SortedSet (Java SE 25 & JDK 25)
Description: declaration: module: java.base, package: java.util, interface: SortedSet
Keywords: java.util.SortedSet interface

# Interface SortedSet<E>

Type Parameters:
  `E` - the type of elements maintained by this set

All Superinterfaces:
  `Collection<E>, Iterable<E>, SequencedCollection<E>, SequencedSet<E>, Set<E>`

All Known Subinterfaces:
  `NavigableSet<E>`

All Known Implementing Classes:
  `ConcurrentSkipListSet, TreeSet`

public interface SortedSet<E> extends Set<E>, SequencedSet<E>
A `Set` that further provides a total ordering on its elements. The elements are ordered using their natural ordering, or by a `Comparator` typically provided at sorted set creation time. The set's iterator will traverse the set in ascending element order. Several additional operations are provided to take advantage of the ordering. (This interface is the set analogue of `SortedMap`.)

All elements inserted into a sorted set must implement the `Comparable` interface (or be accepted by the specified comparator). Furthermore, all such elements must be mutually comparable: `e1.compareTo(e2)` (or `comparator.compare(e1, e2)`) must not throw a `ClassCastException` for any elements `e1` and `e2` in the sorted set. Attempts to violate this restriction will cause the offending method or constructor invocation to throw a `ClassCastException`.

Note that the ordering maintained by a sorted set (whether or not an explicit comparator is provided) must be consistent with equals if the sorted set is to correctly implement the `Set` interface. (See the `Comparable` interface or `Comparator` interface for a precise definition of consistent with equals.) This is so because the `Set` interface is defined in terms of the `equals` operation, but a sorted set performs all element comparisons using its `compareTo` (or `compare`) method, so two elements that are deemed equal by this method are, from the standpoint of the sorted set, equal. The behavior of a sorted set is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the `Set` interface.

All general-purpose sorted set implementation classes should provide four "standard" constructors: 1) A void (no arguments) constructor, which creates an empty sorted set sorted according to the natural ordering of its elements. 2) A constructor with a single argument of type `Comparator`, which creates an empty sorted set sorted according to the specified comparator. 3) A constructor with a single argument of type `Collection`, which creates a new sorted set with the same elements as its argument, sorted according to the natural ordering of the elements. 4) A constructor with a single argument of type `SortedSet`, which creates a new sorted set with the same elements and the same ordering as the input sorted set. There is no way to enforce this recommendation, as interfaces cannot contain constructors.

Note: several methods return subsets with restricted ranges. Such ranges are half-open, that is, they include their low endpoint but not their high endpoint (where applicable). If you need a closed range (which includes both endpoints), and the element type allows for calculation of the successor of a given value, merely request the subrange from `lowEndpoint` to `successor(highEndpoint)`. For example, suppose that `s` is a sorted set of strings. The following idiom obtains a view containing all of the strings in `s` from `low` to `high`, inclusive:

      SortedSet<String> sub = s.subSet(low, high+"\0");

A similar technique can be used to generate an open range (which contains neither endpoint). The following idiom obtains a view containing all of the Strings in `s` from `low` to `high`, exclusive:

      SortedSet<String> sub = s.subSet(low+"\0", high);

This interface is a member of the Java Collections Framework.

Since:
  1.2
See Also:
  - `Set`
  - `TreeSet`
  - `SortedMap`
  - `Collection`
  - `Comparable`
  - `Comparator`
  - `ClassCastException`

- ## Method Summary

  Modifier and Type
  Method
  Description
  `default void`
  `addFirst(E e)`
  Throws `UnsupportedOperationException`.
  `default void`
  `addLast(E e)`
  Throws `UnsupportedOperationException`.
  `Comparator<? super E>`
  `comparator()`
  Returns the comparator used to order the elements in this set, or `null` if this set uses the natural ordering of its elements.
  `E`
  `first()`
  Returns the first (lowest) element currently in this set.
  `default E`
  `getFirst()`
  Gets the first element of this collection.
  `default E`
  `getLast()`
  Gets the last element of this collection.
  `SortedSet<E>`
  `headSet(E toElement)`
  Returns a view of the portion of this set whose elements are strictly less than `toElement`.
  `E`
  `last()`
  Returns the last (highest) element currently in this set.
  `default E`
  `removeFirst()`
  Removes and returns the first element of this collection (optional operation).
  `default E`
  `removeLast()`
  Removes and returns the last element of this collection (optional operation).
  `default SortedSet<E>`
  `reversed()`
  Returns a reverse-ordered view of this collection.
  `default Spliterator<E>`
  `spliterator()`
  Creates a `Spliterator` over the elements in this sorted set.
  `SortedSet<E>`
  `subSet(E fromElement, E toElement)`
  Returns a view of the portion of this set whose elements range from `fromElement`, inclusive, to `toElement`, exclusive.
  `SortedSet<E>`
  `tailSet(E fromElement)`
  Returns a view of the portion of this set whose elements are greater than or equal to `fromElement`.

