Classes and Objects in Java - The Basics

1. What is a Class?

Think of a class as a blueprint or template. Just like a blueprint for a house tells what features the house will have, a class defines what properties and actions its objects will have.

Simple Class Example:

// This is a class named 'Car'
public class Car {
    // Properties (what a car has)
    String color;
    String brand;
    int year;

    // Actions (what a car can do)
    public void startEngine() {
        System.out.println("Car engine started!");
    }

    public void stopEngine() {
        System.out.println("Car engine stopped!");
    }
}

2. What is an Object?

An object is an actual instance created from a class. If a class is a blueprint for a house, then an object is the actual house built from that blueprint. You can create many objects from one class.

Creating and Using Objects:

public class Main {
    public static void main(String[] args) {
        // Creating objects of the Car class
        Car car1 = new Car();  // First car object
        Car car2 = new Car();  // Second car object

        // Setting properties for car1
        car1.color = "Red";
        car1.brand = "Toyota";
        car1.year = 2024;

        // Setting properties for car2
        car2.color = "Blue";
        car2.brand = "Honda";
        car2.year = 2023;

        // Using object methods
        car1.startEngine();  // Output: Car engine started!
        car2.stopEngine();   // Output: Car engine stopped!
    }
}

3. Real-World Example

Let's use something familiar - a Student class:

// Class definition
public class Student {
    // Properties
    String name;
    int age;
    
    // Actions
    public void study() {
        System.out.println(name + " is studying");
    }
    
    public void sleep() {
        System.out.println(name + " is sleeping");
    }
}

// Using the Student class
public class Main {
    public static void main(String[] args) {
        // Creating student objects
        Student student1 = new Student();
        Student student2 = new Student();
        
        // Setting values for student1
        student1.name = "John";
        student1.age = 15;
        
        // Setting values for student2
        student2.name = "Mary";
        student2.age = 16;
        
        // Using the objects
        student1.study();  // Output: John is studying
        student2.sleep();  // Output: Mary is sleeping
    }
}

4. Key Points to Remember:

1. Class:

   • Is a template or blueprint
   • Contains properties (variables) and methods (functions)
   • Starts with public class ClassName

2. Object:

   • Is an instance of a class
   • Created using new keyword
   • Can have its own values for properties
   • Can use all methods defined in the class

3. Creating Objects:

   ClassName objectName = new ClassName();

4. Using Objects:

   objectName.propertyName = value;    // Set property
   objectName.methodName();            // Call method