Is there a no-duplicate List implementation out there?

Question

I know about SortedSet, but in my case I need something that implements List, and not Set. So is there an implementation out there, in the API or elsewhere?

It shouldn't be hard to implement myself, but I figured why not ask people here first?

Answer 1

There's no Java collection in the standard library to do this. LinkedHashSet<E> preserves ordering similarly to a List, though, so if you wrap your set in a List when you want to use it as a List you'll get the semantics you want.

Alternatively, the Commons Collections (or commons-collections4, for the generic version) has a List which does what you want already: SetUniqueList / SetUniqueList<E>.

Answer 2

Here is what I did and it works.

Assuming I have an ArrayList to work with the first thing I did was created a new LinkedHashMap.

LinkedHashSet<E> hashSet = new LinkedHashSet<E>() 

Then I attempt to add my new element to the LinkedHashSet. The add method does not alter the LinkedHasSet and returns false if the new element is a duplicate. So this becomes a condition I can test before adding to the ArrayList.

if (hashSet.add(E)) arrayList.add(E); 

This is a simple and elegant way to prevent duplicates from being added to an array list. If you want you can encapsulate it in and override of the add method in a class that extends the ArrayList. Just remember to deal with addAll by looping through the elements and calling the add method.