什么是 PECS(生产者扩展消费者超级)? [英] What is PECS (Producer Extends Consumer Super)?

问题描述

我在阅读泛型时遇到了 PECS(Producer extends 和 Consumer super 的缩写).

I came across PECS (short for Producer extends and Consumer super) while reading up on generics.

谁能向我解释一下如何使用 PECS 来解决 extends 和 super 之间的混淆?

Can someone explain to me how to use PECS to resolve confusion between extends and super?

推荐答案

tl;dr:是从集合的角度来看.如果你只从一个泛型集合中提取项目，它是一个生产者，你应该使用extends；如果你只往里面塞东西，它就是一个消费者，你应该使用super.如果您使用相同的集合同时执行这两项操作，则不应使用 extends 或 super.

tl;dr: "PECS" is from the collection's point of view. If you are only pulling items from a generic collection, it is a producer and you should use extends; if you are only stuffing items in, it is a consumer and you should use super. If you do both with the same collection, you shouldn't use either extends or super.

假设您有一个将事物集合作为其参数的方法，但您希望它比仅接受一个Collection 更灵活.

Suppose you have a method that takes as its parameter a collection of things, but you want it to be more flexible than just accepting a Collection<Thing>.

案例 1:您想浏览集合并处理每个项目.
那么列表是一个生产者，所以你应该使用一个Collection.

Case 1: You want to go through the collection and do things with each item.
Then the list is a producer, so you should use a Collection<? extends Thing>.

推理是 Collection 可以包含 Thing 的任何子类型，因此当您执行操作时，每个元素都将表现为 Thing.(实际上您不能向 Collection 添加任何内容(除了 null)，因为您无法在运行时知道 Thing 的 特定 子类型> 收藏.)

The reasoning is that a Collection<? extends Thing> could hold any subtype of Thing, and thus each element will behave as a Thing when you perform your operation. (You actually cannot add anything (except null) to a Collection<? extends Thing>, because you cannot know at runtime which specific subtype of Thing the collection holds.)

案例 2:您想向集合中添加内容.
那么列表是一个消费者，所以你应该使用一个Collection.

Case 2: You want to add things to the collection.
Then the list is a consumer, so you should use a Collection<? super Thing>.

这里的推理是不像 Collection，集合 总是可以保存一个 Thing.在这里，您不关心列表中已有的内容，只要它允许添加 Thing 即可；这是什么?超级事物保证.

The reasoning here is that unlike Collection<? extends Thing>, Collection<? super Thing> can always hold a Thing no matter what the actual parameterized type is. Here you don't care what is already in the list as long as it will allow a Thing to be added; this is what ? super Thing guarantees.

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