Java Void type - possible/allowed values?

Question

1) In Java, I can do this:

Void z = null;

Is there any other value except null I can assign to z?

2) Consider the following code snipped:

Callable<Void> v = () -> {
    System.out.println("zzz"); 
    Thread.sleep(1000);
    return null;
}; 

This compiles OK, but if I remove the last statement return null; it doesn't. Why? After all, Void is supposed to mean no return value.

Answer 1

From the docs:

The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void.

So, no.

Void is used by methods having to return an object, but really returning nothing.

A decent example can be observed with some usage of the AsyncTask in Android, in cases where you don't need to return any object after the task is complete.

You would then extend AsyncTask<[your params type], [your progress type], Void>, and return null in your onPostExecute override.

You wouldn't need it in most cases though (for instance, Runnable is typically more suitable than Callable<Void>).

Ansering your question more specifically:

But if I remove the return null it does not compile?! Why?

... because a Void is still an object. However, it can only have value null.

If your method declares it returns Void, you need to (explicitly) return null.