conclude our journey here, I want to thank you for your time and engagement. Sharing my professional and academic experiences in this book has been a rewarding experience. I appreciate your involvement and look forward to seeing how it helps you advance in the tech world.

I'm Vahe Aslanyan, dedicated to making AI and data science education inclusive and accessible. I guide developers towards clear tech understanding in software engineering.

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199+ List of Basic Java Programs For Practice With Solutions PDF

199+ List of Basic Java Programs For Practice With Solutions. Java programs are frequently asked in interviews. These programs can be asked from control statements, array, string, oops etc. Java programs for practice for intermediate. Java basic programs like the Fibonacci series, prime numbers, factorial numbers and palindrome numbers are frequently asked in interviews and exams. Beginners java programs for practice PDF Download. If you are new to Java programming, we will recommend you read our Java tutorial first.

Here we have listed down the 11 Java programming categories in which you can find the 200+ List of Basic Java Programs For Practice With Solutions. We almost covered all the categories in java programming, Ie. Basic Programs, Array Programs, Matrix Programs, String Programs, Conversion Programs, Pattern programs, Singly Linked List Programs, Circular Linked List Programs, Doubly Linked List Programs, Tree Programs and Searching and Sorting Programs.

Top 100 Java Programs PDF – Table of Contents

199+ java programs for practice with category wise.

Below are the top 11 categories. all the Programs List in Java is related to this category.

• Java Basic Programs
• Java Array Programs
• Java Matrix Programs
• Java String Programs
• Java Searching and Sorting Programs
• Java Conversion Programs
• Pattern programs
• Singly Linked List Programs
• Circular Linked List Programs
• Doubly Linked List Programs
• Tree Programs

Basic Java Programs For Practice

• Fibonacci Series in Java
• Prime Number Program in Java
• Palindrome Program in Java
• Factorial Program in Java
• Armstrong Number in Java
• How to Generate Random Numbers in Java
• How to Print Patterns in Java
• How to Compare Two Objects in Java
• How to Create Objects in Java
• How to Print ASCII Value in Java

Array Java Programs For Practice

• Java Program to copy all elements of one array into another array
• Java Program to find the frequency of each element in the array
• Java Program to left rotate the elements of an array
• Java Program to print the duplicate elements of an array
• Java Program to print the elements of an array
• Java Program to print the elements of an array in reverse order
• Java Program to print the elements of an array present on an even position
• Java Program to print the elements of an array present in an odd position
• Java Program to print the largest element in an array
• Java Program to print the smallest element in an array
• Java Program to print the number of elements present in an array
• Java Program to print the sum of all the items of the array
• Java Program to right rotate the elements of an array
• Java Program to sort the elements of an array in ascending order
• Java Program to sort the elements of an array in descending order
• Java Program to Find 3rd Largest Number in an array
• Java Program to Find 2nd Largest Number in an array
• Java Program to Find the Largest Number in an array
• Java to Program Find 2nd Smallest Number in an array
• Java Program to Find the Smallest Number in an array
• Java Program to Remove Duplicate Elements in an array
• Java Program to Print Odd and Even Numbers from an array
• How to Sort an Array in Java

Java Matrix Programs For Practice

Simple Matrix Java Programs for Practice for Beginners and Intermediate.

• Java Program to Add Two Matrices
• Java Program to Multiply Two Matrices
• Java Program to subtract the two matrices
• Java Program to determine whether two matrices are equal
• Java Program to display the lower triangular matrix
• Java Program to display the upper triangular matrix
• Java Program to find the frequency of odd & even numbers in the given matrix
• Java Program to find the product of two matrices
• Java Program to find the sum of each row and each column of a matrix
• Java Program to find the transpose of a given matrix
• Java Program to determine whether a given matrix is an identity matrix
• Java Program to determine whether a given matrix is a sparse matrix
• Java Program to Transpose the matrix

String Programs for Practice

• Java Program to count the total number of characters in a string
• Java Program to count the total number of characters in a string 2
• Java Program to count the total number of punctuation characters that exist in a String
• Java Program to count the total number of vowels and consonants in a string
• Java Program to determine whether two strings are the anagram
• Java Program to divide a string into ‘N’ equal parts.
• Java Program to find all subsets of a string
• Java Program to find the longest repeating sequence in a string
• Java Program to find all the permutations of a string
• Java Program to remove all the white spaces from a string
• Java Program to replace lower-case characters with upper-case and vice-Versa
• Java Program to replace the spaces of a string with a specific character
• Java Program to determine whether a given string is a palindrome
• Java Program to determine whether one string is a rotation of another
• Java Program to find a maximum and minimum occurring character in a string
• Java Program to find the Reverse of the string
• Java program to find the duplicate characters in a string
• Java program to find the duplicate words in a string
• Java Program to find the frequency of characters
• Java Program to find the largest and smallest word in a string
• Java Program to find the most repeated word in a text file
• Java Program to find the number of words in the given text file
• Java Program to separate the Individual Characters from a String
• Java Program to swap two string variables without using a third or temp variable.
• Java Program to print the smallest and biggest possible palindrome word in a given string

Java Searching and Sorting Basic Java Programs For Practice

• Linear Search in Java
• Binary Search in Java
• Bubble Sort in Java
• Selection Sort in Java
• Insertion Sort in Java

Conversion Programs List in Java for Practice

• How to convert String to int in Java
• How to convert int to String in Java
• How to convert String to long in Java
• How to convert long to String in Java
• How to convert String to float in Java
• How to convert float to String in Java
• How to convert String to double in Java
• How to convert double to String in Java
• How to convert String to Date in Java
• How to convert Date to String in Java
• How to convert String to char in Java
• How to convert char to String in Java
• How to convert String to Object in Java
• How to convert an Object to a String in Java
• How to convert int to long in Java
• How to convert long to int in Java
• How to convert int to double in Java
• How to convert double to int in Java
• How to convert char to int in Java
• How to convert int to char in Java
• How to convert String to boolean in Java
• How to convert a boolean to a String in Java
• How to convert date to Timestamp in Java
• How to convert timestamp to Date in Java
• How to convert binary to Decimal in Java
• How to convert decimal to Binary in Java
• How to convert hex to Decimal in Java
• How to convert decimal to Hex in Java
• How to convert octal to Decimal in Java
• How to convert decimal to Octal in Java

Java Simple Java Programs for Practice

Write down the Pattern Programs Java Programs for Practice Online, Beginner must practice these programs for better understanding.

• Java program to print the following spiral pattern on the console
• Java program to print the following pattern
• Java program to print the following pattern 2
• Java program to print the following pattern 3
• Java program to print the following pattern 4
• Java program to print the following pattern 5
• Java program to print the following pattern on the console

Singly Linked List Difficult Java Programs for Practice

• Java Program to create and display a singly linked list
• Java program to create a singly linked list of n nodes and count the number of nodes
• Java program to create a singly linked list of n nodes and display it in reverse order
• Java program to delete a node from the beginning of the singly linked list
• Java program to delete a node from the middle of the singly linked list
• Java program to delete a node from the end of the singly linked list
• Java program to determine whether a singly linked list is the palindrome
• Java program to find the maximum and minimum value node from a linked list
• Java Program to insert a new node in the middle of the singly linked list
• Java program to insert a new node at the beginning of the singly linked list
• Java program to insert a new node at the end of the singly linked list
• Java program to remove duplicate elements from a singly linked list
• Java Program to search an element in a singly linked list

Circular Linked List Java Programs for Practice Intermediate

• Java program to create and display a Circular Linked List
• Java program to create a Circular Linked List of N nodes and count the number of nodes
• Java program to create a Circular Linked List of n nodes and display it in reverse order
• Java program to delete a node from the beginning of the Circular Linked List
• Java program to delete a node from the end of the Circular Linked List
• Java program to delete a node from the middle of the Circular Linked List
• Java program to find the maximum and minimum value node from a circular linked list
• Java program to insert a new node at the beginning of the Circular Linked List
• Java program to insert a new node at the end of the Circular Linked List
• Java program to insert a new node in the middle of the Circular Linked List
• Java program to remove duplicate elements from a Circular Linked List
• Java program to search an element in a Circular Linked List
• Java program to sort the elements of the Circular Linked List

Java Doubly Linked List Programs

16 Advanced Java Programs for Practice for intermediate. You must also C Programs for Practice .

• Java program to convert a given binary tree to a doubly-linked list
• Java program to create a doubly linked list from a ternary tree
• Java program to create a doubly-linked list of n nodes and count the number of nodes
• Java program to create a doubly-linked list of n nodes and display it in reverse order
• Java program to create and display a doubly linked list
• Java program to delete a new node from the beginning of the doubly linked list
• Java program to delete a new node from the end of the doubly linked list
• Java program to delete a new node from the middle of the doubly linked list
• Java program to find the maximum and minimum value node from a doubly linked list
• Java program to insert a new node at the beginning of the Doubly Linked list
• Java program to insert a new node at the end of the Doubly Linked List
• Java program to insert a new node in the middle of the Doubly Linked List
• Java program to remove duplicate elements from a Doubly Linked List
• Java program to rotate doubly linked list by N nodes
• Java program to search an element in a doubly-linked list
• Java program to sort the elements of the doubly linked list

Tree Advanced Java Programs for Practice

• Java Program to calculate the Difference between the Sum of the Odd Level and the Even Level Nodes of a Binary Tree
• Java program to construct a Binary Search Tree and perform deletion and In-order traversal
• Java program to convert Binary Tree to Binary Search Tree
• Java program to determine whether all leaves are at the same level
• Java program to determine whether two trees are identical
• Java program to find the maximum width of a binary tree
• Java program to find the largest element in a Binary Tree
• Java program to find the maximum depth or height of a tree
• Java program to find the nodes which are at the maximum distance in a Binary Tree
• Java program to find the smallest element in a tree
• Java program to find the sum of all the nodes of a binary tree
• Java program to find the total number of possible Binary Search Trees with N keys
• Java program to implement Binary Tree using the Linked List
• Java program to search a node in a Binary Tree

Where Can I Practice Java Exercises?

