In what cases is it advisable to use the HashSet data structure?

HashSet in Java (and Android) is used when you need to store unique elements and ensure quick existence checks. It is advisable to use HashSet if:

• The uniqueness of elements is important (e.g., a list of unique users).
• Fast access, addition, and removal of elements are required (on average O(1)).
• The order of elements does not matter, as HashSet does not guarantee order preservation.

Example of usage:

HashSet<String> set = new HashSet<>();
set.add("apple");
set.add("banana");
if (set.contains("apple")) {
    // quickly check for existence
}

If an ordered set is needed, it is better to use LinkedHashSet or TreeSet.