Question

I have an ArrayList that has a nested ArrayList of Strings and I want to remove duplicates from. I know if I always wanted to remove duplicates, I shouldn’t use an ArrayList, but in some cases duplicates are valid. What is the best way to remove duplicates from the nested ArrayList?

For example, I would like to execute some Java that converts:

[[duplicate], [duplicate], [duplicate], [unique1], [unique2]]

to:

[[duplicate], [unique1], [unique2]]

Answer 1

What aioobe said, using a set, except you will put it in a loop since you have a two demisional array:

Set<String> set = new LinkedHashSet<String>();
for (ArrayList<String> list:yourList) {
    set.addAll (list);
}
ArrayList<String> uniqueList = new ArrayList<String>(set);