_ 类型、结构、联合体和枚举定义。
这个枚举定义了注释类型可应用的不同程序元素。
This enum defines the various program elements that an annotation type can target. Listing 9 show the ElementType enum in its entirety.
当枚举定义的简单类型使用常数的特殊列表时，就会需要这种类型了。
It is needed when there should be a specific list of constant values to be used for the simple type defined by the enumeration.
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