What is PECS (Producer Extends Consumer Super)?

2021-6-3 anglehua

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?

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.

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 (except null) 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.

The principles behind this in computer science is called

  • Covariance: ? extends MyClass,
  • Contravariance: ? super MyClass and
  • Invariance/non-variance: MyClass

The picture below should explain the concept. Picture courtesy: Andrey Tyukin

Covariance vs Contravariance


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