As we step into 2025, Java continues to hold its ground as one of the most powerful and versatile programming languages in the tech industry. From powering enterprise-level applications to forming the backbone of Android development, Java remains a top choice for developers and organizations alike.
Whether you're a recent computer science graduate aiming to kickstart your career or a seasoned software engineer looking to switch roles or scale up, preparing thoroughly for Java interviews is crucial. Employers today are not just looking for candidates who can write code — they’re seeking professionals who possess a strong grasp of Java’s core principles, understand its evolving features, and can apply those concepts to solve real-world problems efficiently.
In this comprehensive article, we’ve curated the top 20 Java interview questions you need to master in 2025. These questions cover a broad range — from fundamental OOP concepts and Java syntax to more advanced topics like memory management, multithreading, new Java updates, and best practices for clean, maintainable code.
Whether you're preparing for product-based companies, startups, or service firms, this list will help you boost your confidence and sharpen your knowledge to ace your next Java interview.
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.
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)?
Java 21 includes features like record patterns, virtual threads, structured concurrency, and pattern matching enhancements—making code cleaner and more efficient.
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.
