Java 泛型(通配符) [英] Java Generics (Wildcards)

问题描述

我有几个关于 Java 中的通用通配符的问题:

I have a couple of questions about generic wildcards in Java:

1. List<有什么区别?扩展 T> 和 List?

什么是有界通配符，什么是无界通配符?

What is a bounded wildcard and what is an unbounded wildcard?

推荐答案

在你的第一个问题中， 和 是有界通配符的例子.无界通配符看起来像 ，基本上意味着 .它松散地意味着泛型可以是任何类型.有界通配符( 或 )通过说它必须extend 特定类型( 称为上限)，或者必须是特定类型的祖先(代码>被称为下界).

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 教程在文章 通配符和通配符带来更多乐趣.

The Java Tutorials have some pretty good explanations of generics in the articles Wildcards and More Fun with Wildcards.

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