Java Thread Programming 1.8.3 - Inter-thread Commu

CubbyHole Example
The class CubbyHole (see Listing 8.9) simulates a cubbyhole. A cubbyhole is a slot that can have only one item in it at a time. One thread puts an item into the slot and another thread takes it out. If a thread tries to put an item into a cubbyhole that is already occupied, the thread blocks until the slot is available. If a thread tries to remove an item from an empty cubbyhole, the thread blocks until an item is added. In this example, the slot is a reference to an object. This technique allows objects to be handed off from one thread to another in a thread-safe manner.
CubbyHole是指一個通道，某一時刻只能容納一個東西，一個線程放，另一個線程取。如果已經有東西，不能再放，直到東西被取出。如果沒有東西，則不能取，直到放進去新東西。下面給出一個安全執行緒的解決方案：
CubbyHole.java
/*
 * Created on 2005-7-14
 *
 * Java Thread Programming - Paul Hyde
 * Copyright ? 1999 Sams Publishing
 * Jonathan Q. Bo 學習筆記
 *
 */
package org.tju.msnrl.jonathan.thread.chapter8;
/**
 * @author Jonathan Q. Bo from TJU MSNRL
 *
 * Email:jonathan.q.bo@gmail.com
 * Blog:blog.111cn.net/jonathan_q_bo
 *      blog.yesky.net/jonathanundersun
 *
 * Enjoy Life with Sun!
 *
 */
public class CubbyHole {
    private Object obj;
    public CubbyHole() {
        obj = null;
    }
  
    public synchronized void putIn(Object obj) throws InterruptedException{
        print("putIn() ... begin");
      
        while(this.obj != null){//等待，直到資料被取出
            print("putIn() wait ... begin");