If a subclass of one of these classes needed to override the default behavior, it could do so by overriding the method.
如果这些类中的任何一个的子类需要覆盖默认行为，那么可以通过覆盖这个方法实现。
This means that no subclass of that class can declare a finalizer.
这意味着该类的任何子类都无法声明终结器。
The sample watchdog code (see Download) defines WatchedPeriodicThread as a subclass of PeriodicThread.
示例 watchdog 代码（参见 下载）定义 WatchedPeriodicThread 作为 PeriodicThread 的子类。
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