There are 199+ Lists of Basic Java Programs For Practice With Solutions for you as a beginner. Tutorials Bookmarks Provides 500 programs to check your coding skills in all different-different categories.

Can I Study Java in 2 Days?

The answer is No In a single word. It’s not possible to learn any programming in just 2 days.

Programs List in Java for Practice

• Java Arrays
• Java Strings
• Java Collection
• Java 8 Tutorial
• Java Multithreading
• Java Exception Handling
• Java Programs
• Java Project
• Java Collections Interview
• Java Interview Questions
• Spring Boot
• 30 OOPs Interview Questions and Answers (2024)
• C++ Interview Questions and Answers (2024)
• Top 100 C++ Coding Interview Questions and Answers (2024)
• Top 50+ Python Interview Questions and Answers (Latest 2024)

Java Interview Questions and Answers

• Java Collections Interview Questions and Answers
• Top 20 Java Multithreading Interview Questions & Answers
• Top 100 Data Structure and Algorithms DSA Interview Questions Topic-wise
• Top 50 Array Coding Problems for Interviews
• Most Asked Problems in Data Structures and Algorithms | Beginner DSA Sheet
• Top 10 Algorithms in Interview Questions
• Machine Learning Interview Questions
• Top 50 Problems on Linked List Data Structure asked in SDE Interviews
• Top 50 Problems on Heap Data Structure asked in SDE Interviews
• Data Analyst Interview Questions and Answers
• SQL Query Interview Questions
• Top Linux Interview Questions With Answer
• MySQL Interview Questions
• Top 50 Django Interview Questions and Answers
• Top 50 Networking Interview Questions (2024)
• Software Testing Interview Questions

Java is one of the most popular programming languages in the world, known for its versatility, portability, and wide range of applications. Java is the most used language in top companies such as Uber, Airbnb, Google, Netflix, Instagram, Spotify, Amazon, and many more because of its features and performance.

In this article, we will provide 200+ Core Java Interview Questions tailored for both freshers and experienced professionals with 3, 5, and 8 years of experience. Here, we cover everything, including core Java concepts, Object-Oriented Programming (OOP), multithreading, exception handling, design patterns, Java Collections, and more, that will surely help you to crack Java interviews.

Table of Content

Java Interview questions for Freshers

Java intermediate interview questions, java interview questions for experienced, java difference interview questions, 1. is java platform independent if then how.

Yes, Java is a Platform Independent language. Unlike many programming languages javac compiler compiles the program to form a bytecode or .class file. This file is independent of the software or hardware running but needs a JVM(Java Virtual Machine) file preinstalled in the operating system for further execution of the bytecode.

Although JVM is platform dependent , the bytecode can be created on any System and can be executed in any other system despite hardware or software being used which makes Java platform independent.

2. What are the top Java Features?

Java is one the most famous and most used language in the real world, there are many features in Java that makes it better than any other language some of them are mentioned below:

• Simple : Java is quite simple to understand and the syntax
• Platform Independent: Java is platform independent means we can run the same program in any software and hardware and will get the same result.
• Interpreted : Java is interpreted as well as a compiler-based language. 
• Robust : features like Garbage collection, exception handling, etc that make the language robust.
• Object-Oriented : Java is an object-oriented language that supports the concepts of class,  objects, four pillars of OOPS, etc. 
• Secured : As we can directly share an application with the user without sharing the actual program makes Java a secure language. 
• High Performance:  faster than other traditional interpreted programming languages.
• Dynamic : supports dynamic loading of classes and interfaces.
• Distributed : feature of Java makes us able to access files by calling the methods from any machine connected.
• Multithreaded : deal with multiple tasks at once by defining multiple threads
• Architecture Neutral : it is not dependent on the architecture.

3. What is JVM?

JVM stands for Java Virtual Machine it is a Java interpreter. It is responsible for loading, verifying, and executing the bytecode created in Java.

Although it is platform dependent which means the software of JVM is different for different Operating Systems it plays a vital role in making Java platform Independent.

To know more about the topic refer to JVM in Java .

4. What is JIT?

JIT stands for (Just-in-Time) compiler is a part of JRE(Java Runtime Environment), it is used for better performance of the Java applications during run-time. The use of JIT is mentioned in step by step process mentioned below:

• Source code is compiled with javac compiler to form bytecode
• Bytecode is further passed on to JVM 
• JIT is a part of JVM, JIT is responsible for compiling bytecode into native machine code at run time.
• The JIT compiler is enabled throughout, while it gets activated when a method is invoked. For a compiled method, the JVM directly calls the compiled code, instead of interpreting it.
• As JVM calls the compiled code that increases the performance and speed of the execution.

To know more about the topic refer to JIT in Java .

5. What are Memory storages available with JVM?

JVM consists of a few memory storages as mentioned below:

• Class(Method) Area: stores class-level data of every class such as the runtime constant pool, field, and method data, and the code for methods.
• Heap: Objects are created or objects are stored. It is used to allocate memory to objects during run time.
• Stack: stores data and partial results which will be needed while returning value for method and performing dynamic linking
• Program Counter Register: stores the address of the Java virtual machine instruction currently being executed.
• Native Method Stack: stores all the native methods used in the application.

To know more about the topic refer to JVM Memory Storages .

6. What is a classloader?

Classloader is the part of JRE(Java Runtime Environment), during the execution of the bytecode or created .class file classloader is responsible for dynamically loading the java classes and interfaces to JVM(Java Virtual Machine). Because of classloaders Java run time system does not need to know about files and file systems.

To know more about the topic refer to ClassLoader in Java.

7. Difference between JVM, JRE, and JDK.

JVM : JVM also known as Java Virtual Machine is a part of JRE. JVM is a type of interpreter responsible for converting bytecode into machine-readable code. JVM itself is platform dependent but it interprets the bytecode which is the platform-independent reason why Java is platform-independent. 

JRE : JRE stands for Java Runtime Environment, it is an installation package that provides an environment to run the Java program or application on any machine.

JDK : JDK stands for Java Development Kit which provides the environment to develop and execute Java programs. JDK is a package that includes two things Development Tools to provide an environment to develop your Java programs and, JRE to execute Java programs or applications.

To know more about the topic refer to the Differences between JVM, JRE, and JDK .

8. What are the differences between Java and C++?

9. explain public static void main(string args[]) in java..

Unlike any other programming language like C, C++, etc. In Java, we declared the main function as a public static void main (String args[]). The meanings of the terms are mentioned below:

• public : the public is the access modifier responsible for mentioning who can access the element or the method and what is the limit.  It is responsible for making the main function globally available. It is made public so that JVM can invoke it from outside the class as it is not present in the current class.
• static : static is a keyword used so that we can use the element without initiating the class so to avoid the unnecessary allocation of the memory. 
• void : void is a keyword and is used to specify that a method doesn’t return anything. As the main function doesn’t return anything we use void.
• main : main represents that the function declared is the main function. It helps JVM to identify that the declared function is the main function.
• String args[] : It stores Java command-line arguments and is an array of type java.lang.String class.

10. What is Java String Pool?

A Java String Pool is a place in heap memory where all the strings defined in the program are stored. A separate place in a stack is there where the variable storing the string is stored. Whenever we create a new string object, JVM checks for the presence of the object in the String pool, If String is available in the pool, the same object reference is shared with the variable, else a new object is created.

11. What will happen if we declare don’t declare the main as static?

We can declare the main method without using static and without getting any errors. But, the main method will not be treated as the entry point to the application or the program.

12. What are Packages in Java?

Packages in Java can be defined as the grouping of related types of classes, interfaces, etc providing access to protection and namespace management.

13. Why Packages are used?

Packages are used in Java in order to prevent naming conflicts, control access, and make searching/locating and usage of classes, interfaces, etc easier.

14. What are the advantages of Packages in Java?

There are various advantages of defining packages in Java.

• Packages avoid name clashes.
• The Package provides easier access control.
• We can also have the hidden classes that are not visible outside and are used by the package.
• It is easier to locate the related classes.

15. How many types of packages are there in Java?

There are two types of packages in Java

• User-defined packages
• Build In packages

16. Explain different data types in Java.

There are 2 types of data types in Java as mentioned below:

• Primitive Data Type
• Non-Primitive Data Type or Object Data type

Primitive Data Type: Primitive data are single values with no special capabilities. There are 8 primitive data types:

• boolean : stores value true or false
• byte : stores an 8-bit signed two’s complement integer
• char : stores a single 16-bit Unicode character
• short : stores a 16-bit signed two’s complement integer
• int : stores a 32-bit signed two’s complement integer
• long : stores a 64-bit two’s complement integer
• float : stores a single-precision 32-bit IEEE 754 floating-point
• double : stores a double-precision 64-bit IEEE 754 floating-point

Non-Primitive Data Type: Reference Data types will contain a memory address of the variable’s values because it is not able to directly store the values in the memory. Types of Non-Primitive are mentioned below:

17. When a byte datatype is used?

A byte is an 8-bit signed two-complement integer. The minimum value supported by bytes is -128 and 127 is the maximum value. It is used in conditions where we need to save memory and the limit of numbers needed is between -128 to 127.

