Save a list of unique Strings in the ArrayList

Why do you need to store it in a List? Do you actually require the data to be ordered or support index-based look-ups?

I would suggest storing the data in a Set. If ordering is unimportant you should use HashSet. However, if you wish to preserve ordering you could use LinkedHashSet.

If you have a List containing duplicates, and you want a List without, you could do:

List<String> newList = new ArrayList<String>(new HashSet<String>(oldList));

That is, wrap the old list into a set to remove duplicates and wrap that set in a list again.