Question

I am breaking my mind to find a solution to the following problem.
I have 4 different ArrayList that get their values from a Database.
They can have size from 0 (including) till what ever.
Each list may have different size and values also.
What I am trying to do effectively is :
Compare all the non 0 size lists and check if they have some common integers and what are those values.

Any ideas?
Thank you!

Answer 1

If you need a collection of common integers for all, excluding empty ones:

List<List<Integer>> lists = ...
Collection<Integer> common = new HashSet<Integer>(lists.get(0));
for (int i = 1; i < lists.size(); i++) {
   if (!lists.get(i).isEmpty()) 
     common.retainAll(lists.get(i));
}

at the end the common will contain integers that common for all of them.