cyclical generics (try 2)

Question

Second attempt at this question (the initial code wasn\'t enough to highlight the issue)

Here is the code that does not compile:

interface Player<         


        

Answer 1

As Paul Bellora points out, you're mixing generic and raw types -- and the correct, fully-generic solution is a bit of a mess and requires a lot of redundancy. There's no nice way (that I know of) to do circular (but not recursive) generics in Java.

Rather than struggling with this, I would make both Player and Game generic on just one parameter, the type of value being played with -- what you had as R.

interface Game {
    void play(Player player);
}

interface Player {
    R takeTurn(Game game);
}

abstract class AbstractGame implements Game {
    public final void play(Player player) {
        final R value;

        value = player.takeTurn(this);
        turnTaken(value);
    }

    protected abstract void turnTaken(R value);
}

class XPlayer implements Player {
    @Override
    public Integer takeTurn(Game game) {
        return 42;
    }
}

class XGame extends AbstractGame {
    @Override
    public void turnTaken(Integer value) {
        System.out.println("value = " + value);
    }
}

public class Main {
    public static void main(String[] argv) {
        XPlayer player = new XPlayer();
        XGame game = new XGame();
        game.play(player);
    }
}

Now, any player who knows how to take R-based moves can play any R-based game.