What do I return if the return type of a method is Void? (Not void!)

问题

Due to the use of Generics in Java I ended up in having to implement a function having Void as return type:

public Void doSomething() {
    //...
}

and the compiler demands that I return something. For now I'm just returning null, but I'm wondering if that is good coding practice...

I'm asking about V‌oid, not v‌oid. The class Void, not the reserved keyword void.

I've also tried Void.class, void, Void.TYPE, new Void(), no return at all, but all that doesn't work at all. (For more or less obvious reasons) (See this answer for details)

So what am I supposed to return if the return type of a function is Void?
What's the general use of the Void class?

回答1:

So what am I supposed to return if the return type of a function has to be Void?

Use return null. Void can't be instantiated and is merely a placeholder for the Class<T> type of void.

What's the point of Void?

As noted above, it's a placeholder. Void is what you'll get back if you, for example, use reflection to look at a method with a return type of void. (Technically, you'll get back Class<Void>.) It has other assorted uses along these lines, like if you want to parameterize a Callable<T>.

Due to the use of generics in Java I ended up in having to implement this function

I'd say that something may be funky with your API if you needed to implement a method with this signature. Consider carefully whether there's a better way to do what you want (perhaps you can provide more details in a different, follow-up question?). I'm a little suspicious, since this only came up "due to the use of generics".

回答2:

There's no way to instantiate a Void, so the only thing you can return is null.

回答3:

return null is the way to go.

回答4:

To make clear why the other suggestions you gave don't work:

Void.class and Void.TYPE point to the same object and are of type Class<Void>, not of Void.

That is why you can't return those values. new Void() would be of type Void but that constructor doesn't exist. In fact, Void has no public constructors and so cannot be instantiated: You can never have any object of type Void except for the polymorphic null.

Hope this helps! :-)

回答5:

If, for obscure reasons, you MUST use this type, then indeed returning null seems to be a sensible option, since I suppose return value will not be used anyway.
The compiler will force you to return something anyway.
And this class doesn't seem to have a public constructor so new Void() is not possible.

回答6:

just like this.

public Class TestClass {
    public void testMethod () {
        return;
    }
}

来源：https://stackoverflow.com/questions/676663/what-do-i-return-if-the-return-type-of-a-method-is-void-not-void

标签

java

generics

void