Question

Given a List of MyClass objects (and a custom Comparitor myComparitor if needed), what good options are there for checking if the List contains two “equal” objects?

Edit: if there are duplicates, return a reference to one or more of the duplicates.

Overriding MyClass.equals(MyClass) in this case is not an option.

My initial thought is to create a hash table of sorts, but I suspect that there’s a non-hack way to accomplish the same thing:

SortedSet mySet = new TreeSet(myComparitor);
mySet.addAll(myList);
// Find duplicates in a sorted set in O(N) time

P.S. Is there a good reference on Markdown?

Answer 1

If the element’s equals(Object) method does not give you the semantic that you require, then HashMap or HashSet are not options. Your choices are:

• Use a TreeMap for de-duping. This is O(NlogN).
• Sort the ArrayList or a copy, then iterate over looking for element i equals element i + 1. This is O(NlogN).
• Find an alternative implementation of hash sets that allows you to provide a separate object to implement equality and hashing. (Neither Apache or Google collections support this, so you’ll need to look further afield.)
• Create a wrapper class for your element type that overrides equals(Object) and hashCode(), and de-dup using a HashSet of wrapped objects. This is O(N), but the constant of proportionality will be larger than a simple HashSet due to creation of wrapper objects.

When de-duping with a Set it is probably better to use a loop rather than addAll. This is necessary if you need to know what all of the duplicates are. If you don’t need to know that, then using a loop allows you to stop when you find the first duplicate. The only case where addAll is likely to perform better is when there are likely to be no duplicates.