Sorting a generic ArrayList of ArrayLists

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?

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
– Iskar Jarak, Commented Nov 7, 2012 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
– Bryan Glazer, Commented Nov 7, 2012 at 2:57

arshajii · Accepted Answer · 2012-11-07 02:50:33Z

5

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));
    }
}

answered Nov 7, 2012 at 2:50

arshajii

130k26 gold badges246 silver badges293 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Bryan Glazer Over a year ago

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>>

arshajii Over a year ago

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.

newacct Over a year ago

for best results use Comparable<? super T>

Collectives™ on Stack Overflow

Sorting a generic ArrayList of ArrayLists

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related