Java：我如何让球像弹跳和弹性球一样？

Question

我有两个类BouncingBall ，另一个叫做ElasticBall 。 这两个类都扩展了BallImpl ，它实现了一个名为Ball的接口。

public interface Ball {
    int DEFAULT_RADIUS = 50;

    int radius();
    Point center();
    void update();
}

public class BouncingBall extends BallImpl {
    public static final int MOVEMENT_SPEED = 12;

    static final int DOWN = 1;
    static final int UP = -1;

    private int direction;

    BouncingBall(int x, int y, int direction) {
        super(x, y);
        this.direction = direction;
    }

    @Override
    public void update() {
        direction = reverseDirectionIfNecessary();
        y = move();
    }

    private int reverseDirectionIfNecessary() {
        if (movingTooHigh() || movingTooLow()) {
            return switchDirection();
        }

        return this.direction;
    }

    private boolean movingTooLow() {
        return y + radius >= BallWorld.BOX_HEIGHT && movingDown();
    }

    private boolean movingTooHigh() {
        return y - radius <= 0 && movingUp();
    }

    private int switchDirection() {
        return movingDown() ? UP : DOWN;
    }

    private int move() {
        return y + (MOVEMENT_SPEED * direction);
    }

    private boolean movingDown() {
        return direction == DOWN;
    }

    private boolean movingUp() {
        return direction == UP;
    }
}

public class ElasticBall extends BallImpl {
    public static final int GROWTH_RATE = 2;

    static final int GROW = 1;
    static final int SHRINK = -1;

    private int growthDirection;

    ElasticBall(int x, int y, int radius, int growthDirection) {
        super(x, y, radius);
        this.growthDirection = growthDirection;
    }

    @Override
    public void update() {
        growthDirection = reverseGrowthDirectionIfNecessary();
        radius = next();
    }

    private int reverseGrowthDirectionIfNecessary() {
        if (growingTooBig() || shrinkingTooSmall()) {
            return switchDirection();
        }

        return this.growthDirection;
    }

    private boolean shrinkingTooSmall() {
        return radius <= 0 && shrinking();
    }

    private boolean growingTooBig() {
        return radius >= Ball.DEFAULT_RADIUS && growing();
    }

    private int switchDirection() {
        return growing() ? SHRINK : GROW;
    }

    private int next() {
        return radius + (GROWTH_RATE * growthDirection);
    }

    private boolean shrinking() {
        return growthDirection == SHRINK;
    }

    private boolean growing() {
        return growthDirection == GROW;
    }
}

我需要创建一个BouncingElasticBall ，它结合了BouncingBall和ElasticBall类的行为。 我对OOP知之甚少，而且我知道Java不允许多重继承，那么如何解决这个问题呢？

提前致谢。

Answer 1

你可以采用的一种方法是不扩展 BallImpl ，而是制作排序插件 。 像这样：

public class BallImpl implements Ball {
    List<BallBehavior> behaviors = ...

    @Override
    public void update() {
       behaviors.forEach(behavior -> behavior.update(this));
    }
    ...
}

public interface BallBehavior {
    void update(BallImpl ballImpl);
}

然后，将弹性和弹跳逻辑写成行为 。

Answer 2

一旦你将层次结构分开，就无法在java中合并它们。

这是一个设计问题：如果您知道ElasticBall和BouncingBall可以组合在一起，您应该创建两个接口Elastic和Bouncing ，两者都扩展接口Ball ，使用对两者都有效的常用方法。

然后可以将公共方法实现设置为公共抽象类，比如说AbstractBall 。 此时，您最终可以详细说明您的三个实现：

• ElasticBall extends AbstractBall implements Elastic
• BouncingBall extendis AbstractBall implements Bouncing
• ElasticBouncingBall extends AbstractBall implements Elastic, Bouncing

通过这种方式，您将能够控制在每个方法中执行的操作，重用常见内容的代码（在抽象类中）。

Answer 3

您可以使用允许多重继承的接口。 为每个球的界面ElasticBall和BouncingBall和执行两者BouncingElasticBall 。