Generics <? super T> and <? extends T>

Question

In the coding below, because the Box<? extends T> extends and the EqualityComparator<? super T> super T? uses so as to make part of the code println true return?

public boolean containsSame
(Box<? extends T> other, EqualityComparator<? super T> comparator){
return comparator.compare(get(),other.get());
}

public interface EqualityComparator<T> {
public boolean compare(T first, T second);
}



Box<Number> nBox = new Box<Number>(42);
Box<Integer> iBox = new Box<Integer>(42);
EqualityComparator<Object> sameObject = new EqualityComparator<Object>() {
public boolean compare(Object o1, Object o2) { return o1 == o2;} 
};
System.out.println(nBox.containsSame(iBox, sameObject));

Answer 1

Using as a basis that answer, that I even posted in the comments, I can bring to this situation saying the following:

When you declare the method containsSame, thus:

public boolean containsSame(Box<? extends T> other, EqualityComparator<? super T> comparator) {
    //...
}

You’re basically saying that: the parameter other must be a Box of objects of a daughter class of T; and the parameter comparator must be a EqualityComparator object mother-class of T.

With this, you created an object, called nBox, with the guy Box<Number>. That is to say:

public boolean containsSame(Box<? extends Number> other, EqualityComparator<? super Number> comparator) {
    //...
}

In that case, the T will be Number.

---

Then you called the method containsSame of nBox and passed as a first argument Box<Integer> and as a second argument EqualityComparator<Object>.

Thus, there should be no errors related to data types. Therefore, the relations defined in the parameters are being met by the arguments. That is to say:

• Integer(the class ?) is a subclass/daughter class of Number (T);
• And Object (also the class ?) is the superclass/mother class of Number (T), as well as of any other class.

Therefore, the method runs smoothly.

I hope I’ve helped!