import java.util.*;

/** 
 * A queue. One thread calls push() to put an object on the queue. 
 * Another calls pop() to get an object off the queue. If there is no
 * data, pop() waits until there is some, using wait()/notify(). 
 * wait() and notify() must be used within a synchronized method or
 * block. In Java 5.0, use a java.util.concurrent.BlockingQueue instead.
 */
public class WaitingQueue<E> {
    LinkedList<E> q = new LinkedList<E>();  // Where objects are stored
    public synchronized void push(E o) {
        q.add(o);         // Append the object to the end of the list
        this.notifyAll(); // Tell waiting threads that data is ready
    }
    public synchronized E pop() {
        while(q.size() == 0) {
            try { this.wait(); }
            catch (InterruptedException ignore) {}
        }
        return q.remove(0);
    }
}