18. Can we declare Pointer in Java?

No, Java doesn’t provide the support of Pointer. As Java needed to be more secure because which feature of the pointer is not provided in Java.

19. What is the default value of byte datatype in Java?

The default value of the byte datatype in Java is 0.

20. What is the default value of float and double datatype in Java?

The default value of the float is 0.0f and of double is 0.0d in Java.

21. What is the Wrapper class in Java?

Wrapper, in general, is referred to a larger entity that encapsulates a smaller entity. Here in Java, the wrapper class is an object class that encapsulates the primitive data types. 

The primitive data types are the ones from which further data types could be created. For example, integers can further lead to the construction of long, byte, short, etc. On the other hand, the string cannot, hence it is not primitive. 

Getting back to the wrapper class, Java contains 8 wrapper classes. They are Boolean, Byte, Short, Integer, Character, Long, Float, and Double. Further, custom wrapper classes can also be created in Java which is similar to the concept of Structure in the C programming language. We create our own wrapper class with the required data types.

22. Why do we need wrapper classes?

The wrapper class is an object class that encapsulates the primitive data types, and we need them for the following reasons:

• Wrapper classes are final and immutable
• Provides methods like valueOf(), parseInt(), etc.
• It provides the feature of autoboxing and unboxing.

23. Differentiate between instance and local variables.

24. what are the default values assigned to variables and instances in java.

In Java When we haven’t initialized the instance variables then the compiler initializes them with default values. The default values for instances and variables depend on their data types. Some common types of default data types are:

• The default value for numeric types (byte, short, int, long, float, and double) is 0.
• The default value for the boolean type is false.
• The default value for object types (classes, interfaces, and arrays) is null.
• The null character, “u0000, ” is the default value for the char type.

25. What is a Class Variable?

In Java, a class variable (also known as a static variable) is a variable that is declared within a class but outside of any method, constructor, or block. Class variables are declared with the static keyword, and they are shared by all instances (objects) of the class as well as by the class itself. No matter how many objects are derived from a class, each class variable would only exist once.

26. What is the default value stored in Local Variables?

There is no default value stored with local variables. Also, primitive variables and objects don’t have any default values.

27. Explain the difference between instance variable and a class variable.

Instance Variable: A class variable without a static modifier known as an instance variable is typically shared by all instances of the class. These variables can have distinct values among several objects. The contents of an instance variable are completely independent of one object instance from another because they are related to a specific object instance of the class.

Class Variable:  Class Variable variable can be declared anywhere at the class level using the keyword static. These variables can only have one value when applied to various objects. These variables can be shared by all class members since they are not connected to any specific object of the class.  

28. What is a static variable?

The static keyword is used to share the same variable or method of a given class. Static variables are the variables that once declared then a single copy of the variable is created and shared among all objects at the class level.

29. What is the difference between System.out, System.err, and System.in?

System.out – It is a PrintStream that is used for writing characters or can be said it can output the data we want to write on the Command Line Interface console/terminal. 

System.err – It is used to display error messages.

Although, System.err have many similarities both of them have quite a lot of difference also, let us check them.

System.in – It is an InputStream used to read input from the terminal Window. We can’t use the System.in directly so we use Scanner class for taking input with the system.in.

30. What do you understand by an IO stream?

Java brings various Streams with its I/O package that helps the user to perform all the input-output operations. These streams support all types of objects, data types, characters, files, etc to fully execute the I/O operations.

31. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

The key difference between them is that byte stream data is read and written by input/output stream classes. Characters are handled by the Reader and Writer classes. In contrast to Reader/Writer classes, which accept character arrays as parameters, input/output stream class methods accept byte arrays. In comparison to input/output streams, the Reader/Writer classes are more efficient, handle all Unicode characters, and are useful for internalization. Use Reader/Writer classes instead of binary data, such as pictures, unless you do so.

32. What are the super most classes for all the streams?

All the stream classes can be divided into two types of classes that are ByteStream classes and CharacterStream Classes. The ByteStream classes are further divided into InputStream classes and OutputStream classes. CharacterStream classes are also divided into Reader classes and Writer classes. The SuperMost classes for all the InputStream classes is java.io.InputStream and for all the output stream classes is java.io.OutPutStream. Similarly, for all the reader classes, the super-most class is java.io.Reader, and for all the writer classes, it is java.io.Writer.

33. What are the FileInputStream and FileOutputStream?

To read and write data, Java offers I/O Streams. A Stream represents an input source or an output destination, which could be a file, an i/o device, another program, etc. FileInputStream in Java is used to read data from a file as a stream of bytes. It is mostly used for reading binary data such as images, audio files, or serialized objects. 

In Java, the FileOutputStream function is used to write data byte by byte into a given file or file descriptor. Usually, raw byte data, such as pictures, is written into a file using FileOutputStream.

34. What is the purpose of using BufferedInputStream and BufferedOutputStream classes?

When we are working with the files or stream then to increase the Input/Output performance of the program we need to use the BufferedInputStream and BufferedOutputStream classes. These both classes provide the capability of buffering which means that the data will be stored in a buffer before writing to a file or reading it from a stream. It also reduces the number of times our OS needs to interact with the network or the disk. Buffering allows programs to write a big amount of data instead of writing it in small chunks. This also reduces the overhead of accessing the network or the disk. 

35. What are FilterStreams?

Stream filter or Filter Streams returns a stream consisting of the elements of this stream that match the given predicate. While working filter() it doesn’t actually perform filtering but instead creates a new stream that, when traversed, contains the elements of initial streams that match the given predicate.

36. What is an I/O filter?

An I/O filter also defined as an Input Output filter is an object that reads from one stream and writes data to input and output sources. It used java.io package to use this filter.

37. How many ways you can take input from the console?

There are two methods to take input from the console in Java mentioned below:

• Using Command line argument
• Using Buffered Reader Class
• Using Console Class
• Using Scanner Class

The program demonstrating the use of each method is given below.

38. Difference in the use of print, println, and printf.

print, println, and printf all are used for printing the elements but print prints all the elements and the cursor remains in the same line. println shifts the cursor to next line. And with printf we can use format identifiers too.

39. What are operators? 

Operators are the special types of symbols used for performing some operations over variables and values.

40. How many types of operators are available in Java? 

All types of operators in Java are mentioned below:

• Arithmetic Operators
• Unary Operators
• Assignment Operator
• Relational Operators
• Logical Operators
• Ternary Operator
• Bitwise Operators
• Shift Operators
• instance of operator

Postfix operators are considered as the highest precedence according to Java operator precedence.

41. Explain the difference between >> and >>> operators.

Operators like >> and >>> seem to be the same but act a bit differently. >> operator shifts the sign bits and the >>> operator is used in shifting out the zero-filled bits.

42. Which Java operator is right associative?

There is only one operator which is right associative which is = operator.

43. What is dot operator?

The Dot operator in Java is used to access the instance variables and methods of class objects. It is also used to access classes and sub-packages from the package.

44. What is covariant return type?

The covariant return type specifies that the return type may vary in the same direction as the subclass. It’s possible to have different return types for an overriding method in the child class, but the child’s return type should be a subtype of the parent’s return type and because of that overriding method becomes variant with respect to the return type.

We use covariant return type because of the following reasons:

• Avoids confusing type casts present in the class hierarchy and makes the code readable, usable, and maintainable.
• Gives liberty to have more specific return types when overriding methods.
• Help in preventing run-time ClassCastExceptions on returns.

45. What is the transient keyword?

The transient keyword is used at the time of serialization if we don’t want to save the value of a particular variable in a file. When JVM comes across a transient keyword, it ignores the original value of the variable and saves the default value of that variable data type.

46. What’s the difference between the methods sleep() and wait()?

47. what are the differences between string and stringbuffer, 48. what are the differences between stringbuffer and stringbuilder, 49. which among string or string buffer should be preferred when there are a lot of updates required to be done in the data.

The string is preferred over StringBuffer as StringBuilder is faster than StringBuffer, but StringBuffer objects are the preferred over as it provides more thread safety.

50. Why is StringBuffer called mutable?

StringBuffer class in Java is used to represent a changeable string of characters. It offers an alternative to the immutable String class by enabling you to change a string’s contents without constantly creating new objects. Mutable (modifiable) strings are created with the help of the StringBuffer class. The StringBuffer class in Java is identical to the String class except that it is changeable.

51. How is the creation of a String using new() different from that of a literal?

String using new() is different from the literal as when we declare string it stores the elements inside the stack memory whereas when it is declared using new() it allocates a dynamic memory in the heap memory. The object gets created in the heap memory even if the same content object is present.

52. What is an array in Java?

An Array in Java is a data structure that is used to store a fixed-size sequence of elements of the same type. Elements of an array can be accessed by their index, which starts from 0 and goes up to a length of minus 1. Array declaration in Java is done with the help of square brackets and size is also specified during the declaration. 

53. On which memory arrays are created in Java?

Arrays in Java are created in heap memory. When an array is created with the help of a new keyword, memory is allocated in the heap to store the elements of the array. In Java, the heap memory is managed by the Java Virtual Machine(JVM) and it is also shared between all threads of the Java Program. The memory which is no longer in use by the program, JVM uses a garbage collector to reclaim the memory. Arrays in Java are created dynamically which means the size of the array is determined during the runtime of the program. The size of the array is specified during the declaration of the array and it cannot be changed once the array is created.

54. What are the types of an array?

There are two types of arrays i.e., Primitive arrays and References Arrays.

• Single-Dimensional Arrays: Arrays that have only one dimension i.e., an array of integers or an array of strings are known as single-dimensional arrays.

• Multi-Dimensional Arrays: Arrays that have two or more dimensions such as two-dimensional or three-dimensional arrays. 

