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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?

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How are you going to handle points with different dimensions with this structure? E.g. What if you have (1,1,1) and (2,1) and have chosen the third element to compare on? – Iskar Jarak Nov 7 '12 at 2:54
That's actually another problem in itself. Right now I just check that all the points are of the proper dimension and throw an error otherwise. – Bryan Glazer Nov 7 '12 at 2:57
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1 Answer

up vote 5 down vote accepted

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));
    }
}
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Thank you that works perfectly. I'm new to generics, could you tell me what the concept at work here is? Especially this: MyComparator<T extends Number & Comparable<T>> – Bryan Glazer Nov 7 '12 at 2:58
Well T extends Number & Comparable<T>> implies that T is a Number and that it is Comparable - which is essentially what you want. This is a good resource to learn about generics. This is good for bounded type parameters. – A. R. S. Nov 7 '12 at 3:02
for best results use Comparable<? super T> – newacct Nov 7 '12 at 3:52
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