| 接口 | 描述 | 
|---|---|
| Elements | 
           对程序元素进行操作的实用方法。 
          | 
| Types | 
           用于操作类型的实用方法。 
          | 
除非在特定实现中另有规定,否则应该预期此程序包中方法返回的集合将被调用者不可修改,并且对并发访问是不安全的。
 除非另有说明,在此包中的方法将抛出一个NullPointerException如果给一个null说法。 
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