55. Why does the Java array index start with 0?

The index of an array signifies the distance from the start of the array. So, the first element has 0 distance therefore the starting index is 0.

56. What is the difference between int array[] and int[] array?

Both int array[] and int[] array are used to declare an array of integers in java. The only difference between them is on their syntax no functionality difference is present between them.

However, it is generally recommended to use Java-style syntax to declare an Array. As it is easy to read and understand also it is more consistent with other Java language constructs.

57. How to copy an array in Java?

In Java there are multiple ways to copy an Array based on the requirements. 

• clone() method in Java: This method in Java is used to create a shallow copy of the given array which means that the new array will share the same memory as the original array.
• arraycopy() method: To create a deep copy of the array we can use this method which creates a new array with the same values as the original array.
• copyOf() method: This method is used to create a new array with a specific length and copies the contents of the original array to the new array.
• copyOfRange() method: This method is very similar to the copyOf() method in Java, but this method also allows us to specify the range of the elements to copy from the original array.

58. What do you understand by the jagged array?

A jagged Array in Java is just a two-dimensional array in which each row of the array can have a different length. Since all the rows in a 2-d Array have the same length but a jagged array allows more flexibility in the size of each row. This feature is very useful in conditions where the data has varying lengths or when memory usage needs to be optimized.   

59. Is it possible to make an array volatile?

In Java, it is not possible to make a volatile. Volatile keywords in Java can only be applied to individual variables but not to arrays or collections. The value of the Variable is always read from and written to the main memory when it is defined as volatile rather than being cached in a thread’s local memory. This makes it easier to make sure that all threads that access the variable can see changes made to it.

60. What are the advantages and disadvantages of an array?

The advantages of Arrays are:

• Direct and effective access to any element in the collection is made possible by arrays. An array’s elements can be accessed using an O(1) operation, which means that the amount of time needed to do so is constant and independent of the array’s size.
• Data can be stored effectively in memory using arrays. The size of an array is known at compile time since its elements are stored in contiguous memory regions.
• Due to the fact that the data is stored in contiguous memory areas, arrays provide quick data retrieval.
• Arrays are easy to implement and understand, making them an ideal choice for beginners learning computer programming.

Disadvantages of Arrays are:

• Arrays are created with a predetermined size that is chosen at that moment. This means that if the array’s size needs to be extended, a new array will need to be made, and the data will need to be copied from the old array to the new array, which can take a lot of time and memory.
• There may be unused memory space in an array’s memory space if the array is not completely occupied. If you have poor recall, this can be a problem.
• Compared to other data structures like linked lists and trees, arrays might be rigid due to their fixed size and limited support for sophisticated data types.
• Because an array’s elements must all be of the same data type, it does not support complex data types like objects and structures.

61. What is an object-oriented paradigm?

Paradigm literally means a pattern or a method. Programming paradigms are the methods to solve a program that is of four types namely, Imperative, logical, functional, and object-oriented. When objects are used as base entities upon which the methods are applied, encapsulation or inheritance functionalities are performed, it is known as an object-oriented paradigm.

62. What are the main concepts of OOPs in Java?

The main concepts of OOPs in Java are mentioned below:

• Inheritance
• Polymorphism
• Abstraction
• Encapsulation 

63. What is the difference between an object-oriented programming language and an object-based programming language?

64. how is the ‘new’ operator different from the ‘newinstance()’ operator in java.

the new operator is used to create objects, but if we want to decide the type of object to be created at runtime, there is no way we can use the new operator. In this case, we have to use the newInstance() method .

65. What are Classes in Java? 

In Java, Classes are the collection of objects sharing similar characteristics and attributes. Classes represent the blueprint or template from which objects are created.  Classes are not real-world entities but help us to create objects which are real-world entities. 

66. What is the difference between static (class) method and instance method?

67. what is this keyword in java.

‘this’ is a keyword used to reference a variable that refers to the current object.

68. What are Brief Access Specifiers and Types of Access Specifiers?

Access Specifiers in Java help to restrict the scope of a class, constructor, variable, method, or data member. There are four types of Access Specifiers in Java mentioned below:

69. What will be the initial value of an object reference which is defined as an instance variable?

The initial value of an object reference which is defined as an instance variable is a NULL value.

70. What is an object?

The object is a real-life entity that has certain properties and methods associated with it. The object is also defined as the instance of a class. An object can be declared using a new keyword.

71. What are the different ways to create objects in Java?

Methods to create objects in Java are mentioned below:

• Using new keyword
• Using new instance
• Using clone() method
• Using deserialization
• Using the newInstance() method of the Constructor class

To know more about methods to create objects in Java refer to this article .

72. What are the advantages and disadvantages of object cloning?

There are many advantages and disadvantages of using object cloning as mentioned below:

Advantages:

• In Java, the ‘=’ assignment operator cannot be used for cloning as it simply creates a copy of reference variables. To overcome such discrepancy the clone() method of Object class can be used over the assignment operator.
• The clone() method is a protected method of class Object which means that only the Employee class can clone Employee objects. This means no class other than Employee can clone Employee objects since it does not know the Employee class’ attributes.
• Code size decreases as repetition decreases. 
• Allows replication (kind of like prototype pattern) manually initilizing each field creates large code if object is complex, its faster with cloning.

Disadvantages:

• As the Object.clone() method is protected, so need to provide our own clone() and indirectly call Object.clone() from it.
• If we don’t have any methods then we need to provide a Cloneable interface as we need to provide JVM information so that we can perform a clone() on our object.if not, we cant clone clone does shallow copy of fields if we just return super.clone() from clone method that can be problematic.

73. What are the advantages of passing this into a method instead of the current class object itself?

There are a few advantages of passing this into a method instead of the current class object itself these are:

• this is the final variable because of which this cannot be assigned to any new value whereas the current class object might not be final and can be changed.
• this can be used in the synchronized block.

74. What is the constructor?

Constructor is a special method that is used to initialize objects. Constructor is called when a object is created. The name of constructor is same as of the class.

75. What happens if you don’t provide a constructor in a class?

If you don’t provide a constructor in a class in Java, the compiler automatically generates a default constructor with no arguments and no operation which is a default constructor.

76. How many types of constructors are used in Java?

There are two types of constructors in Java as mentioned below:

• Default Constructor
• Parameterized Constructor

Default Constructor: It is the type that does not accept any parameter value. It is used to set initial values for object attributes.

Parameterized Constructor: It is the type of constructor that accepts parameters as arguments. These are used to assign values to instance variables during the initialization of objects.

77. What is the purpose of a default constructor?

Constructors help to create instances of a class or can be said to create objects of a class. Constructor is called during the initialization of objects. A default constructor is a type of constructor which do not accept any parameter, So whatever value is assigned to properties of the objects are considered default values.

78. What do you understand by copy constructor in Java?

The copy constructor is the type of constructor in which we pass another object as a parameter because which properties of both objects seem the same, that is why it seems as if constructors create a copy of an object.

79. Where and how can you use a private constructor?

A private constructor is used if you don’t want any other class to instantiate the object to avoid subclassing. The use private constructor can be seen as implemented in the example.

80. What are the differences between the constructors and methods?

Java constructors are used for initializing objects. During creation, constructors are called to set attributes for objects apart from this few basic differences between them are:

• Constructors are only called when the object is created but other methods can be called multiple times during the life of an object.
• Constructors do not return anything, whereas other methods can return anything.
• Constructors are used to setting up the initial state but methods are used to perform specific actions.

81. What is an Interface?

An interface in Java is a collection of static final variables and abstract methods that define the contract or agreement for a set of linked classes. Any class that implements an interface is required to implement a specific set of methods. It specifies the behavior that a class must exhibit but not the specifics of how it should be implemented.

82. Give some features of the Interface.

An Interface in Java programming language is defined as an abstract type used to specify the behavior of a class. An interface in Java is a blueprint of a behavior. A Java interface contains static constants and abstract methods.

Features of the Interface are mentioned below:

• The interface can help to achieve total abstraction.
• Allows us to use multiple inheritances in Java.
• Any class can implement multiple interfaces even when one class can extend only one class.
• It is also used to achieve loose coupling.

83. What is a marker interface?

An Interface is recognized as an empty interface (no field or methods) it is called a marker interface. Examples of marker interfaces are Serializable, Cloneable, and Remote interfaces. 

84. What are the differences between abstract class and interface?

85. What do you mean by data encapsulation?

Data Encapsulation is the concept of OOPS properties and characteristics of the classes that The interface is binded together. Basically, it bundles data and methods that operate on that data within a single unit. Encapsulation is achieved by declaring the instance variables of a class as private, which means they can only be accessed within the class.

86. What are the advantages of Encapsulation in Java?

The advantages of Encapsulation in Java are mentioned below:

• Data Hiding:  it is a way of restricting the access of our data members by hiding the implementation details. Encapsulation also provides a way for data hiding. The user will have no idea about the inner implementation of the class. 
• Increased Flexibility: We can make the variables of the class read-only or write-only depending on our requirements. 
• Reusability: Encapsulation also improves the re-usability and is easy to change with new requirements.
• Testing code is easy: Code is made easy to test for unit testing.

87. What is the primary benefit of Encapsulation? 

The main advantage of Encapsulation in Java is its ability to protect the internal state of an object from external modification or access. It is the is a way of hiding the implementation details of a class from outside access and only exposing a public interface that can be used to interact with the class. The main benefit is of providing a way to control and manage the state and the behavior of an object and also protecting it from modification and unauthorized access at the same time. 

88. What do you mean by aggregation?

Aggregation is a term related to the relationship between two classes best described as a “has-a” relationship. This kind is the most specialized version of association. It is a unidirectional association means it is a one-way relationship. It contains the reference to another class and is said to have ownership of that class.

