泛型什么是<？>其实是指？ [英] Generics what does <?> actually mean?

问题描述

可能存在重复：

什么是List<？>在Java泛型？ -

Java泛型中的问号是什么意思？

b

道歉但试图搜寻<？> 。

关于Java泛型？我知道< A extends B> 和< A super B> ，但我从未见过这个问号

速记<？扩展对象> ，它也被称为无界通配符。因此，您可以在泛型中指定任何类型的对象。

例如， List 类声明为 List<？> ，因为它可以是你想要的任何东西的列表。

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资源：

• Effective Java - Generics.pdf


• Java语言规范 - 类型参数和通配符


• oracle.com - Java教程，通配符

Possible Duplicate:
What does List<?> mean in java generics?
What does the question mark in Java generics' type parameter mean?

Apologies but it was difficult trying to search for <?>.

What does mean in regards to Java generics? I understand <A extends B> and <A super B>, but I have never seen this question mark on its own before.

解决方案

<?> is a shorthand for <? extends Object>, it's also known as an unbounded wildcard. So you can specify any type of object in your generic.

For example the List class is declared as List<?>, because it can be a list of anything you want.

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Resources:

• Effective java - Generics.pdf
• Java language specification - Type Arguments and Wildcards
• oracle.com - Java tutorials, wildcards

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