This class has a number of subclasses that can construct images from different sources.

这个类有许多可以从不同来源构造图像的子类。

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Aggregating allows any number of extensions to be added, without the need for defining all possible combinations of subclasses.

聚合允许添加任意数量的扩展，而无需定义子类的所有可能的组合。

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For example, a method that operates on Numbers might only want to accept instances of number or its subclasses.

比如，一个操作各种Number类的方法可能仅仅需要接收Number类或者它的子类对象。

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