Question

I’ve read around that instead of using instanceof and getClass methods to implement polymorphism, as it’s better object-oriented practice.

If my program looks like the following:

Piece
SlowPiece extends Piece,
FastPiece extends Piece,
SlowFlexible extends SlowPiece,
FastFlexible extends FastPiece

And I have the following rule: SlowPiece's can only move one space, and FastPiece's as many as they'd like.

How would I use polymorphism to ensure that if I’m moving a Piece like Piece piece1 = new SlowPiece(); that it is not moved more than one?

I’ve got a class for the game-board, and it should be able to move the piece, but I’m not sure how to stop it from moving a SlowPiece more than 1 without code-sniffing.

Basically, I get how polymorphism is fantastic for say, listing things, but with actually interacting with them I don’t get how to use it properly.

Answer 1

Make Piece abstract and add an abstract method move() to Piece (you could also user an interface). Implement move() differently (a SlowPiece only moves 1 spot, etc).

public abstract class Piece {
    public abstract void move();
}

public class SlowPiece extends Piece {
    public void move() {
        System.out.println("moving a slow piece");
    }
}

public class FastPiece extends Piece {
    public void move() {
        System.out.println("moving a fast piece");
    }
}

Piece f = new FastPiece();
f.move();       

Piece s = new SlowPiece();
s.move();

Here, move() has all the moving logic; you could even use some overloads to allow for movement variety (like take N/S/W/E as arguments, etc).