More formally, sets contain no pair of elements e1 and e2 such that e1.equals (e2), and at most one null element.

更准确地说，set不包含成对的e1和e2元素，因此如果e1 . equals (e2)，那么最多包含一个null元素。

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An element consists of a pair of tags and the content in between.

元素由一对标记和其中的内容组成。

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Oncreate means that this pair of attributes will only be synchronized during the creation of a new object or element.

oncreate意味着这一对属性只是在新对象或者元素的创建期间才会得到同步化。

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