Java Collection比较通用类，扩展接口，扩展可比

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.