在画布上看不到圆圈

Question

我真的想不通，我需要你的帮助。我创建了一个简单的 小程序。我有一个面板和一个画布。我在画布上画画。我添加画布 到面板。我将面板添加到小程序。我正在尝试在上面画一个圆圈 画布，但我看不到它。如果我调整小程序的大小，我可以看到圆圈 在画布后面闪烁。请帮忙。

        import java.applet.*;
        import java.awt.*;


        public class TestCanvas extends Applet implements Runnable
        {
            Panel myPanel = new Panel();
            int x = 50;
            int y = 50;
            int width = 100;
            int height = 100;
            boolean startBall = false;

            Graphics bufferGraphics;
            Image offscreen;

            Thread t;

            public void init()
            {
                canvas.setSize(500,500);

                //canvas.setMaximumSize(new Dimension(500,500));
                //canvas.setMinimumSize(new Dimension(50,50));
                myPanel.add(canvas);
                add(myPanel);
                //For double buffering
                offscreen = createImage(canvas.getWidth(),canvas.getHeight());
                bufferGraphics = offscreen.getGraphics();

            }

            public void start()
            {
                t = new Thread();
                t.start();
                startBall = true;

            }

            public void stop()
            {
                t = null;
            }

            public void destroy()
            {
                t = null;
            }

            public void run()
            {
                for ( ; ; )
                {
                    try
                    {
                        canvas.repaint();
                        Thread.sleep(100);
                    }catch(InterruptedException e){}



                }
            }

            class myCanvas extends Canvas
            {
                private static final long serialVersionUID = 1L;

                public myCanvas()
                {
                    setBackground(Color.yellow);
                    setSize(500, 300);
                    //setBounds(0, 0, 500, 300);

                }   

            }

            myCanvas canvas = new myCanvas();

            public void paint(Graphics g)
            {
                g = canvas.getGraphics();
                bufferGraphics.clearRect(0,0,canvas.getWidth(), canvas.getHeight());
                bufferGraphics.setColor(Color.black);
                //x++;
                //y++;
                bufferGraphics.setColor(Color.black);
                bufferGraphics.fillOval(x, y, width, height);       
                g.drawImage(offscreen,0,0,null);        
            }

            public void update(Graphics g)
            {
                canvas.paint(g);
            }



        }

Answer 1

它已经到了我不会触及涉及小程序或 AWT 的问题的阶段，直到提出问题的人对他们为什么使用其中任何一个给出了合理的回答。 OTOH，您已经部分实现了这一点，所以这是一个动画球的工作示例。

仔细查看您的代码与此代码之间的差异。

import java.applet.*;
import java.awt.*;

public class TestCanvas extends Applet implements Runnable {

    Panel myPanel = new Panel(new GridLayout());
    int x = 0;
    int y = 0;
    int width = 100;
    int height = 100;
    boolean startBall = false;
    myCanvas canvas = new myCanvas();
    Thread t;

    public void init() {
        myPanel.add(canvas);
        System.out.println("" + this.getLayout());
        /* A Single component added to a no args GridLayout will be stretched 
         to fill the avilable space. */
        this.setLayout(new GridLayout());
        add(myPanel);
        t = new Thread(this);
        t.start();
    }

    public void start() {
        t = new Thread();
        t.start();
        startBall = true;
    }

    public void stop() {
        t = null;
    }

    public void destroy() {
        t = null;
    }

    public void run() {
        for (;;) {
            try {
                canvas.repaint();
                Thread.sleep(100);
            } catch (InterruptedException e) {
            }
        }
    }

    class myCanvas extends Canvas {

        private static final long serialVersionUID = 1L;

        public myCanvas() {
            setBackground(Color.yellow);
        }

        @Override
        public void paint(Graphics g) {
            super.paint(g);
            g.clearRect(0, 0, getWidth(), getHeight());
            g.setColor(Color.black);
            x++;
            y++;
            g.setColor(Color.black);
            g.fillOval(x, y, width, height);
        }
    }
}