List difference in java

Question

I have two ArrayList as follows:

original: 12, 16, 17, 19, 101

selected: 16, 19, 107, 108, 109

Answer 1

 List original;
 List selected;

 List add = new ArrayList(selected);
 add.removeAll(original);

 List remove = new ArrayList(original);
 remove.removeAll(selected);

Be careful with border cases around duplicate elements. Should cardinality be respected? As in, if I had 5, 6 originally and 5, 5, 6 after, should add be 5? The above works better with Sets, since they don't have duplicates (plus contains() lookups are faster since they are indexed by the data they contain).