绘图程序复制 JMenuBar

Question

我用Java编写了一个简单的绘图程序，允许用户在不同的形状，大小和颜色之间进行选择（在JMenuBar中），并用鼠标绘制它们。 在克服了该程序的许多问题之后（我最初是用AWT编写的，但被说服切换到 Swing，所以请原谅任何糟糕的Swing编码，因为这是我第一次使用它），我还有一个问题需要解决。 每当用户在屏幕上绘制内容（默认情况下是带有黑色边框的红色矩形）时，JMenuBar都会在原始JMenuBar下方复制。 我认为 paintComponent 缓冲屏幕的方式很奇怪，但由于我以前从未使用过它，我不知道它是如何工作的。

任何想法如何让它停止复制JMenuBar？

import java.awt.*;
import java.awt.event.*;
import java.lang.*;
import javax.swing.*;
public class E3G04 extends JFrame implements WindowListener, ActionListener, MouseListener, MouseMotionListener
{
    //Variables are declared as volatile to ensure that they're always called from system RAM
    static volatile String type = "rectangle";
    static volatile Boolean fill = true;
    static Color lineColor = Color.BLACK;
    static Color fillColor = Color.RED;
    static int size = 1;
    CanvasEX cx = new CanvasEX();
    static boolean running = true;
    JMenuBar mb = new JMenuBar();
        JMenu fileMenu = new JMenu("File");
            JMenuItem newItem = new JMenuItem("New");
            JMenuItem quitItem = new JMenuItem ("Quit");

    protected E3G04()
    {
        mb.add(fileMenu);
        fileMenu.add(newItem);
        fileMenu.add(quitItem);             
        newItem.setMnemonic('N');
        quitItem.setMnemonic('Q');
        newItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_N, ActionEvent.CTRL_MASK));
        quitItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_Q, ActionEvent.CTRL_MASK));          
        newItem.addActionListener(this);
        quitItem.addActionListener(this);
        cx.setSize(800,600);
        cx.addMouseListener(this);
        cx.addMouseMotionListener(this);
        setJMenuBar(mb);
        setBounds(100,100,800,600);
        setLayout(new BorderLayout());
        add("Center",cx);
        addWindowListener(this);
        setResizable(true);
        setVisible(true);
    }
    public static void main(String [] args)
    {
        new E3G04();
    }

    public void stop()
    {
        newItem.removeActionListener(this);
        quitItem.removeActionListener(this);    
        dispose();
        System.exit(0);
    }
    public void actionPerformed(ActionEvent e)
    {
        Object o = e.getSource();
        if (o == newItem)
        {
            cx.erase = true;
            cx.repaint();
        }
        if (o == quitItem)
        {   
            running = false;
            stop();
        }
    }      
    public void mousePressed(MouseEvent m)
    {
        cx.start = m.getPoint();
        cx.end = cx.start;
        cx.cur = cx.start;
    }
    public void mouseDragged(MouseEvent m)
    {
        cx.cur = m.getPoint();
    }   
    public void mouseReleased(MouseEvent m)
    {
        cx.end = cx.cur;
        cx.repaint();
    }   
    public void itemStateChanged(ItemEvent e)
    {
        Object o = e.getSource();
    }
    public void windowClosing(WindowEvent e)
    {
        running = false;
        stop();
    }
    public void mouseClicked(MouseEvent m){}    
    public void mouseExited(MouseEvent m){}
    public void mouseEntered(MouseEvent m){}    
    public void mouseMoved(MouseEvent m){}
    public void windowClosed(WindowEvent e){}
    public void windowOpened(WindowEvent e){}
    public void windowActivated(WindowEvent e){}
    public void windowDeactivated(WindowEvent e){}
    public void windowIconified(WindowEvent e){}
    public void windowDeiconified(WindowEvent e){}
}

class CanvasEX extends JPanel
{
    Point start = new Point(100,100);
    Point cur = new Point(100,100);
    Point end = new Point(100,100);
    boolean erase = false;

    public void paintComponent(Graphics g)
    {
        Graphics buffer = g;
        if (erase)
        {
            g.dispose();
            erase = false;
        }   
        g.setColor(E3G04.lineColor);
        if (end.x > start.x && end.y > start.y)
            g.fillRect(start.x,start.y, Math.abs(end.x-start.x),Math.abs(end.y-start.y));
        if (end.x > start.x && end.y < start.y)
            g.fillRect(start.x,end.y, Math.abs(end.x-start.x),Math.abs(end.y-start.y));  
        if (end.x < start.x && end.y > start.y)
            g.fillRect(end.x, start.y, Math.abs(end.x-start.x),Math.abs(end.y-start.y));
        if (end.x < start.x && end.y < start.y)
            g.fillRect(end.x, end.y, Math.abs(end.x-start.x),Math.abs(end.y-start.y));
        g.setColor(E3G04.fillColor);
        if (end.x > start.x && end.y > start.y)
            g.fillRect(start.x + E3G04.size,start.y + E3G04.size, Math.abs(end.x-start.x) - 2 * E3G04.size,Math.abs(end.y-start.y) - 2 * E3G04.size);
        if (end.x > start.x && end.y < start.y)
            g.fillRect(start.x + E3G04.size,end.y + E3G04.size, Math.abs(end.x-start.x) - 2 * E3G04.size,Math.abs(end.y-start.y) - 2 * E3G04.size);  
        if (end.x < start.x && end.y > start.y)
            g.fillRect(end.x + E3G04.size, start.y + E3G04.size, Math.abs(end.x-start.x) - 2 * E3G04.size,Math.abs(end.y-start.y) - 2 * E3G04.size);
        if (end.x < start.x && end.y < start.y)
            g.fillRect(end.x + E3G04.size, end.y + E3G04.size, Math.abs(end.x-start.x) - 2 * E3G04.size,Math.abs(end.y-start.y) - 2 * E3G04.size);          
    }
}

Answer 1

你不遵守油漆链（你应该这样做），通过在覆盖的方法中调用super.paintComponent(Graphics g) paintComponent(..)：

@Override
public void paintComponent(Graphics g)
{
     super.paintComponent(g);
    //do rest of painting etc
}