Constructors and this

Setting up an object straight away

Setting each field one by one is tiring. A constructor is a special method that runs automatically when you write new. It has the same name as the class and no return type:

class Student {
    String name;
    int marks;

    Student(String name, int marks) {
        this.name = name;
        this.marks = marks;
    }

    void show() {
        System.out.println(name + " scored " + marks);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student("Asha", 91);
        Student s2 = new Student("Ravi", 78);
        s1.show();
        s2.show();
    }
}

Output

Asha scored 91
Ravi scored 78

What does this mean?

Inside the constructor, the parameter name and the field name have the same name. this.name means “the field of this object”, and plain name means the parameter.

Think of it like this: If everyone in a room is called “Ravi”, saying “my bag” makes it clear whose bag you mean. this is the object saying “my”.

More than one constructor

Constructors can be overloaded too, so there are different ways to build an object:

class Box {
    int width;
    int height;

    Box(int width, int height) {
        this.width = width;
        this.height = height;
    }

    Box(int side) {
        this(side, side);
    }

    int area() {
        return width * height;
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(new Box(3, 4).area());
        System.out.println(new Box(5).area());
    }
}

Output

12
25

this(side, side) calls the other constructor, so we don't repeat code.

Remember: If you write no constructor, Java gives you an empty one for free. Once you write your own, the free one disappears.