Why is void not covariant in Java?

Question

If I have this interface:

public interface Foo {
    void bar();
}

Why can't I implement it like this?

public class FooImpl implements Foo {
    @Override
    public Object bar() {
         return new Object();
    }
}

It seems like void should be covariant with everything. Am I missing something?

Edit: I should have been clearer that I'm looking for the design justification, not the technical reason that it won't compile. Are there negative consequences to making void covariant to everything?

Answer 1

void is only covariant with void because the JLS says so:

A method declaration d1 with return type R1 is return-type-substitutable for another method d2 with return type R2, if and only if the following conditions hold:

• If R1 is void then R2 is void.

• If R1 is a primitive type, then R2 is identical to R1.

• If R1 is a reference type then:

  • R1 is either a subtype of R2 or R1 can be converted to a subtype of R2 by unchecked conversion (§5.1.9), or

  • R1 = |R2|

Answer 2

covariance means that if your methods return type is a Supertype object you can return sub-type Object at runtime, void is not the super type of java.lang.Object(or any Object for that matter except itself as answered by MR NPE)

Answer 3

Technically void cannot be a covariant return type as the caller needs to know the stack layout. Calling a function that returns an object would result of an object-ref on the top of the stack after the INVOKEVIRTUAL/INTERFACE. Void return type leaves nothing on the top of the stack, hence the functions are binary incompatible.

So the JLS rightfully says it's not possible.