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Problem in incrementing

  Asked By: Richard    Date: May 28    Category: Java    Views: 537
  

I have one problem in thread programming.
I have developed program in which one thread
increment by one and the other should decrease
by one. Thereafter they should display on the screen
. However, on the screen number is incremented twice and
decremented twice and not one by one
Can anyone suggest to do it ?


import java.applet.*;
import java.awt.*;

public class AnimationApplet15 extends Applet implements Runnable
{
Thread th1,th2;
private Image im = null;
private boolean animate = true;
boolean b1,b2;
int n1,n2;
public AnimationApplet15()
{

}
public void init()
{
resize(500, 250);
im = createImage(500,250);
b1=true;b2=false;
n1=40;n2=0;
}

public void paint(Graphics g)
{
if ( im!= null)
g.drawImage(im, 0,0, null);
}
public void start()
{
if (th1== null)
{
th1 = new Thread(this);
th1.start();
}
if (th2== null)
{
th2 = new Thread(this);
th2.start();
}
}
public void stop()
{
if (th1!= null)
{
th1.stop();
th1= null;
}

if (th2!= null)
{
th2.stop();
th2 = null;
}
}

public void run()
{

// Get an instance of java.awt.Graphics to draw on image
Graphics display = im.getGraphics();
// Set font for display
Font textFont = new Font("Arial", Font.PLAIN, 20);
display.setFont(textFont);
display.setColor (Color.yellow);
display.fillRect (0,0, 500, 250);
display.setColor (Color.gray);

while (true)
{
if(b1 == true)
{
try
{
synchronized(this)
{
display.setColor (Color.yellow);
display.fillRect (0,0, 500, 250);
display.setColor (Color.gray);
n1--;
display.drawString ("Value of First
Number: "+n1, 100, 100);
display.drawString ("Value of Second
Number: "+n2, 150, 150);
repaint();
th1.sleep(1000);
}
b1 = false;
b2= true;
wait();
notify();
}catch(Exception e) {}
}
if(b2 == true)
{
try
{
synchronized(this)
{
display.setColor (Color.yellow);
display.fillRect (0,0, 500, 250);
display.setColor (Color.gray);
n2++;

display.drawString ("Value of First
Number: "+n1, 100, 100);
display.drawString ("Value of Second
Number: "+n2, 150, 150);
repaint();
th2.sleep(1000);
}

b1= true;
b2 = false;
wait();
notify();
}catch(Exception e) {}
}
}//end of while
}
}

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2 Answers Found

 
Answer #1    Answered By: Sally Pierce     Answered On: May 28

Take a look at this one. By the way, feel free to write two different threads and see how observer can help with this. This code looks much like mandarin to me.

 
Answer #2    Answered By: Erma Henry     Answered On: May 28

Well. I read your code. The problem  is that you depend on how Java chooses
one of waiting threads (to enter monitor) when it receives a notify()
signal; The thing thats is prohibited in Java.

Its is interesting that on JDK 1.5 is seems to work correctly as is (i.e.
increment/decrements by 1) but in JDK 1.6 step changes to 2. This
clarifies that they have different thread/monitor entrance policy.

 
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