Top 25 Java Interview Questions for 2025 (With Answers)

Whether you're a fresher entering the battlefield or a working dev aiming for top-tier product companies, mastering Java interview questions is non-negotiable.
Here are 25 hand-picked, battle-tested Java questions to help you dominate interviews in 2025.

✅ Core Java Concepts

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

• JDK: Java Development Kit – tools + compiler + JRE

• JRE: Java Runtime Environment – JVM + libraries

• JVM: Java Virtual Machine – runs bytecode

2. What are the features of Java?

• Object-Oriented, Platform-Independent, Secure, Robust, Multithreaded, and more.

3. How is Java platform-independent?
Bytecode runs on the JVM, which is available for multiple platforms. Write once, run anywhere.

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

• == compares object references.

• .equals() compares actual content.

5. What is a constructor? Types?

• Special method called when object is created.

• Types: Default, Parameterized, Copy Constructor

✅ OOP Principles & Java Internals

6. What is Polymorphism in Java?

• Compile-time (method overloading)

• Runtime (method overriding)

7. What is Abstraction?
Hiding internal details and showing essential features — via abstract class or interface.

8. Interface vs Abstract Class?

• Interfaces: Full abstraction, default methods (Java 8+).

• Abstract: Partial abstraction, can have constructors & instance variables.

9. What is Encapsulation?
Binding data and code together, achieved by private variables + public getters/setters.

10. Difference between Heap and Stack Memory?

• Heap: Objects, slow, shared.

• Stack: Method calls, fast, thread-local.

✅ Java Collections

11. List vs Set vs Map?

• List: Ordered, allows duplicates (e.g., ArrayList)

• Set: No duplicates (e.g., HashSet)

• Map: Key-value pairs (e.g., HashMap)

12. What is the difference between ArrayList and LinkedList?

• ArrayList: Fast random access, slow inserts/removals

• LinkedList: Slow access, fast insert/delete

13. HashMap vs Hashtable?

• HashMap: Not thread-safe, allows nulls

• Hashtable: Thread-safe, no null keys/values

14. What is ConcurrentHashMap?
Thread-safe Map for concurrent access without full locking — used in multi-threaded apps.

15. Iterator vs ListIterator?

• Iterator: Forward only

• ListIterator: Forward + backward traversal

✅ Multithreading & Concurrency

16. What is the difference between process and thread?

• Process: Independent execution unit

• Thread: Sub-unit of a process, lightweight, shares memory

17. What is synchronization?
Controlling access to shared resources to avoid race conditions — via synchronized blocks/methods.

18. What is volatile keyword?
Ensures visibility of changes to variables across threads — prevents caching issues.

19. Difference between wait(), sleep(), and join()?

• wait(): Releases lock, used in inter-thread comm

• sleep(): Pauses thread, doesn’t release lock

• join(): Waits for a thread to die

20. Thread lifecycle in Java?
New → Runnable → Running → Blocked → Terminated

✅ Advanced Concepts

21. Exception vs Error?

• Exception: Recoverable (e.g., FileNotFoundException)

• Error: Unrecoverable (e.g., OutOfMemoryError)

22. Checked vs Unchecked Exceptions?

• Checked: Caught at compile time

• Unchecked: Caught at runtime

23. What is a lambda expression?
Shorthand for anonymous methods — introduced in Java 8:
(a, b) -> a + b

24. What is Stream API?
Functional-style operations on collections: map(), filter(), reduce(), etc.

25. What is garbage collection in Java?
Automatic memory management — deletes unused objects via JVM’s GC threads.

🧠 Bonus: How to Use These Questions

• Copy them into a Google Doc

• Practice writing answers in your own words

• Pair each question with a code example for mastery

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