What is the Runnable representation in java.util.function?

Question

I was wondering recently as Runnable is very often used in a functional context. At the same time according to javadoc its semantical meaning is very close to multithreading while it is not aways used in such context:

The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread. The class must define a method of no arguments called run. This interface is designed to provide a common protocol for objects that wish to execute code while they are active. For example, Runnable is implemented by class Thread. Being active simply means that a thread has been started and has not yet been stopped.

Is Runnable equivalent to Supplier<Void> ? Or is Runnable equivalent to Function<Void,Void> and why is it not Supplier then ? How does Runnable align with java.util.function package's already present functional interfaces.

`Runnable` is a functional interface by itself, isn't it? There is no need for an equivalent for it in `java.util.function`. — Sweeper, Jun 29 '19 at 09:48
@Sweeper this is not the point of the question.Of course it is. — Alexander Petrov, Jun 29 '19 at 09:49
@Sweeper it is a question of semantics. I can use napkins as baby napkins or as kitchen paper. But even if the material is the same, they come with different names and flavors. — Alexander Petrov, Jun 29 '19 at 09:52
I edited the question I was meaning Supplier not Consumer. So Why it is not supplier or function? — Alexander Petrov, Jun 29 '19 at 09:56
`Runnable` is much older than the functions, and was not retrofitted into the new framework. You are overthinking this. — Thorbjørn Ravn Andersen, Jun 29 '19 at 10:15
@AlexandarPetrov I think `Runnable` is a semantically fine name for the context in which it would be used. It is a functional interface which neither produces something or consume something, so one could say that the method is free of any context, essentially being a block of code to run. I cannot think of any scenario where this name is actually inaccurate. — MC Emperor, Jun 29 '19 at 11:22

score 16 · Accepted Answer · edited Jun 29 '19 at 19:35

16

Runnable is it. There was an intentional choice (see Brian Goetz here) not to add a functional interface to the java.util.function package that is effectively equivalent to Runnable. One of the major ideas behind Java's implementation of lambdas (the idea that all uses of it must be where some functional interface is required, and that the lambda then becomes an implementation of that functional interface) is intended to ensure that Java post-lambda is more or less compatible with APIs designed pre-lambda, without any real need to release an incompatible 'v2' edition with a more Java8+ style API for the majority of existing APIs out there.

Introducing a variant of Runnable solely for it to exist in the java.util.function package would run counter to that idea.

Note that Runnable and Supplier<Void> and Function<Void, Void> might feel very similar but are utterly incompatible in javaland. Void isn't special; it's just a type, same as any other. For example, if you are making a Supplier<Void> lambda, you need to actually return something, and you can only return null; there is no non-hacky way to make instances of the Void type. Whereas for void, you don't have to (and can't).

edited Jun 29 '19 at 19:35

MC Emperor

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answered Jun 29 '19 at 10:47

rzwitserloot

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Thanks for the answer. Just curios if Runnable was positioned in the j.l.function what would it be named still Runnable ? – Alexander Petrov Jun 29 '19 at 10:57
Runnable, perhaps Executable (which, interestingly enough, is a pre-1.8 interface matching Supplier, but that one WAS re-imagined as Supplier. There are reasons for this (Executable is not in java.lang, and throws, which doesn't mesh with what all the j.l.function interfaces do). – rzwitserloot Jun 29 '19 at 14:26
It's actually `java.util.function` instead of `j.l.function`. – MC Emperor Jun 29 '19 at 19:09
Gracias for spotting that and fixing it, @MCEmperor :) – rzwitserloot Nov 04 '20 at 15:57

What is the Runnable representation in java.util.function?

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