为了实现该侦听器类，我们将在主资源管理器窗口中编码一个匿名类。
To implement this listener class, we'll code an anonymous class in the main Explorer Window.
每当打开一个聊天窗口，都会向伙伴列表添加一个侦听器。
Each time a chat window was opened, it would add a listener to the buddy list.
同时，每次当伙伴列表向列表内的侦听器发信号，都会浪费时间去通知那些本来已经被关闭的聊天窗口。
Also, each time the buddy list would signal its list of listeners, it would waste time notifying the chat Windows that had previously been closed.
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