Classes and Objects in Java - OOP Fundamentals

Introduction

Classes and objects are the core concepts of Object-Oriented Programming (OOP) in Java. A class is a blueprint, and an object is an instance of that blueprint.

Defining a Class

public class Student {
    // Fields (attributes)
    String name;
    int age;
    String grade;
    
    // Method (behavior)
    void displayInfo() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Grade: " + grade);
    }
    
    void study(String subject) {
        System.out.println(name + " is studying " + subject);
    }
}

Creating Objects

public class Main {
    public static void main(String[] args) {
        // Create objects
        Student student1 = new Student();
        Student student2 = new Student();
        
        // Set field values
        student1.name = "Rahul";
        student1.age = 20;
        student1.grade = "A";
        
        student2.name = "Priya";
        student2.age = 19;
        student2.grade = "A+";
        
        // Call methods
        student1.displayInfo();
        student1.study("Java Programming");
        
        System.out.println();
        
        student2.displayInfo();
        student2.study("Data Structures");
    }
}

Class with Constructor

public class Car {
    String brand;
    String model;
    int year;
    
    // Constructor
    Car(String brand, String model, int year) {
        this.brand = brand;
        this.model = model;
        this.year = year;
    }
    
    void displayDetails() {
        System.out.println(year + " " + brand + " " + model);
    }
}
public class Main {
    public static void main(String[] args) {
        Car car1 = new Car("Toyota", "Camry", 2023);
        Car car2 = new Car("Honda", "Civic", 2022);
        
        car1.displayDetails();
        car2.displayDetails();
    }
}

Summary

• Classes are blueprints for objects
• Objects are instances of classes
• Fields store object state
• Methods define object behavior
• Constructors initialize objects