Why is my program not showing background color change after an interval?

Question

I have been trying for hours to figure out my mistake in a simple program in java. I wish to display the change in backgroundcolor after 200 ms so I wrote this program but it's not showing correct output.

It's a long program ahead thanks in advance for reading.

public class bgColor extends Applet implements Runnable
{
     Thread timer=null;
     int c,flag;
  public void init()
  {
    c=0;
    setBackground(new Color(255,255,255));  
  }
  public void start()
  {
     if(timer==null)
    {
     timer=new Thread(this);
     timer.start();
    }
  }
 public void paint(Graphics g)
 {
  switch(c)
  {
  case 0:setBackground(Color.white);
         break;
  case 1:setBackground(Color.blue);
         break;
  case 2:setBackground(Color.green);
         break;
  case 3:setBackground(Color.red);
         break;
  }
 if(c==4)
 {
  flag=0;
 }
 else if(c==0)
 {
  flag=1;
 }
 if(flag==0)
 {
   c--;
 }
 else if(flag==1)
 {
   c++;
 }
}
public void stop()
{
   timer=null;
}
public void run()
{
   if(timer!=null)
   {
         repaint();
         c++;
   try
   {
         Thread.sleep(200);
   }
   catch(InterruptedException e){}
   }
         timer=null;
}
}

I basically set up a switch for changing background Color after certain intervals but it's showing only red Color throughout.

It would also be of enormous help if someone could suggest where I can read up how the program runs step by step like I think probably init(), then start() then run() then paint() then stop() but it's purely my guess. I need an authentic source.

Answer 1

Have you tried printing c ?

The thread runs more often than paint since even if you call repaint a million times, it only paints at most once per frame. The sleep helps, but it only reduces the risk of a race condition instead of eliminating the issue.

You also have a race condition where c can go above 4 and flag will never toggle. Try using c >= 4 and c <= 0 instead to avoid this. Based on your description, this is probably your issue.