Question

Here is the case, I have two classes A, B, and a generic interface C

Class A implements Comparable<A> {...}

interface C<T> {...}

//this class is not longer generic type, I do not know if this matter.
Class B extends A implements C<A> {...} 

Then, at other class, I got a B List and sort it as follow

List<B> list = new ArrayList<B>();
Collections.sort(list);

This works perfectly, but now I would like to change the list of B to the generic interface C, so that it can be more general.

List<C<A>> list = new ArrayList<C<A>>();
Collections.sort(list);

This time I got the Error as follow:

Bound mismatch: The generic method sort(List<T>) of type Collections is not
applicable for the arguments (List<C<A>>). The inferred type C<A> is not a
valid substitute for the bounded parameter <T extends Comparable<? super T>>

I have tried the following modifications (of course does not work):

1. change C to interface C<T> extends Comparable<T>{...}
2. change B to class B extends A implements C<A>, Comparable<T> {...}

Can anybody help me?

Answer 1

change C to interface C extends Comparable{…}
Class B extends A implements C {…}

As you would have already seen from the error messages, these two won’t work together as there will be a conflict in B‘s definition w.r.t to Comparable<A> and Comparable<C<A>>.

Since A is already implementing Comparable<A>, you can achieve the following

List<C<A>> list = new ArrayList<C<A>>();
Collections.sort(list);

by defining a Comparator for C<A> as follows:

class CTComparator<T> implements Comparator<C<T>>
{
    @Override
    public int compare(C<T> o1, C<T> o2)
    {
        return 0;
    }
}

and then applying the sort method with this comparator:

List<C<T>> list  = new ArrayList<C<T>>();
Collections.sort(list, comparator);