< 和有什么区别?超级E>和 <?扩展E>? [英] What is a difference between <? super E> and <? extends E>?

问题描述

<有什么区别?super E> 和 ?

例如，当您查看类 java.util.concurrent.LinkedBlockingQueue 时，构造函数具有以下签名:

For instance when you take a look at class java.util.concurrent.LinkedBlockingQueue there is the following signature for the constructor:

public LinkedBlockingQueue(Collection<? extends E> c)

对于方法之一:

public int drainTo(Collection<? super E> c)

推荐答案

第一个 () 说它是某种类型，它是 E 的祖先(超类)";;第二个()说它是某种类型，它是 E 的子类".(在这两种情况下，E 本身都可以.)

The first (<? super E>) says that it's "some type which is an ancestor (superclass) of E"; the second (<? extends E>) says that it's "some type which is a subclass of E". (In both cases E itself is okay.)

所以构造函数使用了 ?扩展 E 形式，因此它保证当它从集合中获取值时，它们都将是 E 或某个子类(即它是兼容的).drainTo 方法试图将值放入集合中，因此集合必须具有 E 或超类的元素类型.

So the constructor uses the ? extends E form so it guarantees that when it fetches values from the collection, they will all be E or some subclass (i.e. it's compatible). The drainTo method is trying to put values into the collection, so the collection has to have an element type of E or a superclass.

举个例子，假设你有一个这样的类层次结构:

As an example, suppose you have a class hierarchy like this:

Parent extends Object
Child extends Parent

和 LinkedBlockingQueue.您可以在 List 中构造这个传递，它将安全地复制所有元素，因为每个 Child 都是一个父级.您无法传入 List