OOP

Constructor, this, static

1. Constructor

A constructor is a special method used to initialize an object.

There are some important rules that need to be followed when creating a constructor:

Rule	Description
Same name as class	Constructor name must match the class name
No return type	A constructor cannot have a return type, not even `void`
Called automatically	It is called automatically when an object is created
Used for initialization	Used to initialize the object's state
Can be overloaded	A class can have multiple constructors with different parameters

class Car {
    String brand;
    int speed;

    Car(String brand, int speed) {
        this.brand = brand;
        this.speed = speed;
    }
}

Car car = new Car("BMW", 120);

Here, Car(String brand, int speed) is a parameterized constructor that initializes the object's brand and speed.

2. this

The this keyword refers to the current object of a class.

There are several common situations where the this keyword is used:

Usage	Description
Refer to instance variable	Used when parameter and instance variable have the same name
Call current method	`this.drive()` calls the current object's method
Call current constructor	`this()` calls another constructor of the same class
Pass current object	`this` can be passed as an argument

class Car {
    String brand;
    int speed;

    Car(String brand, int speed) {
        this.brand = brand;
        this.speed = speed;
    }

    void drive() {
        System.out.println(brand + " is driving at " + speed + " km/h");
    }
}

Here, this.brand refers to the instance variable, while brand refers to the constructor parameter.

3. static

The static keyword is used to define members that belong to the class rather than individual objects.

There are some important points to remember about static members:

Point	Description
Belongs to class	Shared by all objects of the class
Single copy	Only one copy of a static variable exists per class
Access using class name	Can be accessed using `ClassName.member`
No object required	Static members can be accessed without creating an object
Commonly used for utility methods	Example: `Math.max()`

class Car {
    static int numberOfCars = 0;

    Car() {
        numberOfCars++;
    }
}

Car car1 = new Car();
Car car2 = new Car();

System.out.println(Car.numberOfCars); // 2

Here, numberOfCars belongs to the Car class, so it is shared by all Car objects.

Member	Belongs To
Instance variable	Object
Instance method	Object
Static variable	Class
Static method	Class

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