为什么Java不能为每个循环解析变量？

Question

在下面的每个循环中,我在标记的行上得到"鸟无法解析"错误.我已经设置了一个接口Bird,它由一个抽象类BirdType实现,其中Cardinal,Hummingbird,Bluebird和Vulture是子类.getColor()和getPosition()方法在抽象类中定义,而fly()对每个子类都是唯一的.这个客户端代码实际上是由我的教授提供的,用于测试我设置的接口/继承关系.注意我已经测试了接口,抽象类和子类,它们似乎都可以工作.我认为问题在于for-each循环,但如果需要,我可以为其他类提供代码.有什么建议？

import java.awt.*;
public class Aviary {
    public static final int SIZE = 20;
    public static final int PIXELS = 10;

    public static void main(String[] args){
        //create drawing panel
        DrawingPanel panel = new DrawingPanel(SIZE*PIXELS,SIZE*PIXELS);
        //create pen
        Graphics g = panel.getGraphics();

        //create some birds
        Bird[] birds = {new Cardinal(7,4), new Cardinal(3,8),
            new Hummingbird(2,9), new Hummingbird(16,11),
            new Bluebird(4,15), new Bluebird(8,1),
            new Vulture(3,2), new Vulture(18,14)};

        while (true){
            //clear screen
            g.setColor(Color.WHITE);
            g.fillRect(0, 0, SIZE*PIXELS, SIZE*PIXELS);

            //tell birds to fly and redraw birds
            for (Bird bird : birds) 
                bird.fly();
!               g.setColor(bird.getColor());
!               Point pos = bird.getPosition();
                g.fillOval((int)pos.getX()*PIXELS, (int)pos.getY()*PIXELS,
                    PIXELS, PIXELS);
                panel.sleep(500);
        }
    }
}

Answer 1

你需要将你想要在for循环中执行的主体包裹在大括号中

for (Bird bird : birds) {
    bird.fly();
    g.setColor(bird.getColor());
    Point pos = bird.getPosition();
    g.fillOval((int)pos.getX()*PIXELS, (int)pos.getY()*PIXELS,
    PIXELS, PIXELS);
    panel.sleep(500);
}

否则,for循环的主体是后面的下一个语句).所以它相当于

for (Bird bird : birds) 
    bird.fly();

// bird is not in scope anymore
g.setColor(bird.getColor());
Point pos = bird.getPosition();
g.fillOval((int)pos.getX()*PIXELS, (int)pos.getY()*PIXELS,
PIXELS, PIXELS);
panel.sleep(500);

正如Darien在评论中所说,缩进不是Java语言的一部分,它与语法无关.但是您应该使用它来使代码更容易阅读,如Java代码样式约定中所表达的那样.