Top 25 Java Interview Questions (2025 Edition)

When preparing for a Java developer position, it’s crucial to brush up on both theoretical and practical aspects of the language. Below are the top 25 Java interview Questions frequently asked in interviews, along with detailed answers to help you stand out from the competition.

Java Interview Questions

1. What are the main features of Java?

Java is a high-level, object-oriented, platform-independent programming language known for its security and performance.

Key features:

• Object-Oriented: Everything is an object; supports concepts like inheritance and polymorphism.
• Platform Independent: Write Once, Run Anywhere (WORA) through the Java Virtual Machine (JVM).
• Robust and Secure: Exception handling and garbage collection make Java stable and secure.
• Multithreaded: Supports multithreading for high-performance applications.
• Architecture-neutral: Java bytecode runs consistently on any platform.

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

JDK (Java Development Kit):

• Contains tools for developing Java applications.
• Includes JRE and development tools like compiler (javac).

JRE (Java Runtime Environment):

• Provides libraries and JVM required to run Java programs.
• Does not include development tools.

JVM (Java Virtual Machine):

• Converts bytecode into machine code.
• Ensures platform independence by executing Java bytecode on any system.

3. Explain OOP principles in Java.

Java is based on four fundamental Object-Oriented Programming (OOP) concepts:

• Encapsulation: Wrapping data and code into a single unit (class). Provides data hiding.
• Inheritance: Enables a new class to inherit features from an existing class.
• Polymorphism: Allows one interface to be used for a general class of actions.
• Abstraction: Hides implementation details and shows only the essential features.

These principles make Java scalable and maintainable.

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

• == compares object references (memory location).
• .equals() compares object values (actual data).

Example:

String a = new String("test");
String b = new String("test");
System.out.println(a == b);       // false
System.out.println(a.equals(b));  // true

Always use .equals() to compare values in Java.

5. What is a constructor in Java?

A constructor initializes a new object.

Types of constructors:

• Default Constructor: No parameters; created by the compiler if not explicitly defined.
• Parameterized Constructor: Accepts parameters to set initial values.

Characteristics:

• Same name as the class.
• No return type.
• Can be overloaded.

6. Difference between ArrayList and LinkedList?

Both are part of the java.util package, but serve different purposes.

Use ArrayList when you need fast access and LinkedList when frequent insertion/deletion is needed.

7. What is method overloading and overriding?

• Overloading: Multiple methods with the same name but different parameters in the same class.
• Overriding: Subclass provides a specific implementation of a method defined in its superclass.

Benefits:

• Improves code readability.
• Enhances polymorphism.

8. What is the final keyword in Java?

Used to declare constants, prevent method overriding or inheritance.

Uses:

• final variable: Constant, cannot be reassigned.
• final method: Cannot be overridden.
• final class: Cannot be subclassed (e.g., java.lang.String).

9. What are checked and unchecked exceptions?

• Checked Exceptions: Compile-time exceptions (e.g., IOException).
• Unchecked Exceptions: Runtime exceptions (e.g., NullPointerException).

Always handle checked exceptions using try-catch or throws clause.

10. What is multithreading in Java?

Multithreading allows concurrent execution of two or more threads.

Benefits:

• Efficient CPU usage.
• Improved performance for tasks like file I/O or animations.

Use Thread class or Runnable interface to implement.

Java Interview Questions

11. What is the difference between abstract class and interface?

Java 8 and above allows default and static methods in interfaces.

12. What is garbage collection in Java?

Java uses automatic memory management.

Garbage Collector:

• Reclaims memory used by objects no longer referenced.
• Runs in the background.

Use System.gc() to suggest JVM to start GC.

13. Explain the Java memory model.

Java memory is divided into:

• Heap: Stores objects.
• Stack: Stores method calls and local variables.
• Method Area: Stores class structures like metadata.
• PC Register: Current instruction being executed.
• Native Method Stack: For native methods.

14. What is the use of the ‘this’ keyword?

this refers to the current object.

Use cases:

• Differentiate between instance variables and parameters.
• Invoke current class methods or constructors.

15. What is the difference between static and non-static methods?

• Static Method: Belongs to the class, not instances.
• Non-static Method: Requires object creation to access.

Use static when the behavior doesn’t depend on object state.

16. What are Java access modifiers?

Control visibility of classes, methods, and variables.

17. What is the Singleton design pattern in Java?

Restricts the instantiation of a class to one object.

Implementation:

• Private constructor.
• Static method to get instance.
• Static instance variable.

Used in logging, configuration, and driver classes.

18. What is the difference between HashMap and Hashtable?

Use HashMap in single-threaded, Hashtable in multi-threaded environments.

19. What is the use of the ‘super’ keyword?

super refers to the parent class.

Use cases:

• Access parent class variables.
• Invoke parent class methods or constructors.

20. What are immutable objects in Java?

Immutable objects cannot be changed after creation.

Example: String class
To create custom immutable classes:

• Make class final.
• Make fields private and final.
• No setters; only getters.

21. What is the difference between == and equals() for strings?

• == compares references.
• .equals() compares values.

Always use .equals() for string comparison.

22. What is the purpose of the Java Collections Framework?

Provides classes and interfaces to store and manipulate groups of data.

Core Interfaces:

• List
• Set
• Map
• Queue

Benefits:

• Reduces programming effort.
• Increases performance and code quality.

23. How does exception handling work in Java?

Uses five keywords:

• try: Code that may throw exception.
• catch: Handles the exception.
• finally: Executes regardless of exception.
• throw: Manually throw exception.
• throws: Declare exceptions in method signature.

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

25. What is autoboxing and unboxing in Java?

Autoboxing: Converting primitive types to wrapper classes.
Unboxing: Converting wrapper classes to primitive types.

Example:

Integer i = 10; // autoboxing
int j = i;      // unboxing

Conclusion

Mastering these top Java Interview Questions will give you a significant edge in job interviews. Whether you’re a fresher or an experienced developer, understanding these concepts thoroughly can make a big difference. Don’t just memorize—practice them in real-world scenarios. Download Java.

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