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Java 21 makes me actually like Java again (wscp.dev)

submitted 1 year ago by [email protected] to c/programming

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[–] BatmanAoD 4 points 1 year ago (1 children)

They allude to this later, acknowledging that it's sort of a cross between unit and bottom.

[–] aloso 4 points 1 year ago (1 children)

No it's not, it is 100% a unit type (except it's not really a type, since you can only use it as return type and nowhere else)

[–] BatmanAoD 5 points 1 year ago (1 children)

It's not possible to instantiate or assign, which is more like a never type than a unit; and it is not possible to define new types with the same properties, which is also more like bottom than unit. But you're right that it's not actually a true never type since it can't represent function divergence.

I think the truth is just that Java's type system isn't very mathematically disciplined.

[–] aloso 1 points 1 year ago* (last edited 1 year ago) (1 children)

It's not possible to instantiate or assign, which is more like a never type than a unit

Actually, this is because void is not a type, it is just a keyword, a placeholder used instead of the return type when a function doesn't return anything.

If it were a bottom type, that would mean that a method returning void must diverge, which is simply not true.

Also, if it were a bottom type, it would be possible to write an "unreachable" method

void unreachable(void bottom) {
    return bottom;
}

Even though it couldn't be called, it should be possible to define it, if void was a bottom type. But it is not, because void isn't a bottom type, it's no type at all.

[–] BatmanAoD 1 points 1 year ago

The post has been edited; it looks like someone on reddit made essentially the same point. You're right of course that void isn't a true type in Java, but the post now also discusses Void, which I suppose just shows how void infects the type system despite not being a type.