JAVA Tutorial

Classes & Objects in Java

In Java, classes and objects are fundamental concepts of Object-Oriented Programming (OOP). A class is a blueprint for creating objects (instances), while an object is an instance of a class. Let's break down these concepts in more detail.

What is a Class?

A class in Java is a template or blueprint for creating objects. It defines the data (attributes) and methods (functions) that can be applied to objects created from that class.

Syntax of a Class:

class ClassName {
    // Fields (attributes)
    type variableName;

    // Constructor
    public ClassName(type value) {
        this.variableName = value;
    }

    // Method
    public void methodName() {
        // Code to perform an action
    }
}
  

In the example above, `ClassName` is the name of the class. Inside it, we define the attributes (fields) and methods that define the behavior of the objects of this class.

What is an Object?

An object is a specific instance of a class. It is created from a class and has its own unique set of data. Each object can have different values for the same attributes defined in the class.

Creating Objects from a Class:

public class Main {
    public static void main(String[] args) {
        // Creating an object of the class Car
        Car myCar = new Car("Toyota", "Corolla", 2021);
        // Calling a method on the object
        myCar.displayDetails();
    }
}
  

Example of a Class & Object in Java

// Define a Car class
class Car {
    String brand;
    String model;
    int year;

    // Constructor to initialize the attributes
    public Car(String brand, String model, int year) {
        this.brand = brand;
        this.model = model;
        this.year = year;
    }

    // Method to display car details
    public void displayDetails() {
        System.out.println("Brand: " + brand);
        System.out.println("Model: " + model);
        System.out.println("Year: " + year);
    }
}

// Main class to test Car object
public class Main {
    public static void main(String[] args) {
        // Creating an object of the Car class
        Car myCar = new Car("Toyota", "Corolla", 2021);
        
        // Calling the displayDetails method on the object
        myCar.displayDetails();
    }
}
  

In this example, the `Car` class is used to create an object `myCar` that represents a specific car. The `displayDetails()` method is used to print out the attributes of `myCar`.

Key Points to Remember

• Class: A blueprint for creating objects with predefined attributes and behaviors.
• Object: An instance of a class, representing a real-world entity with its own state and behavior.
• Constructor: A special method used to initialize the object's attributes when it is created.
• Methods: Functions defined inside a class that define the actions or behaviors of objects.
• Encapsulation: Classes allow you to group related data and methods together, providing better structure and organization.

Advantages of Using Classes and Objects

• Modularity: Classes help in breaking down complex programs into smaller, manageable sections.
• Code Reusability: Once a class is created, it can be used to create multiple objects with different attributes.
• Maintainability: Changes made to a class will automatically be reflected in all objects created from that class.
• Abstraction: Classes help to hide implementation details and expose only the necessary parts of an object.