public class MainBot  {
   HashSet<String> mySet = loadFromfile();  //a very big set!

   public List<String> doSomething()  {
       List<String> result = new LinkedList<>();
       ...
       List<String> myList = ... //big list!

       for (String s: myList)  {
             if (new UniqueItemCondition(s, mySet).isTrue())  {
                 result.add(s);
             }
       }

       return result;
}
public class UniqueItemCondition  {
          public boolean isTrue(String s, Set<String> initialSet)  {
                return !initialSet.contains(s);
          }
}