My wait() thread never wakes up with notify()

Question

In the code below, I'm trying to wake up the DispatchThread whenever the add method is called in the PriorityQueue. For some reason the notify() method is not unblocking the thread. All I can figure is that the reference to the thread in notify() is incorrect. Can anyone find the problem with the code?
import java.util.TreeSet;
public class PriorityQueue{
  private TreeSet queue;
  private DispatchThread d_Thread;
  private Thread d_thread;
  //default constructor
     PriorityQueue()
       queue = new TreeSet();
     d_Thread = new DispatchThread(this);
     public  void add(Object anobject)
    synchronized (this.d_Thread)
        queue.add(anobject);
      this.d_Thread.notify();
      System.out.println(anobject+" added to queue now");
      try
        Thread.sleep(500);
      catch (InterruptedException e){}
     static void main(String args[])
          PriorityQueue intTest = new PriorityQueue();
          intTest.add(new Integer(25));
          intTest.add(new Integer(35));
          intTest.add(new Integer(6));
class DispatchThread implements Runnable{
  private PriorityQueue queue;
  private Thread d_Thread;
  public DispatchThread(PriorityQueue queue)
    this.queue = queue;
    d_Thread = new Thread(this);
    d_Thread.start();
  public void run()
    while(true)
      synchronized (this.d_Thread)
        try
          System.out.println("about to wait - dispatch thread");
          this.d_Thread.wait();
          System.out.println("just woke up dispatch thread"); //never makes it this far
//do some dispatching operations here
        catch(InterruptedException ie){}
}

Answer 1

When you call the wait method, you use the thread you create in the constructor of DispatchThread.
When you call the notify method, you use another lock, the Dispatch object itself.
And notify and wait are methods defined in Object. It's rarely you would use a thread object as a lock.
You can make you code work if you do this in DispatchThread:
synchronized (this) {
  try {
    System.out.println("about to wait - dispatch thread");
    wait();
}because now you use the same lock both places.