In this case, you could avoid the conditional logic by overloading the invoice method for different types.

在本例中，通过为不同类型重载invoice方法，您可以避免条件逻辑。

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If you define an invoice method on a class, you have no way of knowing that some other user of that class won't define a different, incompatible method also named invoice.

如果您在类上定义invoice方法，则您没有办法知道该类的一些其他用户会不会定义不同的、不兼容的方法，还命名为invoice。

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And this implementation is closed: After you compile the FulfillmentGenerator class, you can't extend the invoice method to new types without editing the source code and recompiling.

此实现是封闭的：在您编译FulfillmentGenerator类以后，在不编辑源代码和重新编译的情况下，您无法将invoice方法扩展到新的类型。

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