要声明静态属性，将关键字static放在可见度修饰符后面，直接位于属性变量前面。

To declare a static property, place the keyword static after the visibility modifier, directly in front of the property variable.

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$onlyOne 之前的静态修饰符将此变量关系到类本身。

The static modifier that precedes $onlyOne ties the variable to the class itself.

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注释是可以添加到代码中的修饰符，可以用于包声明、类型声明、构造函数、方法、字段、参数和变量。

Annotations are modifiers you can add to your code and apply to package declarations, type declarations, constructors, methods, fields, parameters, and variables.

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