最为重要的是，能够轻易地调用在运行时过后所添加的任意行为。
Most important, you can easily invoke any arbitrary behavior, one that has been added after run time.
当想要基于未知规则给一个类添加任意行为时，则需要使用一项不同的技术。
When you want to add arbitrary behavior to a class, based on unknown criteria, you need to use a different technique.
异常处理是每个应用程序的基本要求，以便处理应用程序的任意意外行为。
Exception handling is the basic requirement of every application to handle any unexpected behavior of the application.
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