To increase the usefulness of our generic code, instead of specifying the exact type of a generic type parameter, we can specify an upper (or lower) bound instead.

要提高泛型代码的有效性，可以指定一个上限（或下限），而不是指定某个泛型类型参数的精确类型。 这可以使用有界通配符来实现，它的形式为 “?

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It contains an invariant — that the lower bound is always less than or equal to the upper bound.

它包含了一个不变式——下界总是小于或等于上界。

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That is because in any lower income group, with both an upper and lower bound to the incomes included, the average or mean income will always be much closer to the median.

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