Top 23 Most Asked Java Interview Questions in 2025

Java continues to be a dominant programming language in software development, widely used in enterprise applications, backend systems, and Android development. If you’re preparing for a software engineer or full-stack developer interview, mastering Java is crucial. Even in top multinational corporations (MNCs), interviewers frequently test candidates on Data Structures and Algorithms (DSA) using Java or C++.

To help you succeed, I’ve compiled 23 commonly asked Java interview questions, covering fundamental and advanced concepts. Let’s dive in.

Basic Java Interview Questions

1. What is Java, and why is it not a pure object-oriented language?

Java is a class-based, object-oriented programming language known for its simplicity and portability. The Java Virtual Machine (JVM) enables the “write once, run anywhere” capability. However, Java is not purely object-oriented because:

• It includes primitive data types (e.g., int, char, boolean), which are not objects.
• It allows static methods and variables, which can be used without creating an object.

2. Key Components Required to Run a Java Program

Running a Java program involves three essential components:

• JDK (Java Development Kit): Includes the compiler, debugger, and libraries needed for development.
• JRE (Java Runtime Environment): Provides libraries and resources to run Java applications.
• JVM (Java Virtual Machine): Converts Java bytecode into machine code, making Java platform-independent.

3. What are the Main Features of Java?

Java’s popularity is due to its key features:

• Simple & Easy to Learn: Java’s syntax is beginner-friendly compared to C++.
• Secure: Built-in security mechanisms protect against vulnerabilities.
• Platform-Independent: JVM allows Java programs to run on any OS.
• Object-Oriented: Java follows OOP principles for modular and reusable code.
• Robust: Exception handling and garbage collection ensure reliability.
• Multithreaded: Java can handle multiple tasks simultaneously.

4. What is the Java String Pool?

The String Pool is a special memory area inside the heap where Java stores string literals. If a string already exists in the pool, a reference is returned instead of creating a new object, optimizing memory usage.

5. What is a Wrapper Class?

Wrapper classes (e.g., Integer, Double, Character) allow primitive data types to be used as objects. They are essential for collections like ArrayList, which only store objects. Java also supports autoboxing (automatic conversion from primitive to wrapper) and unboxing (wrapper to primitive conversion).

6. Scenario-Based Question on Collections

Imagine managing an online bookstore:

• ArrayList stores the catalog of books (dynamic list).
• HashSet maintains unique genres (prevents duplicates).
• HashMap maps authors to books (quick lookup).

7. Difference Between this and super Keywords

• this: Refers to the current class instance. Used to differentiate instance variables from parameters.
• super: Refers to the parent class instance. Used to access parent class methods and constructors.

8. Difference Between Static and Instance Methods

• Static Methods: Belong to the class. Called using the class name.
• Instance Methods: Belong to an object. Called using an object reference.

Advanced Java Interview Questions

9. What Are Constructors, and What Are Their Types?

Constructors initialize objects in Java. Types:

• Default Constructor: No parameters; initializes default values.
• Parameterized Constructor: Accepts arguments for custom initialization.

10. Difference Between StringBuffer and StringBuilder

Both support mutable strings, but:

• StringBuffer is synchronized (thread-safe, but slower).
• StringBuilder is non-synchronized (faster, but not thread-safe).

11. Difference Between Abstract Classes and Interfaces

• Abstract Classes: Can contain abstract and concrete methods. Supports single inheritance.
• Interfaces: Contains only abstract methods (pre-Java 8). Supports multiple inheritance.

12. Can We Overload the main() Method in Java?

Yes, the main() method can be overloaded, but the JVM will only call the standard public static void main(String[] args) as the entry point.

13. What is Method Overriding?

Overriding allows a subclass to redefine a superclass method. The method signature must match exactly. Static and private methods cannot be overridden.

14. How Does Exception Handling Work in Java?

Java handles runtime errors using:

• try block: Code that may throw an exception.
• catch block: Handles specific exceptions.
• finally block: Always executes (cleanup operations).

15. What Are the Lifecycle Stages of a Thread?

A thread goes through:

• New → Runnable → Running → Blocked/Waiting → Terminated

16. What is a Singleton Class?

A Singleton ensures only one instance exists throughout the program. Achieved using a private constructor and a static method.

17. Aggregation vs. Composition

• Aggregation: Objects can exist independently (e.g., a car has an engine).
• Composition: Objects are dependent (e.g., a human has a heart).

18. What is an Anonymous Inner Class?

An anonymous inner class is a class without a name, declared and instantiated at the same time, usually for short-lived implementations of interfaces.

19. Implicit vs. Explicit Type Conversion

• Implicit (Widening): Automatic conversion (int → double).
• Explicit (Narrowing): Manual casting (double → (int)).

20. Purpose of the volatile Keyword

volatile ensures visibility of variable changes across threads, preventing caching optimizations.

21. Difference Between System.out, System.err, and System.in

• System.out: Standard output stream (normal messages).
• System.err: Error output stream (error messages).
• System.in: Standard input stream (user input).

22. Access Specifiers in Java

Java provides different levels of access control:

• public: Accessible everywhere.
• private: Accessible only within the same class.
• protected: Accessible in the same package and subclasses.
• Default (package-private): Accessible within the same package.

23. Difference Between final, finally, and finalize

• final: Used to declare constants, prevent method overriding, or prevent inheritance.
• finally: A block that always executes, usually for cleanup.
• finalize(): A method called by the garbage collector before destroying an object (rarely used).

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Mastering these Java interview questions will give you an edge in your next software development job interview. Practice coding, understand the concepts deeply, and stay updated with new Java features.

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