Top 20 Java Interview Questions You Must Know in 2025

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Preparing for interviews? We’ve also curated the Top 20 Java Interview Questions for 2025 to help you crack your next job with confidence. These questions cover essential topics like OOPs, multithreading, memory management, Java 17+ updates, and more.

1. What are the main features of Java?

Java is known for its platform independence, object-oriented structure, robustness, security, and portability. Key features include automatic memory management (Garbage Collection), multithreading, and a rich standard library.

2. Explain the concept of the Java Virtual Machine (JVM).

The JVM is an abstract machine that enables Java bytecode to be executed on any platform. It provides memory management, garbage collection, and ensures platform independence.

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

• JDK (Java Development Kit): Includes tools for Java development.
• JRE (Java Runtime Environment): Used to run Java applications.
• JVM (Java Virtual Machine): Executes Java bytecode.

4. What are the main differences between == and .equals() in Java?

• == compares object references (memory addresses).
• .equals() compares object content (as defined in the class).

5. What is the difference between Stack and Heap memory in Java?

• Stack: Stores method calls and local variables.
• Heap: Stores objects and class instances. Managed by the Garbage Collector.

6. What is the significance of the final keyword in Java?

The final keyword can be used with variables, methods, and classes:

• Variables: Cannot be reassigned.
• Methods: Cannot be overridden
• Classes: Cannot be extended.

Learn More : Why Has Java Become So Popular?

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

• Overloading: Same method name, different parameters (compile-time).
• Overriding: Subclass modifies the behavior of a superclass method (runtime).

8. What is encapsulation in Java?

Encapsulation is the technique of wrapping data (variables) and code (methods) together as a single unit. It is implemented using private variables and public getters/setters.

9. What is the use of the super keyword in Java?

• super is used to access:
• Superclass constructors
• Superclass methods or variables

10. How does Java achieve platform independence?

Java programs are compiled into bytecode (.class files), which can be executed by the JVM on any platform, making Java platform-independent.

11. What are checked and unchecked exceptions in Java?

• Checked Exceptions: Must be handled at compile-time (e.g., IOException).
• Unchecked Exceptions: Occur at runtime (e.g., NullPointerException).

12. What is the difference between ArrayList and LinkedList?

• ArrayList: Uses dynamic arrays; better for random access.
• LinkedList: Uses nodes; better for insertions/deletions.

13. What is a lambda expression in Java?

Introduced in Java 8, lambda expressions allow concise implementation of functional interfaces using the syntax:

• java
• CopyEdit
• (parameter) -> expression

14. What are functional interfaces in Java?

A functional interface has only one abstract method and can be implemented using lambda expressions. Example: Runnable, Callable, Comparator.

15. Explain the Stream API in Java 8.

The Stream API is used for processing collections in a functional style (filter, map, reduce). It improves readability and performance with lazy evaluation.

16. What is the difference between HashMap and Hashtable?

• HashMap: Not synchronized, allows one null key.
• Hashtable: Synchronized, doesn’t allow null keys or values.

17. How does Garbage Collection work in Java?

Java automatically manages memory using Garbage Collection. It identifies and removes unused objects to free up memory, improving performance and preventing memory leaks.

18. What is multithreading in Java?

Multithreading allows concurrent execution of two or more threads. It improves application performance and responsiveness.

19. What are design patterns in Java?

Design patterns are standard solutions to common software design problems. Examples: Singleton, Factory, Observer, and Builder patterns.

20. What’s new in Java 21 (or latest LTS version in 2025)?

Conclusion:

Java interviews in 2025 focus on both foundational knowledge and modern Java features introduced in recent versions. Being well-prepared with these questions can significantly improve your chances of success. Practice coding, understand core concepts deeply, and stay updated with the latest Java advancements.