摆动重绘（）不起作用

Question

 class ballbouncepanel extends JPanel
{
        public void start()
            {

                Timer timer;
                final int FREQ = 45;
                timer = new Timer(FREQ, new ActionListener()
                {
                    public void actionPerformed(ActionEvent evt)
                    {
                        repaint();
                    }
                });
                timer.start();
    }

    Rect rect = new Rect();
        public Dimension getPreferredSize()
        {
            return new Dimension(250,200);
        }
        public void paintComponent(Graphics g)
        {
            super.paintComponent(g);
            rect.draw(g);
            rect.move(g);
            rect.erase(g);
        }
}

class Rect
{
    public int xLocation = 0;
    public int yLocation = 0;
    public int xVelocity = 10;
    public int yVelocity = 10;

    public void draw(Graphics g)
    {
        g.setColor(Color.cyan);
        g.fillRect(xLocation, yLocation, 20, 20);
    }
    public void move(Graphics g)
    {
        xLocation += xVelocity;
        yLocation += yVelocity;
    }
    public void erase(Graphics g)
    {
        g.setColor(Color.white);
        g.fillRect(xLocation, yLocation, 20, 20);
    }
}

新的错误是现在我的重绘方法不起作用。

上面是我想要绘制的框架的代码，我了解使用小程序或 JApplet 进行绘制，但我正在尝试在 Swing 上的小程序中执行我所做的事情，现在我遇到了问题，我查阅了很多关于如何在其中实现图形的教程，但其中大多数只是运行一个主图形，我需要我的在这个特定的框架（BB）中。如果有人可以帮助我理解或指向我的初学者教程，将不胜感激。

Answer 1

我认为您只是忘记调用ballbouncepanel 的start() 方法。另请注意：您的move() 方法没有绘画，所以取出Graphics 参数并在计时器中调用它

也不确定erase 方法应该做什么，但我想你想在计时器的每个滴答声中改变颜色。在这种情况下，只需保留一个 color 变量，然后更改该变量即可。你可以看下面的例子

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class bounceballpanel extends JPanel {
    public void start() {

        Timer timer;
        final int FREQ = 45;
        timer = new Timer(FREQ, new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                rect.move();
                rect.changeColor();
                repaint();
            }
        });
        timer.start();
    }

    Rect rect = new Rect();

    public Dimension getPreferredSize() {
        return new Dimension(250, 200);
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        rect.draw(g);
        //rect.erase(g);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable(){
            public void run() {
                JFrame frame = new JFrame();
                bounceballpanel panel = new bounceballpanel();
                panel.start();
                frame.add(panel);
                frame.pack();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}

class Rect {
    public int xLocation = 0;
    public int yLocation = 0;
    public int xVelocity = 10;
    public int yVelocity = 10;

    Color color = Color.cyan;

    public void draw(Graphics g) {
        g.setColor(color);
        g.fillRect(xLocation, yLocation, 20, 20);
    }

    public void move() {
        xLocation += xVelocity;
        yLocation += yVelocity;
    }

    public void changeColor() {
        if (color == Color.cyan) {
            color = Color.white;
        } else {
            color = Color.cyan;
        }
    }

    /*
    public void erase(Graphics g) {
        g.setColor(Color.white);
        g.fillRect(xLocation, yLocation, 20, 20);
    }*/
}