List<? extends Number> list和List<Number>的区别
If wanting a List that can store objects of all subclasses of Number, you can just use List<Number>.That is not the same as List<? extends Number>, which is a List whose members can only be a specific subclass of Number.
(When I say "subclass of Number", I include Number itself ... obviously?)
http://www.coderanch.com/t/383399/Java-General/java/use-add-method-List-extends
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