Update method

Question

I am writing a simple test application that plots a line depending on some values read from an array. I dont want to redraw the full line everytime a new point is added to the end of the line, instead i just want to leave the points already plotted and just draw a line between the last point and the next point in the array. Here is the code. when i run the code only the last point(line) gets drawn. i read that if you overwrite the update() method the screen should not be cleared.
public class NewPanel extends JPanel {
     int[] array;
     int ii = 0;
     //Graphics g;
     public NewPanel(){
          array = new int[200];
          for(ii = 0; ii <200; ii++ ){
               array[ii] = 100;
               repaint();
          this.setVisible(true);
          this.setBackground(Color.BLACK);
     public void update(Graphics g){
          paint(g);
     public void paint(Graphics g){
          g.setColor(Color.BLACK);
          g.drawLine(ii,array[ii],ii + 1,array[ii + 1]);
}

Answer 1

1/ Calling repaint puts an event onto the window manager's queue. When the window manager's thread gets time, the window is repainted.
The loop increments ii to 200 and puts 200 event onto the queue in a very short space of time.
By the time the window manager gets to take the first event off the queue and call update(), ii already is 200.
To see the update happen, you need to put your animation into a seperate thread, and incorproate a delay in it.
2/ update is called when the background is to be repainted by the component. Overriding it to call paint means that the background is not repainted by the component. It does not mean that anything you draw will stay on the window.
Different platforms apply different amounts of buffering to the window, so what you have drawn may persist until the panel is resized, of it may not persist at all. It may get overwritten if the windon gets covered, it may not.
Swing components override update to delegate drawing the background to paintComponent() anyway, so for swing you only ever need override paintComponent().
To get a persistant image, you would use an Image as a double buffer.
But in this case, as the image data is already in an integer array, the easier thing is to use the drawPolyline method.
The following shows the update happening as data is set by a separate thread, and allows the call to clear the background to be switched on by passing in a command line argument:import java.awt.*;
import javax.swing.*;
import java.util.Random;
public class NewPanel extends JPanel {
  int[] xPoint;
  int[] yPoint;
  int numberPoints;
  boolean clear;
  static final int MAX_POINTS = 100;
  static final int STEP_SIZE = 4;
  static final int LEFT = 12;
  static final int RING_RADIUS = 6;
  public NewPanel (boolean clear){
    xPoint = new int[MAX_POINTS];
    yPoint = new int[MAX_POINTS];
    this.setVisible(true);
    this.setBackground(new Color(255, 255, 220));
    this.clear = clear;
  public void animate () {
    new Thread (new Runnable () {
      public synchronized void run () {
        try {
          Random random = new Random();
          numberPoints = 0;
          while (numberPoints<MAX_POINTS) {
            // some random line data
            if (numberPoints==0) {
              yPoint[numberPoints] = getHeight()/2;
            } else {
              yPoint[numberPoints] = yPoint[numberPoints-1] + 10 - random.nextInt(21);
            xPoint[numberPoints] = STEP_SIZE*numberPoints + LEFT;
            numberPoints++;
            repaint();
            Thread.sleep(100);
        } catch (InterruptedException iex) {
    }).start();
  static final BasicStroke FAT_STROKE = new BasicStroke (3);
  public void paintComponent (Graphics g) {
    // if a command arg is given, clear the background
    if (clear) {
      super.paintComponent(g);
    // draw the line
    if (numberPoints>0) {
      // make it pretty
      Graphics2D g2D = (Graphics2D)g;
      g2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                           RenderingHints.VALUE_ANTIALIAS_ON);
      g2D.setStroke(FAT_STROKE);
      g2D.setColor(Color.black);
      // actually draw it
      g2D.drawPolyline(xPoint, yPoint, numberPoints);
      // highlight current point- if many rings show
      // then the background isn't cleared
      g2D.setColor(Color.red);
      g.drawOval(xPoint[numberPoints-1] - RING_RADIUS,
                 yPoint[numberPoints-1] - RING_RADIUS,
                 2*RING_RADIUS,
                 2*RING_RADIUS);
  public static void main (String[] args) {
    JFrame frame = new JFrame();
    NewPanel panel = new NewPanel(args.length>0);
    frame.getContentPane().setLayout(new BorderLayout());
    frame.getContentPane().add(panel, BorderLayout.CENTER);
    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.setSize(2*LEFT+MAX_POINTS*STEP_SIZE, 300);
    frame.setVisible(true);
    panel.animate();
}Pete