Top 30 Java Interview Questions (With Answers & Examples) 2025 Edition

If you’re preparing for a Java job interview, you must be familiar with the Top 30 Java Interview Questions commonly asked by recruiters. This comprehensive list will help you solidify your fundamentals and brush up on advanced topics.

Java Interview Questions

1. What is Java?

Java is a high-level, class-based, object-oriented programming language developed by Sun Microsystems in 1995. It’s designed to have minimal implementation dependencies.

Key features:

• Platform-independent through JVM
• Object-Oriented
• Secure and robust
• Multi-threaded and dynamic
• Used in web, mobile, and enterprise applications

Its WORA (Write Once, Run Anywhere) nature makes it a top choice for developers worldwide.

2. Explain the concept of Object-Oriented Programming in Java.

Java supports all four pillars of Object-Oriented Programming (OOP):

• Encapsulation – Wrapping data and methods into a single unit (class)
• Inheritance – Acquiring properties from a parent class
• Polymorphism – Same method behaves differently in different situations
• Abstraction – Hiding implementation details and exposing functionality

Using these, Java promotes reusability and modular programming.

3. What is the difference between JDK, JRE, and JVM?

• JDK (Java Development Kit): Contains tools to develop Java programs, including JRE and compilers.
• JRE (Java Runtime Environment): Provides libraries and JVM to run Java programs.
• JVM (Java Virtual Machine): Executes Java bytecode and makes Java platform-independent.

Understanding these components is critical in mastering the Top 30 Java Interview Questions.

4. What are Java Data Types?

Java supports two types of data:

• Primitive Types: int, float, boolean, char, etc.
• Reference Types: Objects, arrays, interfaces

This allows Java to manage both simple and complex data efficiently.

5. Explain the concept of a Class and an Object.

• Class: A blueprint to create objects. It contains variables and methods.
• Object: An instance of a class created using the new keyword.

Example:

class Dog {
  String breed;
}
Dog d = new Dog();

6. What is Inheritance in Java?

Inheritance allows a class to inherit properties and methods of another class using the extends keyword.

• Promotes code reusability
• Supports method overriding
• Enables hierarchical classification

class Animal {}
class Dog extends Animal {}

7. What is Polymorphism?

Polymorphism means “many forms.” Java supports:

• Compile-time Polymorphism: Method overloading
• Run-time Polymorphism: Method overriding

It allows developers to write flexible and maintainable code.

8. What is Encapsulation?

Encapsulation is bundling data (variables) and methods that act on the data into a single unit. It also hides internal details.

Benefits:

• Better control over data
• Code flexibility and maintainability
• Protects data from outside interference

9. What is Abstraction in Java?

Abstraction is the process of hiding internal implementation details and showing only necessary functionality.

• Achieved via abstract classes and interfaces
• Improves security and modularity

10. What are constructors in Java?

A constructor is a special method invoked when an object is created.

• Same name as the class
• No return type
• Can be default or parameterized

class Car {
  Car() {
    System.out.println("Car created");
  }
}

11. What is the difference between == and .equals()?

• == compares object references.
• .equals() compares actual content or values.

For strings, always use .equals() to avoid logical bugs.

12. What is a static keyword?

The static keyword makes a method or variable belong to the class rather than an instance.

• Static methods can be called without creating an object.
• Used for constants and utility methods.

13. What are access modifiers?

Java provides the following access modifiers:

• private
• default
• protected
• public

They control visibility and accessibility of classes and members.

14. What is method overloading?

Method overloading means defining multiple methods with the same name but different parameters.

Example:

void add(int a, int b) {}
void add(double a, double b) {}

15. What is method overriding?

Method overriding occurs when a subclass provides its own version of a method declared in a superclass.

• Must have the same method signature
• Achieved using inheritance

16. What is an interface in Java?

An interface is a reference type that can contain abstract methods and constants. It provides full abstraction.

• Supports multiple inheritance
• Implemented using implements keyword

17. What are exceptions in Java?

Exceptions are events that disrupt the normal flow of execution.

Types:

• Checked Exceptions – Handled at compile-time
• Unchecked Exceptions – Occur at runtime

18. What is the difference between ArrayList and LinkedList?

Both implement the List interface but differ in internal structure.

19. What is multithreading in Java?

Multithreading allows concurrent execution of two or more threads.

• Improves application performance
• Achieved via Thread class or Runnable interface

20. What is synchronization?

Synchronization is the capability to control the access of multiple threads to shared resources.

• Prevents thread interference
• Uses synchronized keyword

21. What is a Java package?

A package is a namespace for organizing classes and interfaces.

• Helps avoid class name conflicts
• Supports access protection

22. What is garbage collection?

Java automatically manages memory using garbage collection, freeing up memory by deleting unused objects.

• Runs in the background
• Improves memory efficiency

23. What is a final keyword?

• final variable – value cannot be changed
• final method – cannot be overridden
• final class – cannot be inherited

24. What is the difference between throw and throws?

• throw – Used to explicitly throw an exception.
• throws – Declares exceptions a method might throw.

25. What is a String in Java?

A String is an object representing a sequence of characters.

• Immutable
• Stored in a special memory area called the String pool

26. What is the difference between StringBuilder and StringBuffer?

27. What is autoboxing and unboxing?

• Autoboxing: Converting primitive types to wrapper classes automatically.
• Unboxing: Converting wrapper class to primitive.

Example:

Integer i = 10; // autoboxing
int x = i;      // unboxing

28. What is the transient keyword?

The transient keyword prevents variables from being serialized. Useful when you don’t want to persist some fields.

29. What is the use of super keyword?

• Refers to the parent class
• Used to call parent constructors or methods

30. What are lambda expressions?

Lambda expressions simplify the syntax for writing anonymous methods. Learn More on Lambda Expression in Java 8.

Syntax:

(parameter) -> expression

Example:

Runnable r = () -> System.out.println("Running");

Final Thoughts

Mastering these Top 30 Java Interview Questions ensures you’re well-prepared for technical rounds. Whether you’re a fresher or experienced developer, revisiting these core concepts is essential for interview success. Download Java. Also check How to setup Java on your machine.

For more such Java tips, bookmark this blog and stay updated!

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• How to Understand Object-Oriented Programming in Java Easily