Factories

Another interesting use of static methods is what we would call a "factory" method.

Say you have a class like the following.

class Position {
    int x;
    int y;

    Position(int x, int y) {
        this.x = x;
        this.y = y;
    }
}
void main() {}

It would be reasonable want to add an overloaded constructor for when y is 0.

class Position {
    int x;
    int y;

    Position(int x, int y) {
        this.x = x;
        this.y = y;
    }

    Position(int x) {
        this.x = x;
        this.y = 0;
    }
}
void main() {}

But now it is impossible to add a constructor that works for when x is 0.

class Position {
    int x;
    int y;

    Position(int x, int y) {
        this.x = x;
        this.y = y;
    }

    Position(int x) {
        this.x = x;
        this.y = 0;
    }

    Position(int y) {
        this.x = 0;
        this.y = y;
    }
}
void main() {}

Using a static method to create a Position - i.e. as a "factory" - is a way around the issue.¹

class Position {
    int x;
    int y;

    Position(int x, int y) {
        this.x = x;
        this.y = y;
    }

    static Position fromX(int x) {
        return new Position(x, 0);
    }

    static Position fromY(int y) {
        return new Position(0, y);
    }
}

class Position {
    int x;
    int y;

    Position(int x, int y) {
        this.x = x;
        this.y = y;
    }

    static Position fromX(int x) {
        return new Position(x, 0);
    }

    static Position fromY(int y) {
        return new Position(0, y);
    }
}

class Main {
    void main() {
        var p1 = new Position(1, 2);
        var p2 = Position.fromX(4);
        var p3 = Position.fromY(5);

        System.out.println(p1.x + ", " + p1.y);
        System.out.println(p2.x + ", " + p2.y);
        System.out.println(p3.x + ", " + p3.y);
    }
}