89. What is the ‘IS-A ‘ relationship in OOPs Java?

‘IS-A’ is a type of relationship in OOPs Java where one class inherits another class.

90. Define Inheritance.

When an object that belongs to a subclass acquires all the properties and behavior of a parent object that is from the superclass, it is known as inheritance.  A class within a class is called the subclass and the latter is referred to as the superclass. Sub class or the child class is said to be specific whereas the superclass or the parent class is generic. Inheritance provides code reusability.

91. What are the different types of inheritance in Java?

Inheritance is the method by which the Child class can inherit the features of the Super or Parent class. In Java, Inheritance is of four types:

• Single Inheritance: When a child or subclass extends only one superclass, it is known to be single inheritance. Single-parent class properties are passed down to the child class. 
• Multilevel Inheritance: When a child or subclass extends any other subclass a hierarchy of inheritance is created which is known as multilevel inheritance. In other words, one subclass becomes the parent class of another.
• Hierarchical Inheritance: When multiple subclasses derive from the same parent class is known as Hierarchical Inheritance. In other words, a class that has a single parent has many subclasses.
• Multiple Inheritance: When a child class inherits from multiple parent classes is known as Multiple Inheritance. In Java, it only supports multiple inheritance of interfaces, not classes.

92. What is multiple inheritance? Is it supported by Java?

A component of the object-oriented notion known as multiple inheritances allows a class to inherit properties from many parent classes. When methods with the same signature are present in both superclasses and subclasses, an issue arises. The method’s caller cannot specify to the compiler which class method should be called or even which class method should be given precedence. 

Note: Java doesn’t support Multiple Inheritance

93. How is inheritance in C++ different from Java?

94. is there any limitation to using inheritance.

Yes, there is a limitation of using Inheritance in Java, as because of inheritance one can inherit everything from super class and interface because of which subclass is too clustered and sometimes error-prone when dynamic overriding or dynamic overloading is done in certain situations.

95. Although inheritance is a popular OOPs concept, it is less advantageous than composition. Explain.

Inheritance is a popular concept of Object-Oriented Programming (OOP), in which a class can inherit the properties and methods from any other class, which is referred to as a Parent or superclass. On the other hand in Composition, a class can contain an instance of another class as a member variable which is often referred to as part or a component. Below are some reasons why composition is more advantageous than inheritance:

• Tight Coupling: Whenever any changes are made to the superclass, these changes can affect the behavior of all its child or Subclasses. This problem makes code less flexible and also creates issues during maintenance. This problem also leads to the Tight coupling between the classes.
• Fragile Base Class Problem: When the changes to the base class can break the functionality of its derived classes. This problem can make it difficult to add new features or modify the existing ones. This problem is known as the Fragile Base class problem.
• Limited Reuse: Inheritance in Java can lead to limited code reuse and also code duplication. As a subclass inherits all the properties and methods of its superclass, sometimes it may end up with unnecessary code which is not needed. This leads to a less maintainable codebase.

96. What is an association?

The association is a relation between two separate classes established through their Objects. It represents Has-A’s relationship.

97. What do you mean by aggregation?

Composition is a restricted form of Aggregation in which two entities are highly dependent on each other. It represents part-of the relationship.

98. What is the composition of Java?

Composition implies a relationship where the child cannot exist independently of the parent. For example Human heart, the heart doesn’t exist separately from a Human.

99. State the difference between Composition and Aggregation.

100. can the constructor be inherited.

No, we can’t inherit a constructor.

101. What is Polymorphism?

Polymorphism is defined as the ability to take more than one form It is of two types namely, Compile time polymorphism or method overloading- a function called during compile time. For instance, take a class ‘area’. Based on the number of parameters it may calculate the area of a square, triangle, or circle. Run time polymorphism or method overriding- links during run time.  The method inside a class overrides the method of the parent class.

102. What is runtime polymorphism or dynamic method dispatch?

Dynamic method dispatch is a resolving mechanism for method overriding during the run time. Method overriding is the one where the method in a subclass has the same name, parameters, and return type as a method in the superclass. When the over-ridden method is called through a superclass reference, java determines which version (superclass or subclass) of that method is to be executed based upon the type of an object being referred to at the time the call occurs. Thus the decision is made at run time. This is referred to as dynamic method dispatch.

103. What is method overriding?

Method overriding, also known as run time polymorphism is one where the child class contains the same method as the parent class. For instance, we have a method named ‘gfg()’ in the parent class. A method gfg() is again defined in the sub-class. Thus when gfg() is called in the subclass, the method within the class id executed. Here, gfg() within the class overridden the method outside.

104. What is method overloading?

Method overriding is a method to achieve Run-time polymorphism in Java. Method overriding is a feature that allows a child class to provide a specific implementation of a method that is already provided by one of its parent classes. When a method in a child class has the same name, the same parameters or signature, and the same return type(or sub-type) as a method in its parent class, then the method in the subclass is said to override the method in the superclass.

105. Can we override the static method?

No, as static methods are part of the class rather than the object so we can’t override them.

106. Can we override the overloaded method?

Yes, since the overloaded method is a completely different method in the eyes of the compiler. Overriding isn’t the same thing at all. The decision as to which method to call is deferred to runtime.

107. Can we overload the main() method?

Yes in Java we can overload the main method to call the main method with the help of its predefined calling method. 

108. What are method overloading and method overriding?

Method Overloading: It is also known as Compile Time Polymorphism. In method overloading two or more methods are shared in the same class with a different signature. 

Method Overriding: Method Overriding occurs when a subclass can provide the implementation of a method which is already defined in the parent class or superclass. The return type, name and arguments must be similar to the methods in superclass.

109. Can we override the private methods?

It is not possible to override the private methods in Java. Method overriding is where the method in the subclass is implemented instead of the method from the parent class. The private methods are accessible only within the class in which it is declared. Since this method is not visible to other classes and cannot be accessed, it cannot be overridden. 

110. Can we change the scope of the overridden method in the subclass?

In Java, it is not possible to modify the overridden method’s scope. The subclass method’s scope must be equal to or wider than the Superclass method’s overridden method’s scope. The overridden method in the subclass, for instance, can have a public scope or a more accessible scope like protected or default if the overridden method in the superclass has a public scope. It cannot, however, have a more exclusive scope like private.

111. Can we modify the throws clause of the superclass method while overriding it in the subclass?

We can modify the throws clause of the Superclass method with some limitations, we can change the throws clause of the superclass method while overriding it in the subclass. The subclass overridden method can only specify unchecked exceptions if the superclass method does not declare any exceptions. If the superclass method declares an exception, the subclass method can declare the same exception, a subclass exception, or no exception at all. However, the subclass method cannot declare a parent exception that is broader than the ones declared in the superclass method.

112. Can you have virtual functions in Java?

Yes, Java supports virtual functions. Functions are by default virtual and can be made non-virtual using the final keyword.

113. What is Abstraction?

Abstraction refers to the act of representing essential features without including background details. The detailed information or the implementation is hidden. The most common example of abstraction is a car, we know how to turn on the engine, accelerate and move, however, the way engine works, and its internal components are complex logic hidden from the general users. This is usually done to handle the complexity.

114. What is Abstract class?

A class declared as abstract, cannot be instantiated i.e., the object cannot be created. It may or may not contain abstract methods but if a class has at least one abstract method, it must be declared abstract.

Example of an abstract class with abstract method:

115. When Abstract methods are used?

An abstract method is used when we want to use a method but want to child classes to decide the implementation in that case we use Abstract methods with the parent classes.

116. How can you avoid serialization in the child class if the base class is implementing the Serializable interface?

Serialization in the child class if the base class is implementing the Serializable interface then we can avoid it by defining the writeObject() method and throwing NotSerializableException().

117. What is Collection Framework in Java?

Collections are units of objects in Java. The collection framework is a set of interfaces and classes in Java that are used to represent and manipulate collections of objects in a variety of ways. The collection framework contains classes(ArrayList, Vector, LinkedList, PriorityQueue, TreeSet) and multiple interfaces (Set, List, Queue, Deque) where every interface is used to store a specific type of data.

118. Explain various interfaces used in the Collection framework.

Collection framework implements

• Collection Interface
• List Interface
• Set Interface
• Queue Interface
• Deque Interface
• Map Interface

Collection interface: Collection is the primary interface available that can be imported using java.util.Collection.

119. How can you synchronize an ArrayList in Java?

An ArrayList can be synchronized using two methods mentioned below:

• Using Collections.synchronizedList()
• Using CopyOnWriteArrayList

Using Collections.synchronizedList():

Using CopyOnWriteArrayList:

• Create an empty List.
• It implements the List interface
• It is a thread-safe variant of ArrayList
• T represents generic

120. Why do we need a synchronized ArrayList when we have Vectors (which are synchronized) in Java?

ArrayList is in need even when we have Vectors because of certain reasons:

• ArrayList is faster than Vectors.
• ArrayList supports multithreading whereas Vectors only supports single-thread use.
• ArrayList is safer to use, as Vectors supports single threads and individual operations are less safe and take longer to synchronize.
• Vectors are considered outdated in Java because of their synchronized nature.

121. Why can’t we create a generic array?

Generic arrays can’t be created because an array carries type information of its elements at runtime because of which during runtime it throw ‘ArrayStoreException’ if the elements’ type is not similar. Since generics type information gets erased at compile time by Type Erasure, the array store check would have been passed where it should have failed.

