Java OOP

Java Classes

A class is a blueprint or template for creating objects. It defines the structure (attributes) and behavior (methods) that an object will have.

public class Car {
    // Fields (attributes or properties)
    String color;
    int speed;

    // Methods (behavior)
    void drive() {
        System.out.println("The car is driving.");
    }
}

In the above example:

• Car is a class with attributes color and speed.
• It also has a method, drive(), defining behavior.

Objects and Object Creation

An object is an instance of a class. When you create an object, you are essentially creating a variable of a particular class type that holds data and can perform actions defined within that class.

Car myCar = new Car();

Here:

• myCar is an object of the class Car.
• The new keyword is used to allocate memory for the object and invoke its constructor.

Constructors

A constructor is a special method that initializes a new object. Constructors have the same name as the class and no return type. They are called automatically when an object is created. If you don’t define a constructor, Java provides a default constructor with no parameters.

public class Car {
    String color;
    int speed;

    // Constructor
    public Car(String color, int speed) {
        this.color = color;
        this.speed = speed;
    }
}

Usage:

Car myCar = new Car("Red", 150);

In this example:

• The constructor sets initial values for color and speed when a Car object is created.

Access Modifiers: Public and Private

Access modifiers control the visibility of classes, methods, and variables. Java provides several modifiers, but the two most commonly used are public and private.

• public: Accessible from any other class.
• private: Only accessible within its own class.

public class Car {
    private String color; // Accessible only within the Car class
    public int speed; // Accessible from any class

    public String getColor() {
        return color; // Accessed through a public method
    }

    public void setColor(String color) {
        this.color = color;
    }
}

In the example:

• The color field is private, so it cannot be accessed directly from outside Car.
• Instead, it can be accessed or modified using getters and setters.

Getters and Setters

Getters and setters are methods that provide controlled access to private fields. This is essential for encapsulation, allowing you to protect and control changes to data.

public class Car {
    private String color;
    private int speed;

    // Getter for color
    public String getColor() {
        return color;
    }

    // Setter for color
    public void setColor(String color) {
        this.color = color;
    }
}

Usage:

Car myCar = new Car();
myCar.setColor("Blue");
System.out.println(myCar.getColor()); // Outputs: Blue

In this example:

• setColor sets the color of myCar.
• getColor retrieves the color of myCar.

The Static Keyword

The static keyword makes a field, method, or class variable belong to the class itself, rather than to instances of the class. Static members are shared across all instances of the class, allowing for values or functions common to every object.

public class Car {
    private static int carCount = 0;

    public Car() {
        carCount++; // Increment the static car count for every new Car created
    }

    // Static method
    public static int getCarCount() {
        return carCount;
    }
}

Usage:

Car car1 = new Car();
Car car2 = new Car();
System.out.println(Car.getCarCount()); // Outputs: 2

In this example:

• carCount is a static field, meaning it is shared by all instances.
• getCarCount() is a static method that retrieves the shared carCount across all instances.

OOP

Object-Oriented Programming (OOP) in Java is a programming paradigm where everything is modeled as objects. The four key principles of OOP are:

1. Encapsulation
2. Inheritance
3. Polymorphism
4. Abstraction

class Car {
    private String model;
    private int year;

    // Getter and Setter methods
    public String getModel() {
        return model;
    }

    public void setModel(String model) {
        this.model = model;
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }
}

        

class Vehicle {
    protected String brand = "Toyota";

    public void honk() {
        System.out.println("Beep beep!");
    }
}

class Car extends Vehicle {
    private String model;

    public Car(String model) {
        this.model = model;
    }

    public void displayInfo() {
        System.out.println(brand + " " + model);
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car("Corolla");
        myCar.honk(); // Output: Beep beep!
        myCar.displayInfo(); // Output: Toyota Corolla
    }
}

        

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

    public int add(int a, int b, int c) {
        return a + b + c;
    }
}

public class Main {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        System.out.println(calc.add(10, 20));   // Output: 30
        System.out.println(calc.add(10, 20, 30)); // Output: 60
    }
}

        

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

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

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

        

abstract class Animal {
    public abstract void sound(); // Abstract method
    
    public void sleep() {
        System.out.println("Sleeping...");
    }
}

class Cat extends Animal {
    @Override
    public void sound() {
        System.out.println("Cat meows");
    }
}

public class Main {
    public static void main(String[] args) {
        Cat myCat = new Cat();
        myCat.sound();  // Output: Cat meows
        myCat.sleep();  // Output: Sleeping...
    }
}