Lower bounded wildcard not checked against upper bounded type parameter

Question

I wonder why does this piece of code compile successfully?

Source code:

abstract class A
{
    public abstract  A

        

Answer 1

This is a surprisingly meaningless piece of code.

All it is saying is that the class A takes a generic type K that is a Number and there is a method useMe that returns an A with some pointless extra restriction on T (other than being a Number obviously).

Here's an implementation to show how little is being said by the sugar:

abstract class A {
    public abstract  A useMe(A k);
}

class B extends A {

    @Override
    public  A useMe(A k) {
        // Not much more you can do here but this.
        return k;
    }

}

The ? super M stuff is just meaningless gobbledegook - all the compiler can derive from it is that both the parameter passed to it and the result returned must be a superclass of a specific unnamed class.

Generics are there to make detection of coding mistakes easy at compile time. Using mumbo-jumbo such as this is just misleading obfuscation.