Most Commonly Asked Java Interview Questions with Answers 

If you are preparing for a software developer job, chances are that Java will be one of the core skills tested. Since its launch in the mid-1990s, Java has been one of the most widely used programming languages in the world. From Android apps to enterprise-level applications, Java powers a huge ecosystem.

When it comes to interviews, companies often test candidates on both fundamental concepts and practical coding skills. To make your preparation easier, we have compiled a list of the most commonly asked Java interview questions with answers that will help you build confidence and stand out in your next interview.

1. What are the main features of Java?

Answer:

• Platform Independent: Write once, run anywhere (WORA) principle.
  
  

• Object-Oriented: Supports encapsulation, inheritance, and polymorphism.
  
  

• Robust: Handles memory management and exceptions efficiently.
  
  

• Secure: Provides a runtime environment with security features like bytecode verification.
  
  

• Multithreaded: Allows concurrent execution of two or more threads.
  
  

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

Answer:

• JVM (Java Virtual Machine): Runs Java bytecode and makes Java platform-independent.
  
  

• JRE (Java Runtime Environment): Provides libraries and JVM to run Java applications.
  
  

• JDK (Java Development Kit): Includes JRE, compiler, and development tools to write and run Java programs.
  
  

3. What are Java data types?

Answer:
Java supports two types of data types:

• Primitive: byte, short, int, long, float, double, boolean, char.
  
  

• Non-Primitive: Strings, arrays, classes, and interfaces.
  
  

4. Explain the concept of OOPs in Java.

Answer:
Java follows Object-Oriented Programming principles:

• Encapsulation: Wrapping data and code together.
  
  

• Inheritance: Reusing code from parent classes.
  
  

• Polymorphism: One interface, many implementations.
  
  

• Abstraction: Hiding internal details and showing only essential features.
  
  

5. Difference between abstract class and interface?

Answer:

• Abstract Class: Can have both abstract and non-abstract methods, supports inheritance.
  
  

• Interface: Contains only abstract methods (before Java 8), supports multiple inheritance.
  
  

6. What is the difference between equals() and == in Java?

Answer:

• == checks reference (memory address) equality.
  
  

• equals() checks object content equality (can be overridden in classes like String).
  
  

7. What are constructors in Java?

Answer:
A constructor is a special method used to initialize objects. It has the same name as the class and no return type. Example:

class Student {

    String name;

    Student(String n) {

        name = n;

    }

}

8. What is the difference between method overloading and overriding?

Answer:

• Overloading: Same method name but different parameter list (compile-time polymorphism).
  
  

• Overriding: Subclass provides its own implementation of a method from parent class (runtime polymorphism).
  
  

9. What are access modifiers in Java?

Answer:

• Public: Accessible everywhere.
  
  

• Private: Accessible only within the class.
  
  

• Protected: Accessible within the package and subclasses.
  
  

• Default: Accessible within the package only.
  
  

10. What is the difference between final, finally, and finalize()?

Answer:

• final: Keyword to declare constants, prevent method overriding, or inheritance.
  
  

• finally: Block in exception handling that always executes.
  
  

• finalize(): Method called by garbage collector before object is destroyed.
  
  

11. What is exception handling in Java?

Answer:
Exception handling ensures smooth program execution even in case of errors. Keywords used are:

• try: Defines a block of code to test for errors.
  
  

• catch: Defines a block of code to handle errors.
  
  

• finally: Always executes code, regardless of exception.
  
  

• throw/throws: Used to declare or throw exceptions.
  
  

12. Difference between checked and unchecked exceptions?

Answer:

• Checked Exceptions: Checked at compile time (e.g., IOException).
  
  

• Unchecked Exceptions: Checked at runtime (e.g., NullPointerException).
  
  

13. What is the difference between ArrayList and LinkedList?

Answer:

• ArrayList: Provides fast random access, slower insert/delete.
  
  

• LinkedList: Provides faster insert/delete but slower random access.
  
  

14. What is the difference between HashMap and Hashtable?

Answer:

• HashMap: Non-synchronized, allows one null key and multiple null values.
  
  

• Hashtable: Synchronized, does not allow null keys or values.
  
  

15. What are threads in Java?

Answer:
A thread is a lightweight process that allows multitasking. In Java, threads can be created by extending Thread class or implementing Runnable interface.

16. What is the difference between process and thread?

Answer:

• Process: Independent execution unit with its own memory.
  
  

• Thread: Subset of a process, shares memory with other threads.
  
  

17. Explain garbage collection in Java.

Answer:
Garbage collection automatically removes unused objects to free memory. The System.gc() method can request garbage collection, though it is not guaranteed.

18. What are Java annotations?

Answer:
Annotations provide metadata to the code. Examples include @Override, @Deprecated, and custom annotations.

19. What is the difference between String, StringBuffer, and StringBuilder?

Answer:

• String: Immutable sequence of characters.
  
  

• StringBuffer: Mutable, thread-safe.
  
  

• StringBuilder: Mutable, non-thread-safe, faster.
  
  

20. Final Tips for Java Interview Preparation

• Revise OOP concepts thoroughly.
  
  

• Practice writing small programs to explain logic clearly.
  
  

• Focus on commonly asked collections and multithreading questions.
  
  

• Brush up on Java 8 features like streams, lambdas, and functional interfaces.
  
  

• Solve coding exercises and practice debugging.
  
  

Conclusion

Preparing for a Java interview requires a mix of theoretical understanding and practical coding practice. Employers often want to see how you apply concepts to real-world problems. By going through this collection of java interview question sets, you will build strong confidence in both fundamentals and advanced topics.

Remember, interviewers look not just at your answers, but at how clearly and confidently you explain them. Keep practicing, work on mini-projects, and stay updated with the latest Java features. With consistent effort, you’ll be ready to ace your next technical interview.