122. Contiguous memory locations are usually used for storing actual values in an array but not in ArrayList. Explain.

The elements of an array are stored in contiguous memory locations, which means that each element is stored in a separate block based on it located within the array. Since the elements of the array are stored in contiguous locations, it can be relatively easy to access any element by its index, as the element address can be calculated based on the location of the element. But Java implements ArrayLists as dynamic arrays, which means that the size can change as elements are removed or added. ArrayList elements are not stored in contiguous memory locations in order to accommodate this dynamic nature. Instead, the ArrayList makes use of a method known as an expandable array in which the underlying array is expanded to a larger size as needed and the elements are then copied to the new location. In contrast to an ArrayList, which has a dynamic size and does not store its elements in contiguous memory locations, an array has a fixed size and its elements are stored there.

123. Explain the method to convert ArrayList to Array and Array to ArrayList.

Conversion of list to arraylist.

There are multiple methods to convert List into ArrayList

Programmers can convert an Array to ArrayList using asList() method of the Arrays class. It is a static method of the Arrays class that accepts the List object.

Conversion of ArrayList to Array

Java programmers can convert ArrayList to

124. How does the size of ArrayList grow dynamically? And also state how it is implemented internally.

Due to ArrayLists array-based nature, it grows dynamically in size ensuring that there is always enough room for elements. When an ArrayList element is first created, the default capacity is around 10-16 elements which basically depends on the Java version. ArrayList elements are copied over from the original array to the new array when the capacity of the original array is full. As the ArrayList size increases dynamically, the class creates a new array of bigger sizes and it copies all the elements from the old array to the new array. Now, the reference of the new array is used internally. This process of dynamically growing an array is known as resizing. 

125. What is a Vector in Java?

Vectors in Java are similar and can store multiple elements inside them. Vectors follow certain rules mentioned below:

• Vector can be imported using Java.util.Vector.
• Vector is implemented using a dynamic array as the size of the vector increases and decreases depending upon the elements inserted in it. 
• Elements of the Vector using index numbers.
• Vectors are synchronized in nature means they only used a single thread ( only one process is performed at a particular time ).
• The vector contains many methods that are not part of the collections framework.

126. How to make Java ArrayList Read-Only?

An ArrayList can be made ready only using the method provided by Collections using the Collections.unmodifiableList() method. 

127. What is a priority queue in Java?

A priority queue is an abstract data type similar to a regular queue or stack data structure. Elements stored in elements are depending upon the priority defined from low to high. The PriorityQueue is based on the priority heap.

128. Explain the LinkedList class.

LinkedList class is Java that uses a doubly linked list to store elements. It inherits the AbstractList class and implements List and Deque interfaces. Properties of the LinkedList Class are mentioned below:

• LinkedList classes are non-synchronized.
• Maintains insertion order.
• It can be used as a list, stack, or queue.

129. What is the Stack class in Java and what are the various methods provided by it?

A Stack class in Java is a LIFO data structure that implements the Last In First Out data structure. It is derived from a Vector class but has functions specific to stacks. The Stack class in java provides the following methods:

• peek(): returns the top item from the stack without removing it
• empty(): returns true if the stack is empty and false otherwise
• push(): pushes an item onto the top of the stack
• pop(): removes and returns the top item from the stack
• search(): returns the 1, based position of the object from the top of the stack. If the object is not in the stack, it returns -1

130. What is Set in the Java Collections framework and list down its various implementations?

Sets are collections that don’t store duplicate elements. They don’t keep any order of the elements. The Java Collections framework provides several implementations of the Set interface, including:

• HashSet: HashSet in Java, stores the elements in a has table which provides faster lookups and faster insertion. HashSet is not ordered.
• LinkedHashSet: LinkedHashSet is an implementation of HashSet which maintains the insertion order of the elements.
• TreeSet: TreeSet stores the elements in a sorted order that is determined by the natural ordering of the elements or by a custom comparator provided at the time of creation.

131. What is the HashSet class in Java and how does it store elements?

The HashSet class implements the Set interface in the Java Collections Framework and is a member of the HashSet class. Unlike duplicate values, it stores a collection of distinct elements. In this implementation, each element is mapped to an index in an array using a hash function, and the index is used to quickly access the element. It produces an index for the element in the array where it is stored based on the input element. Assuming the hash function distributes the elements among the buckets appropriately, the HashSet class provides constant-time performance for basic operations (add, remove, contain, and size).

132. What is LinkedHashSet in Java Collections Framework?

The LinkedHashSet is an ordered version of Hashset maintained by a doubly-linked List across all the elements. It is very helpful when iteration order is needed. During Iteration in LinkedHashSet, elements are returned in the same order they are inserted.

133. What is a Map interface in Java?

The map interface is present in the Java collection and can be used with Java.util package. A map interface is used for mapping values in the form of a key-value form. The map contains all unique keys. Also, it provides methods associated with it like containsKey(), contains value (), etc. 

There are multiple types of maps in the map interface as mentioned below:

• LinkedHashMap

134. Explain Treemap in Java

TreeMap is a type of map that stores data in the form of key-value pair. It is implemented using the red-black tree. Features of TreeMap are :

• It contains only unique elements.
• It cannot have a NULL key 
• It can have multiple NULL values.
• It is non-synchronized.
• It maintains ascending order.

135. What is EnumSet?

EnumSet is a specialized implementation of the Set interface for use with enumeration type. A few features of EnumSet are:

• Faster than HashSet.
• All of the elements in an EnumSet must come from a single enumeration type.
• It doesn’t allow null Objects and throws NullPointerException for exceptions.
• It uses a fail-safe iterator.

Parameter: E specifies the elements.

136. What is BlockingQueue?

A blocking queue is a Queue that supports the operations that wait for the queue to become non-empty while retrieving and removing the element, and wait for space to become available in the queue while adding the element.

Parameters: E is the type of elements stored in the Collection

137. What is the ConcurrentHashMap in Java and do you implement it?

ConcurrentHashMap is implemented using Hashtable.

Parameters : K is the key Object type and V is the value Object type

138. Can you use any class as a Map key?

Yes, we can use any class as a Map Key if it follows certain predefined rules mentioned below:

• The class overriding the equals() method must also override the hashCode() method
• The concurrentHashMap class is thread-safe.
• The default concurrency level of ConcurrentHashMap is 16.
• Inserting null objects in ConcurrentHashMap is not possible as a key or as value.

139. What is an Iterator?

The Iterator interface provides methods to iterate over any Collection in Java. Iterator is the replacement of Enumeration in the Java Collections Framework. It can get an iterator instance from a Collection using the _iterator()_ method.  It also allows the caller to remove elements from the underlying collection during the iteration.

140. What is an enumeration?

Enumeration is a user-defined data type. It is mainly used to assign names to integral constants, the names make a program easy to read and maintain. The main objective of the enum is to define user-defined data types.

141. What is the difference between Collection and Collections?

142. differentiate between array and arraylist in java., 143. what is the difference between array and collection in java, 144. difference between arraylist and linkedlist., 145. differentiate between arraylist and vector in java., 146. what is the difference between iterator and listiterator, 147. differentiate between hashmap and hashtable., 148. what is the difference between iterator and enumeration, 149. what is the difference between comparable and comparator, 150. what is the difference between set and map, 151. explain the failfast iterator and failsafe iterator along with examples for each..

A FailFast iterator is an iterator that throws a ConcurrentModificationException if it detects that the underlying collection has been modified while the iterator is being used. This is the default behavior of iterators in the Java Collections Framework. For example, the iterator for a HashMap is FailFast.

A FailSafe iterator does not throw a ConcurrentModificationException if the underlying collection is modified while the iterator is being used. Alternatively, it creates a snapshot of the collection at the time the iterator is created and iterates over the snapshot. For example, the iterator for a ConcurrentHashMap is FailSafe.

152. What is Exception Handling?

An Exception is an Event that interrupts the normal flow of the program and requires special processing. During the execution of a program, errors and unplanned occurrences can be dealt with by using the Java Exception Handling mechanism. Below are some reasons why Exceptions occur in Java:

• Device failure
• Loss of Network Connection
• Code Errors
• Opening an Unavailable file
• Invalid User Input
• Physical Limitations (out of disk memory)

153. How many types of exceptions can occur in a Java program?

There are generally two types of exceptions in Java:

• ArrayIndexOutOfBoundsExceptions
• ClassNotFoundException
• FileNotFoundException
• IOException
• NullPointerException
• ArithmeticException
• InterruptedException
• RuntimeException
• User-Defined Exceptions: User-defined exceptions are defined by the programmers themselves to handle some specific situations or errors which are not covered by built-in exceptions. To define user-defined exceptions a new class that extends the appropriate exception class must be defined. User-defined Exceptions in Java are used when the built-in exceptions are in Java.

154. Difference between an Error and an Exception.

155. explain the hierarchy of java exception classes..

All exception and error types in Java are subclasses of the class throwable, which is the base class of the hierarchy. This class is then used for exceptional conditions that user programs should catch. NullPointerException is an example of such an exception. Another branch, error is used by the Java run-time system to indicate errors having to do with the JRE. StackOverflowError is an example of one of such error.

156. Explain Runtime Exceptions.

Runtime Exceptions are exceptions that occur during the execution of a code, as opposed to compile-time exceptions that occur during compilation. Runtime exceptions are unchecked exceptions, as they aren’t accounted for by the JVM.

Examples of runtime exceptions in Java include:

• NullPointerException: This occurs when an application attempts to use a null object reference.
• ArrayIndexOutOfBoundsException: This occurs when an application attempts to access an array index that is out of bounds.
• ArithmeticException: This occurs when an application attempts to divide by zero.
• IllegalArgumentException: This occurs when a method is passed on an illegal or inappropriate argument.

