HashSet

Set<String> set = new HashSet<>();
if (set.isEmpty()) {
    System.out.println("Empty!");
}
set.add("dog");
set.add("cat");
set.add("mouse");
set.add("snake");
set.add("bear");
if (set.contains("cat")) {
    System.out.println("Contains cat");
}
set.remove("cat");
for (String element : set) {
    System.out.println(element);
}