Classes and Objects in Java

What are classes and objects

Java is object-oriented, so everything is built from classes and objects. A class is a blueprint that describes data and behaviour. An object is a real thing made from that blueprint. "Car" is a class; your specific car is an object.

Defining a class

class Car {
    String color;       // field (data)
    int speed;          // field (data)

    void drive() {      // method (action)
        System.out.println(color + " car driving");
    }
}

A class groups related data (fields like color) and actions (methods like drive). This blueprint does nothing on its own until we create an object from it.

Creating an object

Car c = new Car();      // create an object
c.color = "Red";        // set its data
c.speed = 80;
c.drive();              // call its method

The new keyword builds an object from the class. c holds a reference to it, and we use the dot (.) to set fields and call methods.

Fields and methods

• Fields (instance variables) store the object's data — color, speed.
• Methods are what the object can do — drive().
• You reach both with the dot operator: c.color, c.drive().

Together, fields and methods describe what an object knows and can do.

Many objects, own data

Car a = new Car();  a.color = "Red";
Car b = new Car();  b.color = "Blue";
a.drive();          // Red car driving
b.drive();          // Blue car driving

Each object made from the class has its own fields. a and b are separate cars with separate colours — changing one does not affect the other.

Methods that use fields

class Student {
    String name;
    int marks;
    void result() {
        String r = (marks >= 40) ? "Pass" : "Fail";
        System.out.println(name + ": " + r);
    }
}
// use:
Student s = new Student();
s.name = "Aman"; s.marks = 75;
s.result();

A method can read the object's own fields (via this implicitly). Here result() uses name and marks to decide pass or fail for that student.

Common mistakes

• Forgetting new. A class is only a blueprint; you need new to make an object.
• Using fields without an object. Non-static fields belong to an object, reached via the object.
• File vs class name. A public class must match the file name.
• Confusing class and object. The class is the design; the object is the actual thing.

Summary

• A class is a blueprint of fields (data) and methods (actions).
• An object is a real instance made with new.
• Access fields and methods with the dot operator.
• Each object keeps its own copy of the fields.
• Classes and objects are the foundation of OOP in Java.

Next, set up objects with Java constructors, or reuse code with inheritance.