Unlike checked exceptions, runtime exceptions do not require a declaration in the throws clause or capture in a try-catch block. However, handling runtime exceptions is advisable in order to provide meaningful error messages and prevent a system crash. Because runtime exceptions provide more specific information about the problem than checked exceptions, they enable developers to detect and correct programming errors more easily and quickly.

157. What is NullPointerException?

It is a type of run-time exception that is thrown when the program attempts to use an object reference that has a null value. The main use of NullPointerException is to indicate that no value is assigned to a reference variable, also it is used for implementing data structures like linked lists and trees. 

158. When is the ArrayStoreException thrown?

ArrayStoreException is thrown when an attempt is made to store the wrong type of object in an array of objects.

159. What is the difference between Checked Exception and Unchecked Exception?

Checked exception:.

Checked Exceptions are the exceptions that are checked during compile time of a program. In a program, if some code within a method throws a checked exception, then the method must either handle the exception or must specify the exception using the throws keyword. 

Checked exceptions are of two types: 

• Fully checked exceptions: all its child classes are also checked, like IOException, and InterruptedException.
• Partially checked exceptions: some of its child classes are unchecked, like an Exception. 

Unchecked Exception:

Unchecked are the exceptions that are not checked at compile time of a program. Exceptions under Error and RuntimeException classes are unchecked exceptions, everything else under throwable is checked. 

160. What is the base class for Error and Exception?

Error is an illegal operation performed by the user which causes abnormality in the program. Exceptions are the unexpected events or conditions that comes while running the program, exception disrupts the normal flow of the program’s instructions.

Errors and Exceptions both have a common parent class which is java.lang.Throwable class.

161. Is it necessary that each try block must be followed by a catch block?

No, It is not necessary to use catch block after try block in Java as we can create another combination with finally block. Finally is the block which runs despite the fact that the exception is thrown or not.

162. What is exception propagation?

Exception propagation is a process in which the exception is dropped from to the top to the bottom of the stack. If not caught once, the exception again drops down to the previous method, and so on until it gets caught or until it reaches the very bottom of the call stack.

163. What will happen if you put System.exit(0) on the try or catch block? Will finally block execute?

System.exit(int) has the capability to throw SecurityException. So, if in case of security, the exception is thrown then finally block will be executed otherwise JVM will be closed while calling System. exit(0) because of which finally block will not be executed.

164. What do you understand by Object Cloning and how do you achieve it in Java?

It is the process of creating an exact copy of any object. In order to support this, a java class has to implement the Cloneable interface of java.lang package and override the clone() method provided by the Object class the syntax of which is:

Protected Object clone() throws CloneNotSupportedException{ return (Object)super.clone();}In case the Cloneable interface is not implemented and just the method is overridden, it results in CloneNotSupportedException in Java.

165. How do exceptions affect the program if it doesn’t handle them?

Exceptions are responsible for abruptly terminating the running of the program while executing and the code written after the exception occurs is not executed.

166. What is the use of the final keyword?

The final keyword is used to make functions non-virtual. By default, all the functions are virtual so to make it non-virtual we use the final keyword.

167. What purpose do the keywords final, finally, and finalize fulfill?

final is a keyword is used with the variable, method, or class so that they can’t be overridden. 

ii). finally

finally is a block of code used with “try-catch” in exception handling. Code written in finally block runs despite the fact exception is thrown or not.

iii). finalize

It is a method that is called just before deleting/destructing the objects which are eligible for Garbage collection to perform clean-up activity.

168. What is the difference between this() and super() in Java?

169. what is multitasking.

Multitasking in Java refers to a program’s capacity to carry out several tasks at once. Threads, which are quick operations contained within a single program, can do this. Executing numerous things at once is known as multitasking.

170. What do you mean by a Multithreaded program?

Multithreaded programs in Java contain threads that run concurrently instead of running sequentially. A computer can use its resources more efficiently by combining multiple tasks at once. Any program with multithreading allows more than one user to simultaneously use the program without running multiple copies. A multithreaded program is designed to run multiple processes at the same time which can improve the performance of a program and allows the program to utilize multiple processors and improves the overall throughput.

171. What are the advantages of multithreading?

There are multiple advantages of using multithreading which are as follows:

• Responsiveness: User Responsiveness increases because multithreading interactive application allows running code even when the section is blocked or executes a lengthy process.
• Resource Sharing: The process can perform message passing and shared memory because of multithreading.
• Economy: We are able to share memory because of which the processes are economical.
• Scalability: Multithreading on multiple CPU machines increases parallelism.
• Better Communication: Thread synchronization functions improves inter-process communication.
• Utilization of multiprocessor architecture
• Minimized system resource use

172. What are the two ways in which Thread can be created?

Multithreading is a Java feature that allows concurrent execution of two or more parts of a program for maximum utilization of the CPU. In general, threads are small, lightweight processes with separate paths of execution. These threads use shared memory, but they act independently, thus if any one thread fails it does not affect the other threads. There are two ways to create a thread:

By extending the Thread class

• By implementing a Runnable interface.

We create a class that extends the java.lang.Thread class . This class overrides the run() method available in the Thread class. A thread begins its life inside run() method. 

By implementing the Runnable interface

We create a new class that implements java.lang.Runnable interface and override run() method. Then we instantiate a Thread object and call the start() method on this object. 

173. What is a thread?

Threads in Java are subprocess with lightweight with the smallest unit of processes and also has separate paths of execution. These threads use shared memory but they act independently hence if there is an exception in threads that do not affect the working of other threads despite them sharing the same memory. A thread has its own program counter, execution stack, and local variables, but it shares the same memory space with other threads in the same process. Java provides built-in support for multithreading through the Runnable interface and the Thread class .

174. Differentiate between process and thread?

A process and a thread are both units of execution in a computer system, but they are different in several ways:

175. Describe the life cycle of the thread?

A thread in Java at any point in time exists in any one of the following states. A thread lies only in one of the shown states at any instant: 

• New: The thread has been created but has not yet started.
• Runnable: The thread is running, executing its task, or is ready to run if there are no other higher-priority threads.
• Blocked: The thread is temporarily suspended, waiting for a resource or an event.
• Waiting: The thread is waiting for another thread to perform a task or for a specified amount of time to elapse.
• Terminated: The thread has completed its task or been terminated by another thread.

176. Explain suspend() method under the Thread class.

The suspend() method of the Thread class in Java temporarily suspends the execution of a thread. When a thread is suspended it goes into a blocked state and it would not be scheduled by the operating system which means that it will not be able to execute its task until it is resumed. There are more safer and flexible alternatives to the suspend() methods in the modern java programming language. This method does not return any value.

177. Explain the main thread under Thread class execution.

Java provides built-in support for multithreaded programming. The main thread is considered the parent thread of all the other threads that are created during the program execution. The main thread is automatically created when the program starts running. This thread executes the main method of the program. It is responsible for executing the main logic of the Java program as well as handling the user input operations. The main thread serves as the base thread from which all other child threads are spawned. 

178. What is a daemon thread?

A daemon thread in Java is a low-priority thread that is used to perform background operations or tasks which are used to perform continuously. such as Garbage collection, Signal dispatches, Action listeners, etc. Daemon threads in Java have lower priority than user threads, which means they can only execute when no user threads are running. Daemon threads in Java are useful features that are required for background tasks that do not require explicit shutdown or finalization. It allows more efficient use of system resource and are used to simplify resources and can simplify long-running tasks.

179. What are the ways in which a thread can enter the waiting state?

Thread is a lightweight process that runs concurrently with the other thread inside a single process. Each thread can execute a different task and share the resources within a single process. Thread in Java can enter the waiting state in many different ways:

• Sleep() method Call: The sleep () method is used to pause the execution of the thread for a specific amount of time. While the thread is paused it goes into the waiting state.
• Wait() method: This method is used to wait a thread until the other thread signals it to wake up. Thread goes into the waiting state until it receives a notification from another thread.
• Join() method: Join() method can be used to wait for thread to finish the execution. Calling thread goes into the waiting state until the target thread is completed.
• Waiting for I/O operations: If the thread is waiting for Input/Output operation to complete, it goes into the waiting state until the operation is finished.
• Synchronization Issues: If there are any synchronization issues in a multi-threaded application, threads may go into the waiting state until the synchronization issues are resolved.

180. How does multi-threading take place on a computer with a single CPU?

Java uses a technique called time-sharing, commonly referred to as time-slicing, to implement multi-threading on computers with a single CPU. The appearance of parallel execution is created by the CPU switching between active threads. The operating system is in charge of allocating CPU time to each thread sequentially and scheduling the threads.

In order to stop threads from interacting with one another and creating race situations or other issues, Java has a number of ways to govern the behavior of threads, including synchronization and locking. It is feasible to create multi-threaded programmers that operate correctly and effectively on a machine with a single CPU by regulating the interaction between threads and making sure that crucial code parts are synchronized. In contrast to running the same program on a computer with multiple CPUs or cores, multi-threading on a single CPU can only give the appearance of parallelism, and actual performance gains may be modest. The operating system divides the CPU time that is available when numerous threads are running on a single CPU into small time slices and gives each thread a time slice to execute. Rapid switching between the threads by the operating system creates the appearance of parallel execution. The switching between threads appears to be immediate because the time slices are often very tiny, on the order of milliseconds or microseconds.

181. What are the different types of Thread Priorities in Java? And what is the default priority of a thread assigned by JVM?

Priorities in threads is a concept where every thread is having a priority which in layman’s language one can say every object is having priority here which is represented by numbers ranging from 1 to 10. There are different types of thread properties in Java mentioned below:

• MIN_PRIORITY
• MAX_PRIORITY
• NORM_PRIORITY 

By default, the thread is assigned NORM_PRIORITY.

