Reordering a collection according to a related list of ids

Question

I have a Collection (unordered) of objects with an id property, and an (ordered) List of ids. The id list is not sorted. I\'d like to crea

Answer 1

If your unordered input is no more specific than Collection and your List of ids is in an arbitrary order (not decreasing numerically or something like that), your simplest and quite performant approach is probably this. The cost is linear, O(m+n) where m is the number of ids in your initially sorted list and n is the number of values to sort.

Map keyed = new HashMap();
for (ValueType value : unsortedCollection) {
    keyed.put(value.getId(), value);
}

List sorted = new ArrayList();
for (IDType id : sortedIds) {
    ValueType value = keyed.get(id);
    if (value != null) {
        sorted.add(value);
    }
}