"E"，"T"和“"之间有什么区别? Java泛型? [英] What is the difference between 'E', 'T', and '?' for Java generics?

问题描述

我遇到了这样的Java代码:

I come across Java code like this:

public interface Foo<E> {}

public interface Bar<T> {}

public interface Zar<?> {}

以上三者之间有什么区别，它们在Java中如何称呼这种类型的类或接口声明?

What is the difference among all three of the above and what do they call this type of class or interface declarations in Java?

推荐答案

前两者没有区别-它们只是为 type参数使用了不同的名称(E或T).

Well there's no difference between the first two - they're just using different names for the type parameter (E or T).

第三个不是有效的声明-?用作通配符，在提供类型 argument 时使用，例如List<?> foo = ...表示foo引用某种类型的列表，但我们不知道什么.

The third isn't a valid declaration - ? is used as a wildcard which is used when providing a type argument, e.g. List<?> foo = ... means that foo refers to a list of some type, but we don't know what.

所有这些都是泛型，这是一个很大的话题.您可能希望通过以下资源来了解它，尽管当然还有更多可用的方法:

All of this is generics, which is a pretty huge topic. You may wish to learn about it through the following resources, although there are more available of course:

• 有关泛型的Java教程
• 泛型语言指南
• Java编程语言中的泛型
• Angelika Langer的 Java泛型常见问题解答(内容广泛，内容广泛，仅供参考)

• Java Tutorial on Generics
• Language guide to generics
• Generics in the Java programming language
• Angelika Langer's Java Generics FAQ (massive and comprehensive; more for reference though)

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