  ### Methods declared in interface Collection

  `parallelStream, removeIf, stream, toArray`

  ### Methods declared in interface Iterable

  `forEach`

  ### Methods declared in interface Set

  `add, addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray`

- ## Method Details

  - ### comparator

    Comparator<? super E> comparator()
    Returns the comparator used to order the elements in this set, or `null` if this set uses the natural ordering of its elements.
    Returns:
      the comparator used to order the elements in this set, or `null` if this set uses the natural ordering of its elements

  - ### subSet

    SortedSet<E> subSet(E fromElement, E toElement)
    Returns a view of the portion of this set whose elements range from `fromElement`, inclusive, to `toElement`, exclusive. (If `fromElement` and `toElement` are equal, the returned set is empty.) The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

    The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

    Parameters:
      `fromElement` - low endpoint (inclusive) of the returned set
      `toElement` - high endpoint (exclusive) of the returned set
    Returns:
      a view of the portion of this set whose elements range from `fromElement`, inclusive, to `toElement`, exclusive
    Throws:
      `ClassCastException` - if `fromElement` and `toElement` cannot be compared to one another using this set's comparator (or, if the set has no comparator, using natural ordering). Implementations may, but are not required to, throw this exception if `fromElement` or `toElement` cannot be compared to elements currently in the set.
      `NullPointerException` - if `fromElement` or `toElement` is null and this set does not permit null elements
      `IllegalArgumentException` - if `fromElement` is greater than `toElement`; or if this set itself has a restricted range, and `fromElement` or `toElement` lies outside the bounds of the range

  - ### headSet

    SortedSet<E> headSet(E toElement)
    Returns a view of the portion of this set whose elements are strictly less than `toElement`. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

    The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

    Parameters:
      `toElement` - high endpoint (exclusive) of the returned set
    Returns:
      a view of the portion of this set whose elements are strictly less than `toElement`
    Throws:
      `ClassCastException` - if `toElement` is not compatible with this set's comparator (or, if the set has no comparator, if `toElement` does not implement `Comparable`). Implementations may, but are not required to, throw this exception if `toElement` cannot be compared to elements currently in the set.
      `NullPointerException` - if `toElement` is null and this set does not permit null elements
      `IllegalArgumentException` - if this set itself has a restricted range, and `toElement` lies outside the bounds of the range

  - ### tailSet

    SortedSet<E> tailSet(E fromElement)
    Returns a view of the portion of this set whose elements are greater than or equal to `fromElement`. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa. The returned set supports all optional set operations that this set supports.

    The returned set will throw an `IllegalArgumentException` on an attempt to insert an element outside its range.

    Parameters:
      `fromElement` - low endpoint (inclusive) of the returned set
    Returns:
      a view of the portion of this set whose elements are greater than or equal to `fromElement`
    Throws:
      `ClassCastException` - if `fromElement` is not compatible with this set's comparator (or, if the set has no comparator, if `fromElement` does not implement `Comparable`). Implementations may, but are not required to, throw this exception if `fromElement` cannot be compared to elements currently in the set.
      `NullPointerException` - if `fromElement` is null and this set does not permit null elements
      `IllegalArgumentException` - if this set itself has a restricted range, and `fromElement` lies outside the bounds of the range

  - ### first

    E first()
    Returns the first (lowest) element currently in this set.
    Returns:
      the first (lowest) element currently in this set
    Throws:
      `NoSuchElementException` - if this set is empty

  - ### last

    E last()
    Returns the last (highest) element currently in this set.
    Returns:
      the last (highest) element currently in this set
    Throws:
      `NoSuchElementException` - if this set is empty

  - ### spliterator

    default Spliterator<E> spliterator()
    Creates a `Spliterator` over the elements in this sorted set.

    The `Spliterator` reports `Spliterator.DISTINCT`, `Spliterator.SORTED` and `Spliterator.ORDERED`. Implementations should document the reporting of additional characteristic values.

    The spliterator's comparator (see `Spliterator.getComparator()`) must be `null` if the sorted set's comparator (see `comparator()`) is `null`. Otherwise, the spliterator's comparator must be the same as or impose the same total ordering as the sorted set's comparator.

