在JPanel 上画图

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import javax.swing.JPanel;
import javax.swing.JButton;
import java.awt.Color;
import java.awt.Point;
import java.awt.Graphics;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import java.awt.FlowLayout;
import java.awt.BorderLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JColorChooser;
 
public class Printer extends JPanel
{
  private JButton changeColorJButton;
  private JButton cleanColorJButton;
  private Point[] points = new Point[100000];
  private JPanel buttonJPanel;
  private int pointCount = 0;
  private int newPointCount = 0;
  private Color color = Color.WHITE;
  private Color newColor;
  
  public Printer()
  {
    this.setLayout( new BorderLayout() );
    changeColorJButton = new JButton("Change Color");
    cleanColorJButton = new JButton("Clean Color");
    buttonJPanel = new JPanel();
    this.setBackground( Color.WHITE );
    buttonJPanel.setLayout( new FlowLayout() );
    buttonJPanel.add( changeColorJButton );
    buttonJPanel.add( cleanColorJButton );
    add( buttonJPanel, BorderLayout.SOUTH );
    
    addMouseMotionListener(
         new MouseMotionAdapter()
         {
           public void mouseDragged( MouseEvent event )
           {
             if ( pointCount < points.length )
             {
               points[ pointCount ] = event.getPoint();
               pointCount++;
               repaint();
             }
           }
           public void mouseClicked( MouseEvent event )
           {
             if ( pointCount < points.length )
             {
               points[ pointCount ] = event.getPoint();
               pointCount++;
               repaint();
             }
           }
         }
        );
    changeColorJButton.addActionListener(
         new ActionListener()
         {
           public void actionPerformed( ActionEvent event )
           {
             newColor = JColorChooser.showDialog( Printer.this, "请选一个颜色", color );
                          if ( newColor == null )
               newColor = color;
             color = newColor;
             newPointCount = pointCount;
           }
         }
        );
    
  }
  
  public void paintComponent( Graphics g )
  {
    super.paintComponent(g);
    
    for( int i = newPointCount ; i < pointCount ; i++ )
    {
      g.setColor( color );
      g.fillOval( points[i].x, points[i].y, 4, 4);
    }
  }
}