java.util · java.base · since Java 1.2

Set

declaration

public interface Set<E> extends Collection<E>

A collection with no duplicates, as judged by equals (or compareTo in sorted sets). add() returning false is the built-in "seen before" test.

Key methods

`boolean add(E e)`	false if an equal element exists — dedup in one call.
`boolean remove(Object o)`	Removes the equal element, if present.
`boolean contains(Object o)`	Equals-based membership test — the reason to reach for a Set at all.
`boolean retainAll(Collection<?> c)`	In-place intersection with another collection.
`static <E> Set<E> of(E...)`	Immutable set literal (throws on duplicate args!).
`static <E> Set<E> copyOf(Collection<E>)`	Immutable defensive copy.