A great Thread example from Nikos Pugunias(http://nikojava.wordpress.com/)

Hi Zhou!
You should be 100% certain that wait() is a method of Object.Consider 2 threads: Thread A carries out a loooong task. Thread B must wait for A's result to go on with its job.This trick between threads happens with a single object that both threads can access: a token. it's like the piece of wood that the athletes running 4x100m in the Olympics: Runner 1 starts, Runner 2 waits for the token from Runner 1, as soon as he gets it Runner 2 starts and so on...

//Here's the first runner,
class Runner1 extends Thread { }

//that holds a wooden stick.

class Runner1 extends Thread {
   Object stick;
   public Runner1(Object stick) { this.stick = stick; }
}

//This runner has a loooong task to do,

class Runner1 extends Thread {
   Object stick;
   public Runner1(Object stick) { this.stick = stick; }
   public void run() { // perform a loooooong task here }
}

//and then he will give the token to another runner.

class Runner1 extends Thread {
   Object stick;
   public Runner1(Object stick) { this.stick = stick; }
   public void run() {
      synchronized (stick) {
         // perform a loooooong task here stick.notify();
      }
   }
}

//Here's the second runner
class Runner2 extends Thread { }

//that can also receive a wooden stick.

class Runner2 extends Thread {
   Object stick;
   public Runner2(Object stick) { this.stick = stick; }
}

//This second runner has also something to do,

class Runner2 extends Thread {
   Object stick;
   public Runner2(Object stick) { this.stick = stick; }
   public void run() {
      // I want to do something after
      // the first runner has finished.
   }
}

// but he'll have to wait for the token first.

class Runner2 extends Thread {
   Object stick;
   public Runner2(Object stick) { this.stick = stick; }
   public void run() {
      // wait for Runner1
      synchronized (stick) {
          try { stick.wait();
          } catch (InterruptedException e) { }
      }
      // I want to do something
      //after the first runner has finished.
   }
}

//+++++++++++++++++++++++++++
//Here's the complete code.
//+++++++++++++++++++++++++++

public class Run {
   public static void main(String[] args) {
      Object token = new Object();
      new Runner1(token).start();
      new Runner2(token).start();
   }
}

class Runner1 extends Thread {
   Object stick;
   public Runner1(Object stick) { this.stick = stick; }
   public void run() {
      synchronized (stick) {
        // perform a loooooong task here stick.notify();
      }
   }
}

class Runner2 extends Thread {
   Object stick;
   public Runner2(Object stick) { this.stick = stick; }
   public void run() {
      // wait for Runner1
      synchronized (stick) {
         try { stick.wait();
         } catch (InterruptedException e) { }
      }
      // at this point Runner1 is guaranteed
      // to have finished his work
   }
}

//*************** Conclusion: ***************//
Using a single object, two threads may schedule in what order they will run in accuracy.In the question you have posted, there two threads: Job and main. They schedule themselves based on the [B]thread[B] instance.It's just a choice to use this instance instead of any other object.Review:When thread A calls token.notify() it says: You may take the token.When thread B calls token.wait() it is waiting to take the token.Always synchronize on the token.