Question

我正在尝试创建一个Java applet，它在applet窗口中弹出几个球，每个都有自己的线程。使用下面的代码绘制所有球，但只有第一个球移动。我做错了什么？

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

import static java.awt.Color.*;

public class BouncingBalls extends Applet implements Runnable {

    List<Ball> ballList = new ArrayList(); // holds Ball objects

    Color[] colors = new Color[]{BLACK, GRAY, WHITE, PINK, RED, ORANGE, YELLOW,
            GREEN, BLUE, CYAN}; // array holding available colors

    static int width, height; // variables for applet dimensions

    int ballCount; // number of balls to be created, set by html parameter

    Random random = new Random(); // random number generator


    public void init() {

        // get window dimensions
        width = getSize().width;
        height = getSize().height;

        //get number of balls from html
        String ballCountString = this.getParameter("ballCount");

        try {
            ballCount = Integer.parseInt(ballCountString);
        } catch (NumberFormatException e) {
            ballCount = 10; // set to 10 by default
        }

        for (int i = 0; i < ballCount; i++) {

            // randomly assign ballDiameter between 1 and 20
            int ballDiameter = random.nextInt(20) + 1;

            // create and add balls to ballList
            ballList.add(new Ball(
                    random.nextInt(width - ballDiameter), // set x coordinate
                    random.nextInt(height - ballDiameter), // set y coordinate
                    ballDiameter, // set ballDiameter
                    random.nextInt(ballDiameter) + 1, // deltaX <= ballDiameter
                    random.nextInt(ballDiameter) + 1, // deltaY <= ballDiameter
                    colors[i % 10] // use remainder to choose colors[] element
                    )
            );

        } // end for

    } // end init


    public void start() {

        for (Ball ball: ballList) {

            Thread t;
            t = new Thread(this);
            t.start();

        } // end for

    } // end start


    public void run() {

        for (Ball ball : ballList) {

            // infinite loop: ball moves until applet is closed
            while (true) {

                ball.move();

                repaint(); // call paint method to draw circle in new location

                // set ball repaint delay using Thread sleep method
                try {
                    Thread.sleep(20); // wait 20 msec before continuing
                } catch (InterruptedException e) {
                    return;
                }

            } // end while

        } // end for

    } // end run

    public void paint(Graphics g) {

        super.paint(g);

        for (Ball ball : ballList) {

            // set current color
            g.setColor(ball.ballColor);

            // draw filled oval using current x and y coordinates and diameter
            g.fillOval(ball.x, ball.y, ball.diameter, ball.diameter);

        } // end for

    } // end paint
}

class Ball {

    int x, y, // coordinates of upper-left corner of circle
        diameter, // circle diameter
        deltaX, deltaY; // number of pixels ball moves each time it's repainted
    Color ballColor;


    public Ball(int x, int y, int diameter, int deltaX, int deltaY,
                Color ballColor) {

        this.x = x;
        this.y = y;
        this.diameter = diameter;
        this.deltaX = deltaX;
        this.deltaY = deltaY;
        this.ballColor = ballColor;

    } // end Ball constructor


    public void move() {

        // update x and y coordinates using delta values
        x += deltaX;
        y += deltaY;

        // reverse x direction when ball reaches boundary
        if (x >= BouncingBalls.width - diameter || x <= 0){
            deltaX = -deltaX;
        } // end if

        // reverse y direction when ball reaches boundary
        if (y >= BouncingBalls.height - diameter || y <= 0) {
            deltaY = -deltaY;
        } // end if

    } // end move

} // end BouncingBalls

Answer 1

您的while(true)应该在for循环之外。它位于迭代器返回的第一个球上。

话虽这么说，你可能想要为每个线程一个球回顾你的逻辑。它实际上似乎创建N个线程（N是球的数量），其中每个线程将移动所有球而不是仅一个。

编辑以解决我的第二点：

假设你有10个球。你启动10个线程，每个线程迭代所有球。

例如：

Thread 1:
public void run(){
   for(Ball b : ballList){
      b.move();
      b.repaint();
   }
}

Thread 2:
public void run(){
   for(Ball b : ballList){
      b.move();
      b.repaint();
   }
}

Thread N:
public void run(){
   for(Ball b : ballList){
      b.move();
      b.repaint();
   }
}

这样做是因为你创建的线程具有相同的this runnable实例，迭代每个球。

 public void start() {

        for (Ball ball: ballList) {

            Thread t;
            t = new Thread(this);
            t.start();

        } // end for

    } // end start

所以我想，如果每个球应该每20毫秒移动1个单位。你应该最终看到每20毫秒在这种情况下每隔20毫秒移动N * 1个单位。

编辑 - 关于建议。

不是将this设置为runnable，而是应该从this类中删除Runnable的实现，并创建一个新的Runnable，它将单个Ball作为参数。

private static class MovingRunnable implements Runnable{
   private final Ball b;
   private MovingRunnable(Ball b){this.b=b;}
   public void run(){
      for(;;){
         b.move(); 
         b.repaint();
         Thread.sleep(20);
      }
   }
}

稍后启动方法

public void start() {
        for (Ball ball: ballList) {
            Thread t;
            t = new Thread(new MovingRunnable(ball));
            t.start();
        } // end for

    } // end start

所以这里每个Ball都有它自己的线程，它拥有自己的Runnable。现在每个球只会每20毫秒一次调用move一次。

但它仍然不完美，因为repaint只能由UI线程调用，每个线程调用它会导致不同的问题（尽管你可能没有注意到任何问题，但值得一说）。 / p>

多线程Java小程序弹跳球

1 个答案: