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

It sounds like your id list has its own order; you're not just using the natural order, right?

Here's the Guava solution:

Ordering.explicit(idList)
     // constructs a "fluent Comparator" that compares elements in the
     // explicitly specified order
  .onResultOf(new Function() {
    public Id apply(MyObject o) { return o.getId(); }
   }) // make this a Comparator that compares on IDs
  .sortedCopy(myObjects); // get the sorted copy of the collection

That's it. Nothing to it. (Disclosure: I contribute to Guava.)

Alternately, if you know IDs are unique, it might just say

Map objectsById =
  Maps.uniqueIndex(myObjects, GET_ID_FUNCTION); // defined elsewhere
List sortedObjects = Lists.newArrayList();
for (Id id : sortedIds) 
  sortedObjects.add(objectsById.get(id));