顶级模块声明会将整个文件包括在模块中。

A top-level module declaration includes the whole file in the module.

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其次，模块声明要公开哪些包，因此没有公开的包是私有的。

Second, a module declares which packages it exposes, so packages that are not exposed are private.

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顶级模块声明只能作为文件中的第一个声明出现。

A top-level module declaration can appear only as the first declaration in a file.

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