Java Comparator for comparing instances of a generic class

generics, java

Solution

Why am I getting unchecked call warnings about the `compareTo()` methods?

Because you are using raw type `Tuple`, instead of parameterized type. To use a parameterized type `Tuple` however, you would need to make the `CustomTupleComparator` class generic itself:

public class CustomTupleComparator<T1 extends Comparable<? super T1>, 
                                   T2 extends Comparable<? super T2>> 
             implements Comparator<Tuple<T1, T2>>
{
    @Override
    public int compare(Tuple<T1, T2> o1, Tuple<T1, T2> o2)
    {
        int result = o1.getSecond().compareTo(o2.getSecond());

        return result != 0 ? result
                           : -o1.getFirst().compareTo(o2.getFirst());
    }
}

Also, you should change your `Tuple` class, to make the type parameters use generic `Comparable`, and not raw type:

public class Tuple<T1 extends Comparable<? super T1>, 
                   T2 extends Comparable<? super T2>> { }

Note: I've used `Comparable<? super T1>` instead of `Comparable<T1>` so as to make it work for subtypes too, where the super types are implementing `Comparable<SuperType>`.

Also, since `Comparable`s are consumers, as they consume types that they are comparing, we use `? super T1`.

Problem

Original source