How to sort a List of ArrayLists (not arrays)?

Question

I have an adjacency list implemented using a List[] of ArrayLists. I want to sort the List in descending order of the size of the ArrayLists. I figured I would do this by writing a Comparator... But how would I do this? or is this just not possible and I should do it another way?

Collections.sort(adjacency, new Comparator<ArrayList<Integer>()>() {
    public int compare(ArrayList<Integer> p1, ArrayList<Integer> p2) {
        return Integer.compare(p1.length, p2.length);
    }
});

The code on top is not functioning. I tried with just an ArrayList, List[], List as a Comparator type. Is there a wrapper class for list? Sorry if this may sound uneducated.

This is how I made the adjacency list:

List<Integer>[] adjacency;
adjacency = (List<Integer>[]) new List[size];
for (int i = 0; i < size; ++i) {
    adjacency[i] = new ArrayList<Integer>();
}

Thank you.

Shouldn't p1.length be p1.size()? So you have an array of ArrayLists and you're trying to use Collections.sort to sort the array...? — MadProgrammer
– MadProgrammer, Commented Mar 16, 2017 at 21:53

ΦXocę 웃 Пepeúpa ツ · Accepted Answer · 2017-03-16 21:56:30Z

2

The code on top is not functioning.

The code is not working because p1 and p2 are ArrayLists and they dont have a field named length, they have the method size(), that is what you need.

return Integer.compare(p1.size(), p2.size());

edited Mar 16, 2017 at 21:56

answered Mar 16, 2017 at 21:55

ΦXocę 웃 Пepeúpa ツ

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1 Comment

MadProgrammer Over a year ago

I'd be seriously careful before you answer this, and have a REALLY close look at how they are creating adjacency ;)

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