什么是<？超级T>句法？ [英] What is <? super T> syntax?

问题描述

我无法理解以下语法：

public class SortedList< T extends Comparable< ? super T> > extends LinkedList< T >

我看到类SortedList扩展了LinkedList。我只是不知道是什么

I see that class SortedList extends LinkedList. I just don't know what

T extends Comparable< ? super T>

表示。

我对它的理解到目前为止，类型T必须是实现Comparable的类型......但是什么是< ？超级T> ？

My understanding of it so far is that type T must be a type that implements Comparable...but what is < ? super T >?

推荐答案

超级泛型与扩展相反。而不是说可比较的泛型类型必须是T的子类，它说它必须是T的超类。区别很重要因为extends告诉你你可以从类中得到什么（你至少得到了这个，也许是一个子类）。 super告诉你你可以把什么放进课堂（最多这个，也许是超类）。

super in Generics is the opposite of extends. Instead of saying the comparable's generic type has to be a subclass of T, it is saying it has to be a superclass of T. The distinction is important because extends tells you what you can get out of a class (you get at least this, perhaps a subclass). super tells you what you can put into the class (at most this, perhaps a superclass).

在这种特殊情况下，它所说的是类型必须实现其自身或其超类的可比性。所以考虑java.util.Date。它实现了 Comparable< Date> 。但是java.sql.Date怎么样？它还实现 Comparable< java.util.Date> 。

In this specific case, what it is saying is that the type has to implement comparable of itself or its superclass. So consider java.util.Date. It implements Comparable<Date>. But what about java.sql.Date? It implements Comparable<java.util.Date> as well.

如果没有超级签名，SortedList将不会能够接受java.sql.Date的类型，因为它不实现自身的Comparable，而是实现它自己的超类。

Without the super signature, SortedList would not be able accept the type of java.sql.Date, because it doesn't implement a Comparable of itself, but rather of a super class of itself.

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