如何排序ArrayLists列表（不是数组）？

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.

Answer 1

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());