Set: HashSet, LinkedHashSet, TreeSet

HashSet is a HashMap in disguise: each element is a key in the HashMap, and every value is the same shared dummy constant PRESENT.

HashSet is unordered O(1), LinkedHashSet is insertion-ordered O(1), and TreeSet is sorted O(log n) – each backed by the corresponding Map implementation.

HashSet is unordered with O(1) operations; LinkedHashSet adds insertion-order iteration at the cost of slightly more memory per element.

Wrap the list in a LinkedHashSet to remove duplicates while preserving insertion order; wrap back in ArrayList if you need a List.

Converting a List to a Set removes duplicates, and the Set implementation you choose determines whether elements are unordered (HashSet), insertion-ordered (LinkedHashSet), or sorted (TreeSet).