Class & Object

Class and Object are the foundation of Object-Oriented Programming (OOP) in Java. Every Java program is built using classes and objects, making this a must-master topic for interviews and real-world development.

What Is a Class?

A class is a blueprint or template used to create objects. It defines:

• Properties (variables / fields)
• Behaviors (methods)

class Car {
    String color;
    int speed;

    void drive() {
        System.out.println("Car is driving");
    }
}
          

👉 A class does not occupy memory until an object is created.

What Is an Object?

An object is a real instance of a class.

• Represents real-world entities
• Occupies memory
• Can access class members

Car c = new Car();
          

Here:

• Car → class
• c → reference variable
• new Car() → object creation

Real-World Analogy

Structure of a Class in Java

class ClassName {
    // variables
    dataType variableName;

    // methods
    returnType methodName() {
        // logic
    }
}
          

Example: Class & Object Program

class Student {
    String name;
    int age;

    void display() {
        System.out.println(name + " " + age);
    }

    public static void main(String[] args) {
        Student s1 = new Student();
        s1.name = "John";
        s1.age = 20;
        s1.display();
    }
}
          

Object Creation Process (Behind the Scenes)

Student s1 = new Student();
          

Steps:

1. JVM loads Student class
2. Memory allocated in heap
3. Instance variables initialized with default values
4. Constructor is called
5. Reference s1 points to the object

Class Variables vs Instance Variables

class Test {
    static int x = 10;  // class variable
    int y = 20;         // instance variable
}
          

Multiple Objects of a Class

Student s1 = new Student();
Student s2 = new Student();
          

• Each object has its own copy of instance variables
• Methods are shared

Accessing Class Members

Using Object

s1.display();
          

Using Class Name (static members)

Test.x;
          

Object Reference Behavior

Student s1 = new Student();
Student s2 = s1;
          

• Both references point to the same object
• Changes via one reference affect the other

new Keyword Explained

• Allocates memory
• Creates object
• Calls constructor
• Returns object reference

Anonymous Object

Object without reference variable.

new Student().display();
          

✔ Used for one-time use

Why Java Is Object-Oriented (Key Reason)

• Code reusability
• Encapsulation
• Abstraction
• Inheritance
• Polymorphism

Common Beginner Mistakes

• Confusing class with object
• Accessing non-static members using class name
• Forgetting new keyword
• Assuming class occupies memory
• Creating unnecessary objects

Interview-Ready Answers

Short Answer

A class is a blueprint that defines properties and behaviors, while an object is an instance of a class.

Detailed Answer

In Java, a class defines the structure and behavior of objects using variables and methods. An object is a runtime instance of a class that occupies memory and can access class members. Multiple objects can be created from a single class.

Key Takeaway

Class defines “what” an object is.

Object represents “how” it exists in memory.

Mastering class and object concepts is mandatory before learning constructors, inheritance, and polymorphism.