What is a Class in Java?

A class in Java is a blueprint for creating objects. It defines the properties (fields/attributes) and behaviors (methods) that the objects created from the class can have.

The components of a class are:

• Fields: Variables that store the state or attributes of an object.

• Methods: Functions inside a class that define its behavior.

• Constructor: A special method used to initialize objects.

Defining a Class

public class Car {
    // Fields (attributes)
    String brand;
    String color;
    int year;

    // Constructor is the method that runs when the class is instantiated
    public Car(String brand, String color, int year) {
        this.brand = brand;
        this.color = color;
        this.year = year;
    }

    // Method (behavior)
    public void displayDetails() {
        System.out.println("Brand: " + brand + ", Color: " + color + ", Year: " + year);
    }

}

Creating an Object

To use a class, create an instance (object) of it.

public class Main {
    public static void main(String[] args) {
        // Create an object of the Car class
        Car myCar = new Car("Tesla", "Red", 2023);

        // Access the object's methods
        myCar.displayDetails();  // Output: Brand: Tesla, Color: Red, Year: 2023
    }
}

Access Modifiers

Access modifiers control the visibility of fields and methods:

• public – Accessible from anywhere.

• private – Accessible only within the class.

• protected – Accessible within the package and by subclasses.

• (default) – Accessible within the package.

public class Person {
    private String name;  // Private field

    public String getName() {
        return name;  // Public method to access private field
    }

    public void setName(String name) {
        this.name = name;
    }
}

Static Fields and Methods

Static members belong to the class rather than any specific object.

public class MathUtils {
    public static int add(int a, int b) {
        return a + b;
    }
}

Usage:

int sum = MathUtils.add(5, 10);  // No need to create an object
System.out.println(sum);  // Output: 15

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Inheritance

Java allows one class to inherit from another.

class Vehicle {
    public void start() {
        System.out.println("Vehicle is starting");
    }
}

class Car extends Vehicle {
    public void honk() {
        System.out.println("Car is honking");
    }
}

Usage:

Car myCar = new Car();

myCar.start();  // Inherited from Vehicle
myCar.honk();   // Defined in Car

Encapsulation

Encapsulation restricts direct access to fields by using getters and setters.

public class BankAccount {
    private double balance;

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }
}

Polymorphism

Polymorphism allows methods to behave differently based on the object calling them.

class Animal {
    public void makeSound() {
        System.out.println("Animal makes a sound");
    }
}

class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal myAnimal = new Dog();
        myAnimal.makeSound();  // Output: Dog barks
    }
}

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Abstract Classes and Interfaces

Abstract Class

abstract class Shape {
    abstract void draw();
}

class Circle extends Shape {
    void draw() {
        System.out.println("Drawing a circle");
    }
}

Interface

interface Animal {
    void makeSound();
}

class Cat implements Animal {
    public void makeSound() {
        System.out.println("Cat meows");
    }
}

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This covers the fundamentals of Java classes. Try these examples and explore more concepts like this keyword, method overloading, and more advanced topics as you gain experience!