Top 25 Interview Questions on Java Language for 2025
Java continues to be one of the most popular programming languages, especially for backend development, Android applications, and large-scale systems. As we approach 2025, companies are on the lookout for skilled Java developers who can handle the complexities of modern applications. To help you prepare, we’ve compiled the Top 25 Interview Questions on Java Language for 2025, along with detailed answers that will help you ace your Java interview.
1. What is Java?
Answer: Java is a high-level, object-oriented programming language developed by Sun Microsystems (now owned by Oracle). It follows the Write Once, Run Anywhere (WORA) principle, meaning that Java applications can run on any device that has a Java Virtual Machine (JVM).
2. What are the main features of Java?
Answer: Some key features of Java include:
- Object-Oriented: Everything in Java is treated as an object.
- Platform-Independent: Java is designed to be platform-independent, thanks to the JVM.
- Multithreaded: Java supports multithreading to perform multiple tasks simultaneously.
- Automatic Garbage Collection: Java automatically manages memory through garbage collection.
- Rich API: Java provides a wide array of built-in libraries and APIs.
3. What is the difference between JDK, JRE, and JVM?
Answer:
- JVM (Java Virtual Machine): It is a virtual machine that runs Java bytecode and provides platform independence.
- JRE (Java Runtime Environment): It includes the JVM and the libraries required to run Java applications.
- JDK (Java Development Kit): It includes the JRE, tools for Java development like the compiler (
javac), and other utilities.
4. What is the difference between == and equals() in Java?
Answer:
==: Compares the memory addresses (references) of two objects, not their content.equals(): Compares the actual contents of two objects, typically overridden in classes likeString.
5. What are the access modifiers in Java?
Answer: Java has four access modifiers:
public: The member is accessible from anywhere.private: The member is accessible only within the same class.protected: The member is accessible within the same package and subclasses.default(no modifier): The member is accessible only within the same package.
6. What is an interface in Java?
Answer: An interface is a reference type in Java that can contain constants, method signatures, default methods, and static methods. Interfaces define a contract that the implementing class must follow. A class can implement multiple interfaces.
7. What is the difference between an abstract class and an interface?
Answer:
- Abstract class: Can have both abstract and concrete methods, and a class can inherit from only one abstract class.
- Interface: Can only have abstract methods (until Java 8, which introduced default methods), and a class can implement multiple interfaces.
8. What is polymorphism in Java?
Answer: Polymorphism refers to the ability of one object to take many forms. It allows methods to behave differently based on the object they are called on. In Java, it’s achieved through method overloading (compile-time polymorphism) and method overriding (runtime polymorphism).
9. What is the difference between method overloading and method overriding?
Answer:
- Method Overloading: Defining multiple methods with the same name but different parameters in the same class.
- Method Overriding: Providing a specific implementation of a method in a subclass that is already defined in the parent class.
10. What are constructors in Java?
Answer: A constructor is a special method used to initialize objects. It is called when an object of a class is created. Constructors can be default (with no parameters) or parameterized (with parameters).
11. What is the use of the final keyword in Java?
Answer: The final keyword can be used in three contexts:
- Final variable: The value of the variable cannot be changed after initialization.
- Final method: The method cannot be overridden in subclasses.
- Final class: The class cannot be subclassed.
12. What is a thread in Java?
Answer: A thread in Java is a lightweight process that enables multitasking. Threads allow a program to perform multiple operations concurrently, and Java supports thread creation through Thread class or Runnable interface.
13. What is synchronization in Java?
Answer: Synchronization is a mechanism in Java to ensure that only one thread can access a resource at a time. It is typically used to prevent data inconsistency when multiple threads are working with the same resource.
14. What is the difference between ArrayList and LinkedList in Java?
Answer:
- ArrayList: Implements a dynamic array, provides fast access by index, but slower insertions and deletions.
- LinkedList: Implements a doubly linked list, providing faster insertions and deletions but slower access by index.
15. What is a Java package?
Answer: A package in Java is a namespace that organizes classes and interfaces. It helps avoid naming conflicts and makes code modular. Java has built-in packages like java.util and java.io.
16. What is the difference between String, StringBuilder, and StringBuffer in Java?
Answer:
- String: Immutable, meaning once created, its value cannot be changed.
- StringBuilder: Mutable, but not synchronized, making it faster for single-threaded applications.
- StringBuffer: Mutable and synchronized, making it suitable for multi-threaded applications.
17. What is the purpose of the transient keyword in Java?
Answer: The transient keyword is used to indicate that a field should not be serialized. It is used in Java serialization to prevent certain data from being saved.
18. What is the super keyword in Java?
Answer: The super keyword is used to refer to the immediate parent class of the current object. It is used to access parent class methods, constructors, or fields.
19. What is exception handling in Java?
Answer: Exception handling in Java is a mechanism to handle runtime errors. It uses try, catch, throw, throws, and finally blocks. This ensures that the program doesn’t crash and handles errors gracefully.
20. What is the difference between throw and throws in Java?
Answer:
throw: Used to throw an exception explicitly from within a method or block.throws: Declares the exceptions that a method might throw, allowing the caller to handle them.
21. What is the this keyword in Java?
Answer: The this keyword refers to the current instance of a class. It is used to differentiate between instance variables and parameters with the same name and to invoke the current object’s methods.
22. What is Java’s garbage collection?
Answer: Garbage collection in Java is the process of automatically reclaiming memory by deleting unused objects. It helps manage memory efficiently and reduces memory leaks.
23. What is the difference between public, protected, and private in Java?
Answer:
public: Accessible from any class.protected: Accessible within the same package and subclasses.private: Accessible only within the same class.
24. What is the clone() method in Java?
Answer: The clone() method is used to create a copy of an object. It belongs to the Object class and requires that the class implements the Cloneable interface.
25. What is an Enum in Java?
Answer: An Enum is a special class in Java that represents a group of constants (unchangeable variables). Enums are used to define fixed sets of constants, such as days of the week or months of the year.
Conclusion
Java is a versatile and powerful language that remains a top choice for developers worldwide. By understanding these Top 25 Interview Questions on Java Language for 2025, you’ll be well-prepared for your next job interview. Practice these concepts, understand the core principles of Java, and stay updated with new advancements in Java to boost your career as a Java developer.