Java Awt笔工具：图形消失

Question

我会尝试用钢笔工具实现Java Applet。 当我开始小程序，我可以按下并释放鼠标正确绘制我的线在窗口上。几秒钟后消失的一切。 修复此问题的一些提示？

import java.applet.Applet; 
import java.awt.BorderLayout; 
import java.awt.Color; 
import java.awt.Graphics; 
import java.awt.Panel; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 
import java.awt.event.AdjustmentEvent; 
import java.awt.event.AdjustmentListener; 
import java.awt.event.MouseEvent; 
import java.awt.event.MouseListener; 
import java.awt.event.MouseMotionListener; 


public class MarkPen extends Applet implements ActionListener, AdjustmentListener,  MouseListener, MouseMotionListener { 

private int x = 0; 
private int y = 0; 
private int prevx = 0; 
private int prevy = 0; 

private boolean dragging = true; 
private Panel drawPanel = new Panel(); 

public void init() 
{ 
    setLayout(new BorderLayout()); 

    drawPanel.setBackground(Color.white); 
    add(drawPanel, "Center"); 
    drawPanel.addMouseMotionListener(this); 
    drawPanel.addMouseListener(this); 

} 


public void setGraphicalDefaults(MouseEvent e) 
{ 
    x = e.getX(); 
    y = e.getY(); 
    prevx = e.getX(); 
    prevy = e.getY(); 
} 

public void mouseDragged(MouseEvent e) 
{ 
    Graphics g = drawPanel.getGraphics(); 
     g.setColor(Color.black); 

     /* 
      IF i'm not dragging... 
     */ 
     if (dragging) 
     { 
      setGraphicalDefaults(e); 
      dragging = false; 
      g.drawLine(prevx,prevy,x,y); 
     } 

     /* 
      If mouse moved.. 
     */ 
     if (x != e.getX() || y != e.getY()) 
     { 
      x = e.getX(); 
      y = e.getY(); 

      g.drawLine(prevx,prevy,x,y); 

      prevx = x; 
      prevy = y; 
     } 
} 

public void mouseReleased(MouseEvent e) 
{ 
    dragging = true; 
} 
public void mouseEntered(MouseEvent e){} 
public void mouseExited(MouseEvent e){} 
public void mouseMoved(MouseEvent e){} 
public void mousePressed(MouseEvent e){} 
public void mouseClicked(MouseEvent e){} 
public void adjustmentValueChanged(AdjustmentEvent arg0){} 
public void actionPerformed(ActionEvent e){} 

} 

新版本：我纠正了我的代码，并在此下方有工作版本

import java.applet.Applet; 
    import java.awt.BorderLayout; 
    import java.awt.Color; 
    import java.awt.Graphics; 
    import java.awt.Graphics2D; 
    import java.awt.event.ActionEvent; 
    import java.awt.event.ActionListener; 
    import java.awt.event.AdjustmentEvent; 
    import java.awt.event.AdjustmentListener; 
    import java.awt.event.MouseEvent; 
    import java.awt.event.MouseListener; 
    import java.awt.event.MouseMotionListener; 
    import java.awt.image.BufferedImage; 

    import javax.swing.JComponent; 
    import javax.swing.JPanel; 



    public class MarkPen extends Applet implements ActionListener, AdjustmentListener, MouseListener, MouseMotionListener { 

    private int x = 0; 
    private int y = 0; 
    private int prevx = 0; 
    private int prevy = 0; 

    private JPanel drawPanel = new JPanel(); 

    public void init() 
    { 
     setLayout(new BorderLayout()); 

     drawPanel.setBackground(Color.white); 
     add(drawPanel, "Center"); 
     drawPanel.addMouseMotionListener(this); 
     drawPanel.addMouseListener(this); 
     drawPanel.add(new Comp()); 
     drawPanel.setDoubleBuffered(true); 

    } 

    public void setGraphicalDefaults(MouseEvent e) 
    { 
     x = e.getX(); 
     y = e.getY(); 
     prevx = e.getX(); 
     prevy = e.getY(); 
    } 

    public class Comp extends JComponent{ 
     public void paintComponent(Graphics g) { 
      super.paintComponent(g); 
      g = drawPanel.getGraphics(); 
      g.setColor(Color.black); 

      g.drawLine(prevx,prevy,x,y); 
      prevx = x; 
      prevy = y; 
      g.dispose(); 
      repaint(); 
     } 
    } 


    public void mouseDragged(MouseEvent e) 
    { 
     if (x != e.getX() || y != e.getY()) 
     { 
      x = e.getX(); 
         y = e.getY(); 
      }  
     } 

     public void mousePressed(MouseEvent e){ 
      setGraphicalDefaults(e); 
     } 

     public void mouseReleased(MouseEvent e){} 
     public void mouseEntered(MouseEvent e){} 
     public void mouseExited(MouseEvent e){} 
     public void mouseMoved(MouseEvent e){} 
     public void mouseClicked(MouseEvent e){} 
     public void adjustmentValueChanged(AdjustmentEvent arg0){} 
     public void actionPerformed(ActionEvent e){}  

    } 

Answer 1

这是有道理的图形正在消失，因为你产生不稳定的一个组件上调用getGraphics()图形对象，一个不会持久。不要这样做。而是在paint(...)覆盖中绘制。更好的办法是使用Swing GUI并绘制JPanel的paintComponent(...)方法或绘制paintComponent(...)方法中显示的 BufferedImage。