Java泛型（通配符） [英] Java Generics (Wildcards)
问题描述
在Java中我有几个关于泛型通配符的问题：
$ b
I have a couple of questions about generic wildcards in Java:
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列表与LT ;?延伸T>和列表< ;? super T>？
什么是有界通配符？什么是无界通配符？ b $ b
What is a bounded wildcard and what is an unbounded wildcard?
推荐答案
在第一个问题中，<扩展T> 和<？ super T> 是有界通配符的例子。无界通配符看起来像<？> ，并且基本上意味着< ;?扩展对象> 。它松散地表示泛型可以是任何类型。有界通配符（< ;? extends T> 或<？super T> ）对类型通过说它要么扩展特定类型（< ;? extends T> 被称为上限），要么必须是一个特定类型的祖先（< ;? super T> 被称为下限）。
In your first question, <? extends T> and <? super T> are examples of bounded wildcards. An unbounded wildcard looks like <?>, and basically means <? extends Object>. It loosely means the generic can be any type. A bounded wildcard (<? extends T> or <? super T>) places a restriction on the type by saying that it either has to extend a specific type (<? extends T> is known as an upper bound), or has to be an ancestor of a specific type (<? super T> is known as a lower bound).
Java教程在通配符中对泛型有一些非常好的解释， a>和更多有趣的通配符。
The Java Tutorials have some pretty good explanations of generics in the articles Wildcards and More Fun with Wildcards.
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