java - 循环泛型(尝试2)

第二次尝试this问题(初始代码不足以突出问题)

这是无法编译的代码:

interface Player<R, G extends Game>
{
    R takeTurn(G game);
}

interface Game<P extends Player>
{
    void play(P player);
}

abstract class AbstractGame<R, P extends Player>
    implements Game<P>
{
    public final void play(final P player)
    {
        final R value;

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

    protected abstract void turnTaken(R value);
}

public class XPlayer
    implements Player<Integer, XGame>
{
    @Override
    public Integer takeTurn(final XGame game)
    {
        return (42);
    }
}

public class XGame<P extends Player<Integer, XGame>>
    extends AbstractGame<Integer, XPlayer>
{
    @Override
    protected void turnTaken(final Integer value)
    {
        System.out.println("value = " + value);
    }
}

public class Main
{
    public static void main(final String[] argv)
    {
        final XPlayer player;
        final XGame   game;

        player = new XPlayer();
        game   = new XGame();
        game.play(player);
    }
}

我遇到的是试图让AbstractGame中的play方法进行编译。似乎我必须与Game和Player一起在扩展/实现中添加泛型，但我一生都无法理解。

play方法必须在AbstractGame类中是最终方法，并且无法进行强制转换，而且我不想编写另一种方法(如turnTaken)来使它能够正常工作。

编辑:这里要求的是编译的代码，但需要强制转换:

interface Player<R, P extends Player<R, P, G>, G extends Game<R, G, P>>
{
    R takeTurn(G game);
}

interface Game<R, G extends Game<R, G, P>, P extends Player<R, P, G>>
{
    void play(P player);
}

abstract class AbstractGame<R, G extends Game<R, G, P>, P extends Player<R, P, G>>
    implements Game<R, G, P>
{
    public final void play(final P player)
    {
        final R value;

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

    protected abstract void turnTaken(R value);
}

class XPlayer
    implements Player<Integer, XPlayer, XGame>
{
    @Override
    public Integer takeTurn(final XGame game)
    {
        return (42);
    }
}

class XGame
    extends AbstractGame<Integer, XGame, XPlayer>
{
    @Override
    protected void turnTaken(final Integer value)
    {
        System.out.println("value = " + value);
    }
}

class Main
{
    public static void main(final String[] argv)
    {
        final XPlayer player;
        final XGame   game;

        player = new XPlayer();
        game   = new XGame();
        game.play(player);
    }
}

最佳答案

混合泛型和原始类型是行不通的。如果需要这些接口互相引用，则它们还需要引用自己:

interface Player<R, P extends Player<R, P, G>, G extends Game<R, G, P>>
{
    R takeTurn(G game);
}

interface Game<R, G extends Game<R, G, P>, P extends Player<R, P, G>>
{
    void play(P player);
}

尽管这看起来很头疼，但我不确定您为什么需要它。

编辑:

我能够基于上述实现您的AbstractGame:

abstract class AbstractGame<R, P extends Player<R, P, AbstractGame<R, P>>>
    implements Game<R, AbstractGame<R, P>, P>
{
    public final void play(final P player)
    {
        final R value;

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

    protected abstract void turnTaken(R value);
}

但是我不能完全使用XGame和XPlayer封闭电路:

public class XGame
    extends AbstractGame<Integer, XPlayer> //compile error on XPlayer
{

    protected void turnTaken(Integer value) { }
}

public class XPlayer
    implements Player<Integer, XPlayer, XGame> //compile error on XGame
{
    @Override
    public Integer takeTurn(final XGame game)
    {
        return (42);
    }
}

问题似乎是XGame和XPlayer的每个通用声明都需要彼此正确。这是您的设计真正具有周期性的地方。如果编译器“假设”每一个都是正确的，那么理论上它将起作用。但事实并非如此。

编辑2:

这个怎么样:

interface Game<R, G extends Game<R, G>>
{
    void play(Player<R, G> player);
}

interface Player<R, G extends Game<R, G>>
{
    R takeTurn(G game);
}

abstract class AbstractGame<R, G extends AbstractGame<R, G>>
    implements Game<R, G>
{
    public final void play(final Player<R, G> player)
    {
        final R value;

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

    protected abstract G self();

    protected abstract void turnTaken(R value);
}

public final class XGame extends AbstractGame<Integer, XGame>
{
   protected XGame self() {
      return this;
   }

   protected void turnTaken(Integer value) { }
}

public class XPlayer implements Player<Integer, XGame>
{
    @Override
    public Integer takeTurn(final XGame game)
    {
       return (42);
    }
}

此处的关键是在self()中声明一个抽象方法AbstractGame，该方法返回G类型的实例。扩展类必须使用自己的类型来解析继承的type参数，并实现self()以返回this。这仅适用于内部代码，因为扩展类很容易说谎，例如:

public class EvilGame extends AbstractGame<Integer, AnotherGame> { ... }

有关此模式的更多详细信息，请参见我的答案here和this post。