Top Core Java Interview Questions You Must Know in 2025

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1. What is Java?

Java is a high-level programming language created by James Gosling in 1991. It enables developers to create software that runs across various platforms and devices, including websites and mobile applications.

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2. What are the Features of Java?                                                                      Imp Questions for Fresher

     

Simple: Java is easy to learn and use. It’s based on C++, but without the confusing parts like pointers.                  

Object-Oriented: Everything in Java is organized as objects and classes, which makes coding cleaner and easier to manage.

Platform Independent: You can write Java code on one computer, and it will run on any other computer that has Java installed. This is known as “Write Once, Run Anywhere.”

Secure: Java has built-in security features that help protect your computer and data from harm.

Robust: Java is reliable. It takes care of memory and handles errors well, which makes programs less likely to crash.

Multithreading: Java can handle multiple tasks at once, which helps make programs run faster, especially for things  like games and web servers.

Portable: Java programs can run on different types of computers without needing to be changed. The same program can work on Windows, Mac, or Linux.

High Performance: While Java is not as fast as some other languages, it does a good job of speeding things up while it runs.

Distributed: Java is great for networked environments. It helps programs talk to other computers over the internet or a local network.

Dynamic: Java can adapt while it’s running, like loading new classes or changing behavior, without restarting the program.

Architecture Neutral: Java is designed to work on any computer system, no matter what kind of hardware is used.

   

3. What is OOP(Object-Oriented Programming)?                                                                        Imp Questions for Fresher 

OOP stands for Object-Oriented Programming. It focuses on creating programs using classes and objects. Java OOP is a way of writing programs using objects. Here are the 6 main concepts: 

Class: A class is like a blueprint for creating objects. It defines what the object will look like and what it can do (its properties and actions).

Object: An object is an instance of a class. Think of it as something you create using the class blueprint. For example, if you have a Car class, you can create different car objects like myCar or yourCar.

Inheritance: Inheritance allows one class to inherit properties and behaviors from another class. This helps in reusing code. For example, a Bike class can inherit from a general Vehicle class, so you don’t have to write common code again.

Polymorphism: Polymorphism means the ability to take many forms. In Java, you can use the same function name to do different things, depending on the object calling it.

Encapsulation: Encapsulation is about hiding the internal details of an object and only exposing the necessary parts. You protect data by using private variables and provide access through public methods.

Abstraction: Abstraction means focusing on the important things and ignoring the unnecessary details. In Java, you can use abstract classes or interfaces to define the important behaviors without worrying about how they are implemented.

    

4. What is a Constructor?

A constructor is a special method used to initialize objects in Java. It shares the class name, has no return type, and cannot use keywords like abstract, static, or final.

Types of Constructors:

• Default Constructor: Parameter-less constructor that initializes default values.
• Parameterized Constructor: Accepts parameters to initialize objects with specific values.

Constructor Chaining:

This refers to one constructor calling another. It can happen within the same class using the this keyword or between different classes using the super keyword.

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5. What is an Abstract Class?
An abstract class in Java is a class that contains abstract methods (methods without a body) as well as concrete methods. It provides partial abstraction and is used to define common behavior for subclasses.

Achieving Abstraction:

• Abstract Class: Offers 0-100% abstraction.
• Interface: Offers 100% abstraction.

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6. What is Association and Composition?

Association refers to relationships between two or more objects. It comes in two types:

• Aggregation: A weak relationship where objects can exist independently (e.g., Student and College).

       Program Example:

         class Student {

int id;

String name;

String school_name;

//create constructor

System.out.println(“\nStudent name is “ + name);

System.out.println(“Student Id is “ + id);

System.out.println(“Student belongs to the “ + school_name + “School”);

}}

class school {

String schoolName;

int noOfStudents;

// create constructor

}

public class AggregationClass

public static void main(String[] args) {

Student n1 = new Student(1, “Shekher”, “Galgotia”);

Student n2 = new Student(2, “Trilok”, “IEC”);

Student n3 = new Student(3, “Aryan”, “ABC”);

}}

• Composition: A strong relationship where one object cannot exist without the other (e.g., Department and College).

Benefits of Association:

• Code reusability
• Cost-effectiveness
• Reducing redundancy

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7. What is JDK?                                                                                              Very Imp Questions for Fresher 

The JDK (Java Development Kit) is a software development kit used to develop Java applications. It includes tools like the Java compiler, Java Runtime Environment (JRE), and APIs needed to create and run Java programs.