Question

An abstract class is declared as

 myClass<E extends Number, D extends Number>

Sublasses are

 final mySubClass<Double,Integer>
 final myOtherSubclass<Double,Double>

Imagine there is a subclass that will not have a parametrized procedure for the first type, I mean, type would be ignored. Is there any way to set it to Void, Null or something?

final mySubClassThatDontUseFirstType<Void,Integer>

For example, if it had a inner List<E> inside, this list would be List because would be never used.

Answer 1

Is there any way to set it to Void, Null or something?

No, not really. What ever you choose it needs to be a subtype of Number (that’s simply a requirement of the Java Language).

What you could do is to create an uninstantiable class VoidNumber

class VoidNumber extends Number {

    // Private constructor
    private VoidNumber() {
    }

    ...
}

and use that to document the fact that it’s not used.