Here are the Top 20 Java Interview Questions and Answers (2025 Updated), suitable for freshers and experienced developers:
✅ Core Java Interview Questions
1. What is Java?
Answer:
Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It is platform-independent due to the JVM (Java Virtual Machine) and is used for building desktop, web, and mobile applications.
2. What are the features of Java?
Answer:
- Object-Oriented
- Platform-Independent
- Robust and Secure
- Multithreaded
- High Performance
- Distributed
3. What is the difference between JDK, JRE, and JVM?
| Component | Description |
|---|---|
| JDK | Java Development Kit – contains JRE + development tools |
| JRE | Java Runtime Environment – runs Java code |
| JVM | Java Virtual Machine – executes bytecode |
4. What is the difference between == and .equals()?
Answer:
==compares object references..equals()compares object content (can be overridden).
Top 20 Java Interview Questions and Answers
5. What is the difference between Stack and Heap memory in Java?
Answer:
- Stack: Stores method-level local variables, operates in LIFO.
- Heap: Stores objects and instance variables, shared across threads.
6. What is method overloading and overriding?
Answer:
- Overloading: Same method name, different parameters (compile-time).
- Overriding: Subclass redefines a parent class method (runtime).
7. What is constructor chaining?
Answer:
Constructor chaining is calling one constructor from another using this() or super().
8. What is a static keyword in Java?
Answer:
It denotes that a method or variable belongs to the class rather than instances of it.
9. What is the difference between an abstract class and an interface?
| Feature | Abstract Class | Interface |
|---|---|---|
| Methods | Can have both abstract and concrete methods | Only abstract methods (Java 7), default/static allowed (Java 8+) |
| Inheritance | Single inheritance | Multiple inheritance |
| Variables | Can have instance variables | Only constants (public static final) |
Top 20 Java Interview Questions and Answers
10. What is a final keyword in Java?
Answer:
- final variable: Value cannot be changed.
- final method: Cannot be overridden.
- final class: Cannot be extended.
✅ OOPs and Advanced Java Questions
11. Explain the four pillars of OOP.
Answer:
- Encapsulation – Hiding internal state using access modifiers.
- Abstraction – Hiding implementation details via abstract classes or interfaces.
- Inheritance – Code reuse by inheriting properties from another class.
- Polymorphism – Method overloading/overriding to perform different operations.
12. What are exceptions in Java?
Answer:
Exceptions are unwanted or unexpected events during program execution.
- Checked: Handled during compile-time (e.g., IOException).
- Unchecked: Occurs at runtime (e.g., NullPointerException).
13. What is the difference between throw and throws?
| Keyword | Purpose |
|---|---|
throw | Used to explicitly throw an exception |
throws | Declares exceptions in method signature |
14. What is the difference between ArrayList and LinkedList?
| Feature | ArrayList | LinkedList |
|---|---|---|
| Structure | Dynamic array | Doubly linked list |
| Access | Fast (index) | Slow (sequential) |
| Insertion/Deletion | Slow | Fast |
Top 20 Java Interview Questions and Answers
15. What is multithreading in Java?
Answer:
Multithreading is a process of executing multiple threads simultaneously. Java provides Thread class and Runnable interface to implement it.
16. What is synchronization in Java?
Answer:
Synchronization ensures only one thread accesses a block/method at a time to avoid race conditions. It’s done using the synchronized keyword.
17. What is a deadlock?
Answer:
Deadlock is a situation where two or more threads are waiting for each other to release resources, leading to a standstill.
18. What is garbage collection in Java?
Answer:
Garbage collection automatically removes unused objects to free up memory. It runs in the background using the gc() method.
19. What are lambda expressions?
Answer:
Introduced in Java 8, lambda expressions allow treating functionality as method arguments or passing behavior.
Syntax:
(parameter) -> { body }
20. What are streams in Java 8?
Answer:
Streams are used to process collections of objects in a functional style (map, filter, reduce). They support parallel operations.
Top 20 Java Interview Questions and Answers
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Top 20 Java Interview Questions and Answers
Top 20 Java Interview Questions and Answers
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