Image flickers when repainting

Question

Hello,
I'm trying to make an applet where a line moves sideways as the slider is moved. The code below is working but the image flickers when repainting:
import java.awt.Graphics;
import java.awt.Color;
import java.awt.Scrollbar;
import java.awt.event.*;
public class testSlider extends java.applet.Applet implements AdjustmentListener {
  int w, h; // Applet width and height
  Scrollbar sb;
  boolean scrollChanged = false;
  int xUnit, yUnit;
  public void init(){
    //field variables initialization
    w  = this.getSize().width ;
    h  = this.getSize().height ;
    xUnit = (int) Math.round(0.1*w);
    yUnit = (int) Math.round(0.1*h);
    sb = new Scrollbar(Scrollbar.HORIZONTAL, xUnit, 1, xUnit, 9*xUnit);
    //add Scrollbar
    sb.setBounds(5, 9*yUnit,this.getWidth() -15, 15) ;
    this.setLayout(null);
    this.add(sb);
    sb.addAdjustmentListener(this ) ;
  public void paint(Graphics g){
    System.out.println("paint...") ;
    g.setColor(Color.GRAY ) ;
    g.fillRect(0,0,w, h);
    g.setColor(Color.BLACK) ;
    //set new Origin
    g.translate(w/2, 8*yUnit) ;
    if (scrollChanged ) {
      g.setColor(Color.YELLOW  ) ;
      g.drawLine(sb.getValue()-w/2, -7*yUnit, sb.getValue()-w/2, 0  );
  //AdjustmentListener method override
  public void adjustmentValueChanged(AdjustmentEvent e){
    scrollChanged=true;
    repaint();
Can someone tell me how to prevent this flickering?
Thank you very much.

Answer 1

g.fillRect(0,0,w, h);This needs to modify quite a few pixels so it is going to be on the slow side.
Try this: store the coordinates of the previous line you drew. When repainting, if you have drawn a line previously, first draw the old line with the background color, then draw the new line, and finally store the coordinates of the new line so you can paint over it later.
I haven't tried that myself though.
An alternate is to use double buffering. Google: java double buffering. But that may be slowish and CPU intensive because that too needs to touch a lot of pixels.