由于将值放入到比较器来决定两个元素的相对顺序，get - put原则告诉我们在这里需要使用一个超通配符。

Because we are putting values into the comparator to determine the relative ordering of two elements, the get-put principle tells us we want to use a super wildcard here.

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当您希望指定一个回调对象(例如一个比较器)或存放某个值的数据结构，可以使用下限通配符。

Lower-bounded wildcards show up when you want to specify a callback object, such as a comparator, or a data structure into which you are going to place a value.

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为了让模式作者更容易创建比较自然的内容模型，XML Schema 1.1引入了弱通配符(weakened wildcard)的概念。

To make it easier for schema authors to create more natural content models, XML schema 1.1 has introduced the concept of a weakened wildcard.

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