Repaint a canvas

Question

Hi everybody,
I have to implement a frame in which you can draw some lines. Those lines are the edges of cells for a mobile network.
But when I drew a line, it disappeared immediately. How can I resolve this ?
The schematic is heavy to load, so I want to save it in a "virtual canvas", to make it easy to repaint the canvas. I don't know how do that.
Here is my code for the frame :
public class FAPShowMap extends Canvas implements MouseListener {
     private Graphics gr ;
     private Frame frame ;
     public FAPShowMap(Vector ed) {
          super() ;
          frame = new Frame("Carte des cellules et antennes") ;
          frame.setSize(600,600) ;
          frame.setResizable(false) ;
          frame.addWindowListener(new GenericWindowListener()) ;
          frame.add(this) ;
          frame.pack() ;
          this.addMouseListener(this) ;
          frame.setVisible(true) ;
          this.addNotify() ;
          paint(new Vector()) ;
     public void paint(Vector ed) {
          gr = this.getGraphics() ;
          gr.drawLine(3,3,200,200) ;
          this.paint(gr) ;
     public void mouseClicked(MouseEvent e) {
          this.repaint() ;
          gr.drawLine(50,50,600,100) ;
          this.paint(gr) ;
     public void mousePressed(MouseEvent e) {}
     public void mouseReleased(MouseEvent e) {}
     public void mouseEntered(MouseEvent e) {}
     public void mouseExited(MouseEvent e) {}
     public static void main(String[] args) {
          new FAPShowMap(null) ;
I will be very glad if you can help me
Thank you
Agni

Answer 1

Hi Agni,
Reread my previous post. In particular, it mentions the probable cause of your problem. Note that it's not caused by overloading the paint method; it's what you're doing inside the overloaded method.
Changing your overloaded paint method to be:public void paint(Vector ed)
   gr = this.getGraphics() ;
   gr.setColor(java.awt.Color.black);
   gr.drawLine(3,3,200,200) ;
}should work, but with a problem. Namely, when the component repaints itself, the line will be erased. Take a look at the tutorials for the explanation of why this happens and how to correct for it.
I strongly recommend seeking a definitive source for Java programming if you are just starting. The "Core" series from Sun MicroSystems Press is very good. Jumping in (especially with graphics) may cause more aggrevation than progress.
Regards,
Nick