类中的常数字段类似于全局或函数常数，不同的只是它的作用范围是该类，而且它可以用多个修饰符来指定其可见性和用法。

A constant field in a class is similar to a global or function constant, except that it is scoped to the class and it can have various modifiers governing its visibility and usage.

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类本身就可作为全局作用范围变量的名字空间。

Classes are themselves worthy namespaces for scoping globally accessible variables.

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