Question

HashMap<ArrayList<Integer>, String>

Iterating though this structure, I get:

[1 1 0], "gh"
[0 4 2], "gh"
[0 5 2], "qh"

However, I want to sort this on the key (in increasing order), to get

[0 4 2], "gh"
[0 5 2], "qh"
[1 1 0], "gh"

The first component is sorted first, then the second, then the third.

Answer 1

HashMaps have no inherent ordering.

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

Use a different data structure, or externally sort the keys yourself, before iterating. Either way, you’ll need to write a Comparator<List<Integer>> since List<E> does not extend Comparable<List<E>>

List<List<Integer>> keys = new ArrayList<>(map.keySet());
Collections.sort(keys, new Comparator<List<Integer>>() {
    @Override public int compare(List<Integer> a, List<Integer> b) {
        // TODO handle edge cases, like lists of differing size

        for (int i=0; i<Math.min(a.size(), b.size()); i++) {
            int result = Integer.compare(a.get(i), b.get(i));
            if (result != 0) {
                return result;
            }
        }

        return 0;
    }
});

for (List<Integer> key : keys) {
    String value = map.get(key);
    // do something interesting with the value
}

N.B. it is generally a Bad Idea to use mutable keys in a map of any type. Personally, I’ve never had a need to – at least use an immutable list of integers instead.