java - JPanel 只显示一个对象

Question

我从 Ball 类中制作了 20 个对象，因为我需要 20 个球在屏幕上弹跳，但现在它只显示 1 个球弹跳。 我认为这与添加了 20 个 JPanel 并且它们彼此重叠有关，但我不完全确定。

  package com.company;

import javax.swing.*;
import java.awt.*;
import java.util.Random;
/**
 * Created by John on 25/02/2015.
 */
public class Ball extends JComponent{
    int _speedX;
    int _speedY;
    int _size;
    int _x;
    int _y;
    Color _color;
    int _windowX;
    int _windowY;

    Ball(int x, int y, int sz, int sX, int sY, Color c, int windowX, int windowY){
        _x = x;
        _y = y;
        _speedX = sX;
        _speedY = sY;
        _size = sz;
        _color = c;
        _windowX = windowX;
        _windowY = windowY;
        setForeground(_color);
    }

    public void update(){

            _x = _x + _speedX;
            _y = _y + _speedY;

            if (_x<0 || _x>_windowX-_size){
                _speedX*=-1;
            }

            if (_y<=0 || _y>_windowY-_size){
                _speedY*=-1;
            }



            this.repaint();


    }

    public static int randInt(int min, int max) {

        // NOTE: Usually this should be a field rather than a method
        // variable so that it is not re-seeded every call.
        Random rand = new Random();

        // nextInt is normally exclusive of the top value,
        // so add 1 to make it inclusive
        int randomNum = rand.nextInt((max - min) + 1) + min;
        return randomNum;
    }

    public void paint(Graphics g) {

        super.paint(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.fillOval(_x, _y, _size, _size);
    }

    public static void main(String[] args) {
        // write your code here

        JFrame frame = new JFrame("Title"); //create a new window and set title on window
        frame.setSize(600, 600); //set size of window
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //set the window to close when the cross in the corner is pressed
        frame.setVisible(true); //make the window visible
        JPanel panel = new JPanel();
        frame.add(panel);

        Ball[] balls = new Ball[20];

        for(int i = 0; i<20;i++){

            balls[i] = new Ball(randInt(0,600),randInt(0,600),randInt(10,20),randInt(1,8), randInt(1,8),Color.yellow,600,600);
            panel.add(balls[i]);

        }



    while(true){

        for(int i = 0; i < balls.length; i++) {
            balls[i].update();

        }
        panel.repaint();
        try { Thread.sleep(10); } catch (InterruptedException e) { e.printStackTrace(); }
    }

}

}

Answer 1

建议:

• 不要让 Ball 继承 JComponent 或 JPanel，而应使其成为逻辑非组件类。
• 创建一个扩展 JPanel 的 Drawing 类并在其 paintComponent 中进行绘制。方法，而不是它的paint(...)方法。这将使动画更加流畅。
• 创建 ArrayList<Ball>你的 Ball 对象并将它们绘制在 paintComponent(...) 内通过迭代列表来重写方法。
• 移动列表中的球也是如此。
• 使用 Swing Timer 来驱动动画，而不是 while (true)环形。如果您在该循环中犯了一个错误，您将卡住 GUI，使其无法运行，因此 Swing Timer 是一种更安全的方法。