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

问题描述

我遇到过这样的 Java 代码:

I come across Java code like this:

public interface Foo<E> {}

public interface Bar<T> {}

public interface Zar<?> {}

以上三者有什么区别，他们在Java中称这种类型的类或接口声明是什么?

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

嗯，前两个没有区别 - 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，用于提供类型参数，例如<代码>列表foo = ... 表示 foo 指的是某种类型的列表，但我们不知道是什么.

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，这是一个非常大的话题.您可能希望通过以下资源了解它，当然还有更多可用资源:

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:

• Java 泛型教程
• 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(大量而全面；但更多供参考)

• 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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