Synchronous painting

Question

Hello!
I need help in writing a program which uses a synchronous painting. My program looks like this:
import javax.swing.*;
import java.awt.*;
import java.util.*;
public class Draw extends JPanel{
private JFrame frame = new JFrame ();
public Draw (){
frame.getContentPane ().add (this,BorderLayout.CENTER);
frame.pack ();
frame.setSize (new Dimension (300,200));
frame.setVisible (true);
public static void main (String[] args)
{Draw draw = new Draw ();}
public void paintComponent (Graphics gr){
super.paintComponent (gr);
gr.setColor (Color.blue);
Random r = new Random(System.currentTimeMillis ());
for(int i=0 ; i<2000000 ; i++){
int x1 = r.nextInt (this.getWidth ());
int y1 = r.nextInt (this.getHeight ());
int x2 = r.nextInt (this.getWidth ());
int y2 = r.nextInt (this.getHeight ());
gr.drawLine (x1,y1,x2,y2);
How can I modify the code so that I see every line drawn after I call drawLine(...) ?
Thanks!

Answer 1

It inpossible to tell from your query whether you want to progress towards a solid blue panel or only have one line showing at a time.
The following code shows both. It is rigged toward showing all lines without clearing. To change to one line showing follow the comment directionsimport javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.util.Random;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Draw extends JPanel implements ActionListener
   private JFrame frame = new JFrame ();
   private int x1 = 0,x2 = 0,y1 = 0,y2 = 0;
   private Random r;
   private Graphics g;       //comment out for single line
   public Draw ()
      super(false); //not double buffered
      r = new Random(System.currentTimeMillis ());
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setContentPane(this);
      frame.pack ();
      frame.setSize (new Dimension (300,200));
      frame.setVisible (true);
      drawLines();
   private void drawLines()
      g = getGraphics();            //comment out for single line
      g.setColor (Color.blue);      //comment out for single line
      int delay = 30; //milliseconds - adjustable
      //forget 2 million, this will run forever if you let it
      new javax.swing.Timer(delay, this).start();
   public static void main (String[] args)
   {Draw draw = new Draw ();}
   public void actionPerformed(ActionEvent evt)
      x1 = r.nextInt (this.getWidth ());
      y1 = r.nextInt (this.getHeight ());
      x2 = r.nextInt (this.getWidth ());
      y2 = r.nextInt (this.getHeight ());
      g.drawLine (x1,y1,x2,y2);      //change to repaint(); for single line
   public void paintComponent (Graphics gr)
      super.paintComponent (gr);
      drawLines();            //comment out for single line
      gr.setColor (Color.blue);
      gr.drawLine (x1,y1,x2,y2);
}