Question

I have a list of points. I represented this as an ArrayList of ArrayLists. I’m trying to use generics so that the list can store Integers, Floats, etc. and treat them the same. So I have:

ArrayList<ArrayList<? extends Number>>. 

I need to compare these lists based on one element in each inner list.

So to do that I wrote a custom Comparator that I’m using in Collections.sort

The Comparator looks like:

int compare(ArrayList<? extends Number> a, ArrayList<? extends Number> b )
{
    return a.get(index).compareTo(b.get(index))
}

This doesn’t compile of course. The compiler complains that there is no compareTo method for the wildcard class.

Is what I’m trying to do even possible?

Answer 1

How about defining your Comparator class as:

class MyComparator<T extends Number & Comparable<T>> 
                                          implements Comparator<ArrayList<T>> {
    @Override
    public int compare(ArrayList<T> a, ArrayList<T> b) {
        return a.get(index).compareTo(b.get(index));
    }
}