Constructors

Defining constructors

A constructor has the class name and no return type. If you write none, Java supplies a no-arg default. this(...) calls another constructor in the same class.

public class Point {
    private final int x, y;

    public Point(int x, int y) { this.x = x; this.y = y; }
    public Point() { this(0, 0); }   // delegate
}

Overloaded constructors

Like methods, constructors can be overloaded — several with the same class name but different parameter lists. Callers pick whichever shape suits them.

public class Rectangle {
    private final double width, height;

    public Rectangle(double width, double height) {
        this.width = width; this.height = height;
    }
    public Rectangle(double side) {   // square
        this(side, side);
    }
}

new Rectangle(4, 5);
new Rectangle(3);      // 3x3 square

Initialization order

When an object is created, Java runs: field default values → instance initializer blocks and field initializers, in source order → the constructor body. In a subclass, the superclass constructor always runs first (implicitly super() if you don't call it).