java.util · java.base · since Java 1.2

TreeSet

declaration

public class TreeSet<E> extends AbstractSet<E>
        implements NavigableSet<E>, Cloneable, Serializable

Red-black-tree set: always sorted, O(log n) operations, and NavigableSet range/nearest queries (floor, ceiling, subSet).

Key methods

`E first() / E last()`	Extremes in sort order — throws NoSuchElementException if empty.
`E floor(E) / E ceiling(E)`	Nearest ≤ / ≥ element in sort order, or null.
`E lower(E) / E higher(E)`	Nearest strictly < / > element, or null.
`E pollFirst() / E pollLast()`	Retrieve-and-remove the extremes — null-returning, not throwing.
`NavigableSet<E> headSet/tailSet/subSet(…)`	Range views, live and write-through; boolean flags control endpoint inclusion.
`NavigableSet<E> descendingSet()`	Reverse-order view backed by the same tree.