Why can't I explicitly return void from a method?

Question

void run() {     ...     if (done) return cancel();     ... } 

where cancel() return void. This won't compile... and I can almost understand why. But if I want to return a void from a void, why not? Instead, I end up writing something like this:

if (done) {     cancel();     return; } 

I'm not looking for code style suggestions, I want to know why Java expressly prohibits this type of void return. Any info is appreciated, thanks.

Answer 1

It's an interesting question. Since java enforces a return type (void is a return type) your first statement seems to make sense. I would take this only for convention. Since void is a placeholder and not an object, it was probably decided to leave it out for language coherency or compiler simplicity.

From JLS

A return statement with no Expression must be contained in the body of a method that is declared, using the keyword void, not to return any value (§8.4), or in the body of a constructor (§8.8).

further

To be precise, a return statement with no Expression always completes abruptly, the reason being a return with no value

Answer 2

A return statement with an expression returns the value of that expression. The type of cancel() is a void expression - it doesn't have a value.

Logically you want to execute cancel(), and then return - so that's what you have to say. The two actions (calling cancel() and then returning) are logically distinct.

Now Java could have a sort of "unit" type instead of void - but that would affect rather more than just return values.