182. Why Garbage Collection is necessary in Java?

For Java, Garbage collection is necessary to avoid memory leaks which can cause the program to crash and become unstable. There is no way to avoid garbage collection in Java. Unlike C++, Garbage collection in Java helps programmers to focus on the development of the application instead of managing memory resources and worrying about memory leakage. Java Virtual Machine (JVM) automatically manages the memory periodically by running a garbage collector which frees up the unused memory in the application. Garbage collection makes Java memory efficient because it removes unreferenced objects from the heap memory.

183. What is the drawback of Garbage Collection?

Apart from many advantages, Garbage Collector has certain drawbacks mentioned below:

• The main drawback to Garbage collection is that it can cause pauses in an application’s execution as it works to clear the memory which slows down the performance of the application. 
• The Process of Garbage collection is non-deterministic which makes it difficult to predict when garbage collection occurs which causes unpredictable behavior in applications. For Example, if we write any program then it is hard for programmers to decide if the issue is caused by garbage collection or by any other factors in the program. 
• Garbage collection can also increase memory usage if the program creates and discards a lot of short-lived objects.

184. Explain the difference between a minor, major, and full garbage collection.

The Java Virtual Machine (JVM) removes objects that are no longer in use using a garbage collector which periodically checks and removes these objects. There are different types of garbage collection in the JVM, each with different characteristics and performance implications. The main types of garbage collection are:

• Minor garbage collection: Also known as young generation garbage collection, this type of garbage collection is used to collect and reclaim memory that is used by short-lived objects (objects that are quickly created and discarded). 
• Major garbage collection: Also known as old-generation garbage collection, this type of garbage collection is used to collect and reclaim memory that is used by long-lived objects (objects that survive multiple minor garbage collections and are promoted to the old generation).
• Full garbage collection: During full garbage collection, memories from all generations are collected and reclaimed, including memories of young and old. A full garbage collection normally takes longer to complete than a minor or major garbage collection which causes that app to pause temporarily.

185. How will you identify major and minor garbage collections in Java?

Major garbage collection works on the survivor space and Minor garbage collection works on the Eden space to perform a mark-and-sweep routine. And we can identify both of them based on the output where the minor collection prints “GC”, whereas the major collection prints “Full GC” for the case where the garbage collection logging is enabled with “-XX:PrintGCDetails” or “verbose:gc”.

186. What is a memory leak, and how does it affect garbage collection?

In Java Memory leaks can be caused by a variety of factors, such as not closing resources properly, holding onto object references longer than necessary, or creating too many objects unnecessarily. There are situations in which garbage collector does not collect objects because there is a reference to those objects. In these situations where the application creates lots of objects and does not use them and every object has some valid references, a Garbage collector in Java cannot destroy the objects. These useless objects which do not provide any value to the program are known as Memory leaks. Memory leaks can impact garbage collection negatively by preventing the garbage collector from reclaiming unused memory. This behavior will lead to slow performance or sometimes system failure. In a program, it is important to avoid memory leaks by managing resources and object references properly. 

187. Name some classes present in java.util.regex package.

Regular Expressions or Regex in Java is an API used for searching and manipulating of strings in Java. It creates String patterns that can extract the data needed from the strings or can generalize a pattern.

There are 3 Classes present in java.util.regex mentioned below:

• Pattern Class: Can define patterns
• Matcher Class: Can perform match operations on text using patterns
• PatternSyntaxException Class: Can indicate a syntax error in a regular expression pattern.

Also, apart from the 3 classes package consists of a single interface MatchResult Interface which can be used for representing the result of a match operation.

188. Write a regular expression to validate a password. A password must start with an alphabet and followed by alphanumeric characters; Its length must be in between 8 to 20.

Explanation:

• ^ used for starting character of the string.
• (?=.*[0-9]) used for a digit must occur at least once.
• (?=.*[a-z]) used for a lowercase alphabet must occur at least once.
• (?=.*[A-Z]) used for an upper case alphabet that must occur at least once in the substring.
• (?=.*[@#$%^&-+=()] used for a special character that must occur at least once.
• (?=\\S+$) white spaces don’t allow in the entire string.
• .{8, 20} used for at least 8 characters and at most 20 characters.
• $ used for the end of the string.

189. What is JDBC?

JDBC standard API is used to link Java applications and relational databases. It provides a collection of classes and interfaces that let programmers to use the Java programming language to communicate with the database. The classes and interface of JDBC allow the application to send requests which are made by users to the specified database. There are generally four components of JDBC by which it interacts with the database:

• JDBC Driver manager
• JDBC Test Suite
• JDBC-ODBC Bridge Drivers

190. What is JDBC Driver?

JDBC Driver is a software component that is used to enable a Java application to interact with the database. JDBC provides the implementation of the JDBC API for a specific database management system, which allows it to connect the database, execute SQL statements and retrieve data. There are four types of JDBC drivers:

• JDBC-ODBC Bridge driver
• Native-API driver
• Network Protocol driver
• Thin driver

191. What are the steps to connect to the database in Java?

There are certain steps to connect the database and Java Program as mentioned below:

• Import the Packages
• Load the drivers using the forName() method 
• Register the drivers using DriverManager 
• Establish a connection using the Connection class object
• Create a statement
• Execute the query
• Close the connections

192. What are the JDBC API components?

JDBC API components provide various methods and interfaces for easy communication with the databases also it provides packages like java Se and java EE which provides the capability of write once run anywhere (WORA).

193. What is JDBC Connection interface?

Java database connectivity interface (JDBC) is a software component that allows Java applications to interact with databases. To enhance the connection, JDBC requires drivers for each database.

194. What does the JDBC ResultSet interface?

JDBC ResultSet interface is used to store the data from the database and use it in our Java Program. We can also use ResultSet to update the data using updateXXX() methods. ResultSet object points the cursor before the first row of the result data. Using the next() method, we can iterate through the ResultSet.

195. What is the JDBC Rowset?

A JDBC RowSet provides a way to store the data in tabular form. RowSet is an interface in java that can be used within the java.sql package. The connection between the RowSet object and the data source is maintained throughout its life cycle. RowSets are classified into five categories based on implementation mentioned below:

• CachedRowSet
• FilteredRowSet

196. What is the role of the JDBC DriverManager class?

JDBC DriverManager class acts as an interface for users and Drivers. It is used in many ways as mentioned below: 

• It is used to create a connection between a Java application and the database. 
• Helps to keep track of the drivers that are available.
• It can help to establish a connection between a database and the appropriate drivers.
• It contains all the methods that can register and deregister the database driver classes.
• DriverManager.registerDriver() method can maintain the list of Driver classes that have registered themselves.

197. Differentiate between Iterable and Iterator.

198. differentiate between list and set., 199. differentiate between list and map., 200. differentiate between queue and stack., 201. differentiate between priorityqueue and treeset., 202. differentiate between the singly linked list and doubly linked list., 203. differentiate between failfast and failsafe., 204. differentiate between hashmap and treemap., 205. differentiate between queue and deque., 206. differentiate between hashset and treeset., java interview questions – faqs, q1. what is a java developer’s salary in india.

According to various resources, The average salary of a Java Backend Developer is more than 14 lakhs per annum which is 30% higher than any other developer role . Here you can also check our latest course on Java Backend Development !

Q2. What does Java Developer do?

A Java developer writes code, designs software solutions, and builds applications using the Java programming language. They collaborate with teams, solve problems, and ensure code quality for efficient and reliable software development.

Q3. What are the essential skills required for a Java developer?

A Java developer should have a strong understanding of core Java concepts such as object-oriented programming, data types, control structures, and exception handling. Additionally, knowledge of frameworks like Spring, Hibernate, and web development technologies like Servlets and JSP is beneficial. Other than Technical Skills Problem-solving, debugging, and critical thinking skills are also highly valued.

Q4. How can I prepare for a Java interview?

To prepare for a Java interview, start by reviewing fundamental Java concepts and practice coding exercises. Study common interview questions related to core Java, data structures, algorithms, and multithreading from GeeksforGeeks Interview Section . Additionally, brush up on design patterns, database connectivity, and web development frameworks. Practising coding challenges on platforms like GeekforGeeks Practice Portal can also be helpful.

Q5. How can I stand out in a Java interview?

To stand out in a Java interview, demonstrate a deep understanding of Java concepts and practical applications. Showcase your problem-solving skills by explaining your approach to complex scenarios and providing efficient solutions. Additionally, highlight any relevant projects or contributions you’ve made to the Java community. Showing enthusiasm, good communication, and a willingness to learn can also leave a positive impression.

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• Intro Video
• Lecture 01: Introduction
• Lecture 02: Java Programming Steps
• Lecture 03: Java Tools and Resources
• Lecture 04: Demonstration-I
• Lecture 05: Java Applet Programming
• Lecture 06: Demonstration-II
• Lecture 07: Encapsulation
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• Lecture 10: Demonstration-IV
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• Lecture 12 : Demonstration-V
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• Lecture 23 : Exception Handling-I
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• Lecture 26 : Demonstration-X
• Lecture 27 : Multithreading-I
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• Lecture 29 : Demonstration-XI
• Lecture 30 : I-O Stream-I
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• Lecture 32: I-O Stream-III
• Lecture 33: Demonstration-XII
• Lecture 34: Applet Programming—I
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• Lecture 37: Demonstration-XIII
• Lecture 38: Demonstration-XIV
• Lecture 39: AWT Programming—I
• Lecture 40: AWT Programming—II
• Lecture 41 : Demonstration-XV
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• Lecture 43 : Swing—I
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