About Wait() and notify() method

[Sanjeev Narula]

Can anyone explain about wait() and notify() method giving some example.

Thanks in advance.

[Chandra Bhatt]

Hi Sanjeev,

See the code below:

package threads;
 
public class WhyWait extends Thread{
    static int sum;
    /** Creates a new instance of WhyWait */
    public WhyWait() throws Exception{
        
    }
    public void run() {
        synchronized(this) { //comment this 
            for(int i=1; i<=10; i++) {
                sum =sum+i;
            }
           notify(); //comment this
        } //comment this
 
    }
    
    public static void main(String... args) throws Exception {
        WhyWait ww = new WhyWait();
        ww.start();
        
        synchronized(ww) { //Line 1
            try {
                ww.wait(); 
                System.out.println("sum = " + sum); 
            }catch(InterruptedException ie) { 
                ie.printStackTrace();
                System.exit(1);
            }
        } //Line 2        
            System.out.println("sum = " + sum); 
    }
}

Try to run this, with Line1 and Line 2 and without Line 1 and Line 2.
Without Line 1 - Line 2, the output is indeterministic but with wait(), the output is surely "sum = 55".
with wait, main waits for thread ww to notify it before it could print the
result.



See the book for more detail!


Regards,
cmbhatt

[m ali]

Hi Chandra,

Thanks for the nice example.

[Keith Lynn]

Here is another example.

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.regex.*;
 
public class Pie extends JApplet {
   private Thread[] threads;
   private JCheckBox[] buttons;
   private MyPanel[] panels;
 
   private class MyPanel extends JPanel implements Runnable {
      private Point upperLeftCorner;
      private Color color;
      private int startAngle;
      private boolean keepGoing;
      private boolean suspended;
      private int index;
 
      public MyPanel(Point upperLeftCorner) {
         this.upperLeftCorner = upperLeftCorner;
 
         setSize(100,100);
 
         setLocation(upperLeftCorner);
 
         keepGoing = true;
      }
 
      public void setColor(Color color) {
         this.color = color;
      }
 
      public void setIndex(int index) {
         this.index = index;
      }
 
      public void setSuspended(boolean suspended) {
         this.suspended = suspended;
      }
 
      public void paintComponent(Graphics g) {
         super.paintComponent(g);
 
         g.setColor(new Color(240,240,240));
 
         g.fillRect(0,0,getWidth(),getHeight());
 
         g.setColor(color);
 
         g.fillArc(0,0,100,100,startAngle,45);
      }
 
      public void run() {
         while (keepGoing) {
            Thread.yield();
 
            pause(1);
 
            synchronized (Pie.this) {
               while (suspended)
                  try {
                     Pie.this.wait();
                  } catch (InterruptedException ie) {
                  }
            }
 
            startAngle += 45;
 
            repaint();
         }
      }
   }
 
   public void init() {
      Container container = getContentPane();
 
      container.setLayout(null);
 
      Color[] colors = new Color[]{new Color(200,0,0),
                                   new Color(0,200,0),
                                   new Color(0,0,200),
                                   new Color(255,255,0)};
 
      panels = new MyPanel[]{new MyPanel(new Point(0,0)),
                             new MyPanel(new Point(200,0)),
                             new MyPanel(new Point(0,200)),
                             new MyPanel(new Point(200,200))};
 
      buttons = new JCheckBox[panels.length];
 
      ItemListener listener = new ItemEventHandler();
 
      for (int counter=0;counter<buttons.length;counter++) {
         buttons[counter] = new JCheckBox("Panel " + counter);
         buttons[counter].setSize(70,20);
         buttons[counter].setLocation(50+80*counter,150);
         buttons[counter].addItemListener(listener);
 
         container.add(buttons[counter]);
      }
 
      threads = new Thread[panels.length];
 
      for (int counter=0;counter<panels.length;counter++) {
         panels[counter].setColor(colors[counter]);
 
         panels[counter].setIndex(counter);
 
         container.add(panels[counter]);
 
         threads[counter] = new Thread(panels[counter]);
         threads[counter].start();
      }
   }
     
   private class ItemEventHandler implements ItemListener {
 
      public void itemStateChanged(ItemEvent e) {
         if (e.getSource() instanceof JCheckBox) {
            JCheckBox box = (JCheckBox)e.getSource();
 
            Pattern pattern = Pattern.compile("\\d");
 
            Matcher matcher = pattern.matcher(box.getText());
 
            if (matcher.find()) {
               int index = Integer.parseInt(matcher.group());
 
               panels[index].setSuspended(e.getStateChange() == ItemEvent.SELECTED);
 
               synchronized (Pie.this) {
                  Pie.this.notifyAll();
               }
            }            
         }
      }
 
   }
 
   private void pause(double seconds) {
      try {
         Thread.sleep((int)(seconds*1000));
      } catch (InterruptedException ie) {
      }
   }
}

You can see the applet in action at

http://home.comcast.net/~klynn47/Pie.html