When you create your own key object for use in a Hashtable, you must override the Object.equals() and Object.hashCode() methods since Hashtable uses a combination of the key's hashCode() and equals() methods to store and retrieve its entries quickly. It's also a general rule that when you override equals(), you always override hashCode().

重写equals():
     1.没有重写equals()意味着什么？
     2. 设计equals()方法
     3.equals()约定

 重写hashCode():
     1.理解散列码
     2.设计hashCode()
     3.hashCode()约定重写equals():