Question

I have this interface

public interface IDataPoint<T> extends Comparable<T>  {
    public T getValue();
}

and this implementation…

public class IntegerDataPoint implements IDataPoint<Integer> {

    // ... some code omitted for this example

    public int compareTo(Integer another) {
         // ... some code
    }
}

and another class…

public class HeatMap<X extends IDataPoint<?> {
    private List<X> xPoints;
}

Now I would like to use Collections.max (and similar) on the xPoints list, but that does not work, probably because I got my generics all messed up.

Any suggestions how this could be solved (without a Comparator)?

Collections.max(xPoints);

gives me this error:

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

Answer 1

The problem is that Collections.max(Collection<? extends T>) wants the T’s to be comparable to themselves not some other type.

In your case IntegerDataPoint is comparable to Integer, but not IntegerDataPoint

You cannot easily fix this because IntegerDataPoint is not allowed to implement Comparable<Integer> and Comparable<IntegerDataPoint> at the same time.