    Specified by:
      `spliterator` in interface `Collection<E>`
    Specified by:
      `spliterator` in interface `Iterable<E>`
    Specified by:
      `spliterator` in interface `Set<E>`
    Implementation Requirements:
      The default implementation creates a late-binding spliterator from the sorted set's `Iterator`. The spliterator inherits the fail-fast properties of the set's iterator. The spliterator's comparator is the same as the sorted set's comparator.

      The created `Spliterator` additionally reports `Spliterator.SIZED`.

    Implementation Note:
      The created `Spliterator` additionally reports `Spliterator.SUBSIZED`.
    Returns:
      a `Spliterator` over the elements in this sorted set
    Since:
      1.8

  - ### addFirst

    default void addFirst(E e)
    Throws `UnsupportedOperationException`. The encounter order induced by this set's comparison method determines the position of elements, so explicit positioning is not supported.
    Specified by:
      `addFirst` in interface `SequencedCollection<E>`
    Implementation Requirements:
      The implementation in this interface always throws `UnsupportedOperationException`.
    Parameters:
      `e` - the element to be added
    Throws:
      `UnsupportedOperationException` - always
    Since:
      21

  - ### addLast

    default void addLast(E e)
    Throws `UnsupportedOperationException`. The encounter order induced by this set's comparison method determines the position of elements, so explicit positioning is not supported.
    Specified by:
      `addLast` in interface `SequencedCollection<E>`
    Implementation Requirements:
      The implementation in this interface always throws `UnsupportedOperationException`.
    Parameters:
      `e` - the element to be added.
    Throws:
      `UnsupportedOperationException` - always
    Since:
      21

  - ### getFirst

    default E getFirst()
    Gets the first element of this collection.
    Specified by:
      `getFirst` in interface `SequencedCollection<E>`
    Implementation Requirements:
      The implementation in this interface returns the result of calling the `first` method.
    Returns:
      the retrieved element
    Throws:
      `NoSuchElementException` - if this collection is empty
    Since:
      21

  - ### getLast

    default E getLast()
    Gets the last element of this collection.
    Specified by:
      `getLast` in interface `SequencedCollection<E>`
    Implementation Requirements:
      The implementation in this interface returns the result of calling the `last` method.
    Returns:
      the retrieved element
    Throws:
      `NoSuchElementException` - if this collection is empty
    Since:
      21

  - ### removeFirst

    default E removeFirst()
    Removes and returns the first element of this collection (optional operation).
    Specified by:
      `removeFirst` in interface `SequencedCollection<E>`
    Implementation Requirements:
      The implementation in this interface calls the `first` method to obtain the first element, then it calls `remove(element)` to remove the element, and then it returns the element.
    Returns:
      the removed element
    Throws:
      `NoSuchElementException` - if this collection is empty
      `UnsupportedOperationException` - if this collection implementation does not support this operation
    Since:
      21

  - ### removeLast

    default E removeLast()
    Removes and returns the last element of this collection (optional operation).
    Specified by:
      `removeLast` in interface `SequencedCollection<E>`
    Implementation Requirements:
      The implementation in this interface calls the `last` method to obtain the last element, then it calls `remove(element)` to remove the element, and then it returns the element.
    Returns:
      the removed element
    Throws:
      `NoSuchElementException` - if this collection is empty
      `UnsupportedOperationException` - if this collection implementation does not support this operation
    Since:
      21

  - ### reversed

    default SortedSet<E> reversed()
    Returns a reverse-ordered view of this collection. The encounter order of elements in the returned view is the inverse of the encounter order of elements in this collection. The reverse ordering affects all order-sensitive operations, including those on the view collections of the returned view. If the collection implementation permits modifications to this view, the modifications "write through" to the underlying collection. Changes to the underlying collection might or might not be visible in this reversed view, depending upon the implementation.
    Specified by:
      `reversed` in interface `SequencedCollection<E>`
    Specified by:
      `reversed` in interface `SequencedSet<E>`
    Implementation Requirements:
      The implementation in this interface returns a reverse-ordered SortedSet view. The `reversed()` method of the view returns a reference to this SortedSet. Other operations on the view are implemented via calls to public methods on this SortedSet. The exact relationship between calls on the view and calls on this SortedSet is unspecified. However, order-sensitive operations generally behave as if they delegate to the appropriate method with the opposite orientation. For example, calling `getFirst` on the view might result in a call to `getLast` on this SortedSet.
    Returns:
      a reverse-ordered view of this collection, as a `SortedSet`
    Since:
      21