Top 10 Java Interview Questions & Answers

1. What is Java and what are its key features?
Java is a high-level, object-oriented programming language that is designed to run on any platform. Its key features include platform independence, automatic memory management, strong type checking, and support for multithreading.

2. What is the difference between JDK, JRE, and JVM?
- JDK (Java Development Kit) is a software development kit that includes tools for developing, debugging, and monitoring Java applications.
- JRE (Java Runtime Environment) is a runtime environment that allows you to run Java applications. It includes the JVM (Java Virtual Machine) and necessary libraries.
- JVM (Java Virtual Machine) is an abstract machine that executes Java bytecode. It provides platform independence by running the same bytecode on different platforms.

3. What are the differences between abstract classes and interfaces in Java?
- An abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods.
- A class can extend only one abstract class, but it can implement multiple interfaces.
- An abstract class can have instance variables, while an interface cannot.
- An abstract class can provide default implementations for some methods, while an interface cannot.

4. What are the access modifiers in Java?
Java has four access modifiers:
- public: accessible from anywhere
- private: accessible only within the same class
- protected: accessible within the same package or subclasses
- default (no modifier): accessible within the same package

5. What is the difference between a class variable and an instance variable in Java?
- A class variable, also known as a static variable, is shared among all instances of a class. It is declared with the static keyword and is accessed using the class name.
- An instance variable is unique to each instance of a class. It is declared without the static keyword and is accessed using the instance name.

6. What is the difference between method overloading and method overriding in Java?
- Method overloading occurs when multiple methods in the same class have the same name but different parameters. The compiler determines which method to call based on the number and types of arguments.
- Method overriding occurs when a subclass provides a different implementation of a method that is already defined in its superclass. The method in the subclass must have the same name, return type, and parameters as the method in the superclass.

7. What is a constructor in Java?
A constructor is a special method that is used to initialize objects of a class. It has the same name as the class and does not have a return type. Constructors are called automatically when an object is created using the new keyword.

8. What are exceptions in Java and how do you handle them?
Exceptions are events that occur during the execution of a program that disrupts the normal flow of instructions. In Java, exceptions are represented by objects. They can be handled using try-catch blocks. The code that may throw an exception is placed within the try block, and any potential exceptions are caught and handled in the catch block.

9. What is multithreading in Java?
Multithreading is a feature of Java that allows multiple threads of execution to run concurrently within a single program. Each thread represents an independent flow of control, allowing for parallel execution of tasks. Multithreading can improve performance by utilizing available CPU resources effectively.

10. What is the difference between ArrayList and LinkedList in Java?
- ArrayList is implemented as a resizable array, while LinkedList is implemented as a doubly linked list.
- ArrayList provides fast random access to elements using indexes, while LinkedList provides fast insertion and deletion at both ends.
- ArrayList uses more memory than LinkedList because it needs to allocate memory for a fixed-size array, while LinkedList only needs to allocate memory for each element and its references.

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