什么是PECS（制片人扩大消费者超级）？ [英] What is PECS (Producer Extends Consumer Super)?

问题描述

我遇到了PECS（简称​​ Producer extends 和Consumer super ））阅读泛型。

有人可以向我解释如何使用PECS解决 extends 和 super之间的混淆？

解决方案

tl; dr：收藏的观点。如果你只是从一个通用集合中抽取项目，它是一个生产者，你应该使用 extends ;如果你只是填充物品，它是一个消费者，你应该使用 super 。如果你同时使用同一个集合，则不应该使用 extends 或 super 。

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假设你有一个方法以参数为参数集合，但你希望它比接受 Collection< Thing> 。

案例1：您想要浏览收藏并对每件物品进行操作。

然后，列表是生产者，因此您应该使用集合<？扩展Thing> 。

推理是一个 Collection< ;?扩展Thing> 可以包含 Thing 的任何子类型，因此每个元素将表现为 Thing 集合< ;? extends Thing> 中添加任何东西，因为你不能在运行时知道特定的子类型>物品集合成立。）

情况2：您想要将物品添加到集合中。 >
然后列表是一个消费者，所以您应该使用集合<超级事物> 。

这里的推理与 Collection <？ extends Thing> ， Collection <？不管实际的参数化类型是什么，超级事物> 总是可以持有 Thing 。在这里，只要它允许添加 Thing ，您就不会在乎列表中的内容;这是？超级事情保证。

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

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

解决方案

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.

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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>.

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>.

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 to a Collection<? extends Thing>, because you cannot know at runtime which specific subtype of Thing the collection holds.)

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

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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