how to undo the last action printed on an applet using undo button

[srinivas bolloju]

hi,
how to undo the last action printed on an applet using undo button....
im having an applet in which i can draw some lines,draw some rectangle's etc, using some buttons. i want to introduce a new button in my applet "undo". the function of this button is ,when clicked it will undo the (erase) lact action performed.
can anybody help me how to do this "undo" functionality?

[Manfred Leonhardt]

Hi Srinivas,
The example below might help you out.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
class JFrameDraw extends JFrame
{
  private int xLoc = 0;
  private int yLoc = 90;
  public JFrameDraw()
  {
    JPanel p = new JPanel();
    JButton b = new JButton( "Draw" );
    b.addActionListener(
      new ActionListener()
      {
        public void actionPerformed( ActionEvent ae )
        {
          xLoc += 10;
          yLoc += 10;
          DrawRect( xLoc, yLoc );
        }
      });
    p.add( b );
    b = new JButton( "Undo" );
    b.addActionListener(
      new ActionListener()
      {
        public void actionPerformed( ActionEvent ae )
        {
          DrawRect( xLoc, yLoc );
          xLoc -= 10;
          yLoc -= 10;
        }
      });
    p.add( b );
    getContentPane().add( p, "North" );
    setSize( 500, 500 );
    addWindowListener(
      new WindowAdapter()
      {
        public void windowClosing( WindowEvent we )
        {
          System.exit( 0 );
        }
      });
  }
  private void DrawRect( int x1, int y1 )
  {
    Graphics g = getGraphics();
    g.setColor( Color.black );
    [B]g.setXORMode( getBackground() );[/B]
    g.drawRect( x1, y1, 100, 100 );
  }
  public static void main(String[] args) 
  {
    new JFrameDraw().setVisible( true );
  }
}

The important line is show in bold.
Regards,
Manfred.

[Daniel Searson]

Draw all of your shapes etc. onto a BufferedImage or an offscreen Image created using the method in the Panel class. Before you draw anything onto the image, draw a copy of the image onto another offscreen image or BufferedImage. When the user hits Undo, simply revert to the last image copy that was made.
This is easily done with BufferedImage - take a look at the documentation for the API.
Hope that helps.