TreeSet 类　　外部有序地遍历成员；附加实现了SortedSet, 支持子集等要求顺序的操作 成员要求实现caparable接口，或者使用 Comparator构造TreeSet。成员一般为同一类型。 -TreeSet orderly traversal outside the class members　additional implements SortedSet, and other requirements in order to support a subset of the members requested the operation to achieve caparable interface, or use a Comparator construction TreeSet. Members of the general for the same type.