Methods that require return even having Void in "signature"

Question

I am doing some tests to understand in practice the functioning of the classSwingWorker in the swing, and I noticed that the method doInBackground demands a return null, even starting the form variable below:

SwingWorker<Void, Void> worker = new SwingWorker<Void, Void>() {
    @Override
    protected Void doInBackground() throws Exception {

        return null;

    }
};

In declaring SwingWorker<Void, Void>, the first parameter refers precisely the return of the method cited, but even if it is Void, he forces to return null(If you do not provide the null return, the IDE displays a semantic error and does not compile the code). See running on IDEONE.

If the signature is protected Void doInBackground(), Why does it require feedback anyway? There is some difference between Void and void to justify this behaviour?

Answer 1

void is the keyword used to indicate that a method does not return anything.
In turn Void is a class, so, following the rules, the compiler forces something to be returned.

Void is the version "Boxing" of the "primitive type"⁽¹⁾ void. Like void indicates no value either Void cannot be instantiated.

When a method has in its signature Void as a type of return Void be instantiated, the method has to return null(which somehow indicates no return/value).

The class Swingworker uses its first type of parameter to indicate the type to be returned by the method doInBackground(), when there is nothing to return is used Void for the type of return.
Generics may only use reference types.

_{⁽¹⁾ void is not one primitive type}