Top 25 Java Interview Questions and Answers (2026 Guide)

1. What is Java?

Java is a high-level, object-oriented, class-based, and platform-independent programming language developed by Sun Microsystems (now owned by Oracle). It follows the principle of Write Once, Run Anywhere (WORA), which means Java code can run on any system that has a Java Virtual Machine (JVM).

Java is secure, robust, multithreaded, portable, and highly scalable. It is widely used in web applications, enterprise applications, Android apps, cloud computing, fintech systems, and large enterprise solutions.

2. What is JVM?

JVM (Java Virtual Machine) is a virtual engine that runs Java bytecode and converts it into machine-level code. It makes Java platform-independent because the same bytecode can run on any operating system that supports JVM.

JVM is responsible for memory management, garbage collection, class loading, security management, and exception handling.

Java Code → Compiler → Bytecode → JVM → Machine Code

3. Difference between JDK, JRE, and JVM?

JVM: Executes Java bytecode and provides platform independence.

JRE: Provides runtime environment including JVM and required libraries to run Java applications.

JDK: Development kit that contains JRE + tools like compiler (javac), debugger, and documentation generator.

In simple words: JDK > JRE > JVM.

4. What is OOP?

OOP (Object-Oriented Programming) is a programming paradigm based on objects and classes. It improves modularity, security, and code reusability.

• Encapsulation – Data hiding
• Inheritance – Code reuse
• Polymorphism – Many forms
• Abstraction – Hiding implementation

5. What is Encapsulation?

Encapsulation is the process of binding data and methods into a single unit called a class. It restricts direct access to data using private variables and provides controlled access using getter and setter methods.

class Student {
  private int age;
  public int getAge() { return age; }
}

Encapsulation improves data security and maintainability.

6. What is Inheritance?

Inheritance allows one class to acquire properties and methods of another class using the extends keyword. It promotes code reusability and reduces redundancy in applications.

7. What is Polymorphism?

Polymorphism allows methods to perform different tasks based on context. It is of two types:

Compile-time: Method Overloading

Runtime: Method Overriding

8. What is Abstraction?

Abstraction hides internal implementation details and shows only essential features. It is achieved using abstract classes and interfaces in Java.

9. Abstract Class vs Interface?

Abstract Class: Can have abstract and non-abstract methods, constructors, and instance variables.

Interface: Supports full abstraction and multiple inheritance. Methods are public by default.

10. Overloading vs Overriding?

Overloading: Same method name with different parameters (compile-time).

Overriding: Child class modifies parent class method (runtime).

11. Difference between == and equals()?

== compares memory references.

equals() compares object content.

12. String vs StringBuilder vs StringBuffer?

String: Immutable and thread-safe.

StringBuilder: Mutable and faster but not thread-safe.

StringBuffer: Mutable and thread-safe.

13. What is Exception Handling?

Exception handling in Java is a mechanism to handle runtime errors using try, catch, finally, throw, and throws keywords.

try {
  int a = 10/0;
} catch(Exception e){
  System.out.println(e);
}

14. Checked vs Unchecked Exceptions?

Checked: Compile-time exceptions like IOException.

Unchecked: Runtime exceptions like NullPointerException.

15. final vs finally vs finalize?

final: Prevents modification.

finally: Always executes after try-catch.

finalize: Called before garbage collection (deprecated).

16. What is Constructor?

A constructor initializes objects. It has the same name as the class and runs automatically when an object is created.

17. What is static keyword?

The static keyword makes variables and methods belong to the class rather than objects. It helps in memory management.

18. What is Garbage Collection?

Garbage Collection automatically removes unused objects from memory to optimize performance and prevent memory leaks.

19. What is Multithreading?

Multithreading allows multiple threads to run simultaneously to improve application performance and CPU utilization.

20. What is Synchronization?

Synchronization controls access to shared resources in multithreaded environments to avoid race conditions.

21. What is Collection Framework?

The Java Collection Framework provides classes and interfaces like List, Set, Map, and Queue to store and manipulate data dynamically.

22. ArrayList vs LinkedList?

ArrayList: Faster for searching, uses dynamic array.

LinkedList: Faster insertion and deletion, uses doubly linked list.

23. What is HashMap?

HashMap stores key-value pairs using hashing technique. It allows one null key and multiple null values.

24. HashMap vs ConcurrentHashMap?

HashMap: Not thread-safe.

ConcurrentHashMap: Thread-safe and used in multithreaded environments.

25. List vs Set vs Map?

List: Allows duplicates and maintains order.

Set: Does not allow duplicates.

Map: Stores key-value pairs with unique keys.