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

问题描述

  public interface Foo< E> {} 
 
 public interface Bar< T> {} 
 
 public interface Zar<？> {} 
  

以上所有三点之间的区别是什么，他们称这种类型为什么或接口声明在Java？

解决方案

那么前两个之间没有区别 - 他们只是使用不同的名称 type parameter （ E 或 T ）。

第三个不是有效的声明 - ？用作通配符，用于提供类型参数，例如列表与LT;？> foo = ... 表示 foo 指的是某种类型的列表，但我们不知道是什么。

所有这些都是泛型，这是一个相当大的话题。您可能希望通过以下资源了解它，但当然还有更多可用：

• 关于泛型的Java教程 ：//download.oracle.com/javase/1.5.0/docs/guide/language/generics.htmlrel =noreferrer>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;更多供参考）
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I come across Java code like this:

public interface Foo<E> {}

public interface Bar<T> {}

public interface Zar<?> {}

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

解决方案

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

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.

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 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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