Module java.base
Package java.util.concurrent

Class ConcurrentSkipListSet<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractSet<E>
java.util.concurrent.ConcurrentSkipListSet<E>
Type Parameters:
E - the type of elements maintained by this set
All Implemented Interfaces:
Serializable, Cloneable, Iterable<E>, Collection<E>, NavigableSet<E>, Set<E>, SortedSet<E>
public class ConcurrentSkipListSet<E> extends AbstractSet<E> implements NavigableSet<E>, Cloneable, Serializable
A scalable concurrent NavigableSet implementation based on a ConcurrentSkipListMap. The elements of the set are kept sorted according to their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.

This implementation provides expected average log(n) time cost for the contains, add, and remove operations and their variants. Insertion, removal, and access operations safely execute concurrently by multiple threads.

Iterators and spliterators are weakly consistent.

Ascending ordered views and their iterators are faster than descending ones.

Beware that, unlike in most collections, the size method is not a constant-time operation. Because of the asynchronous nature of these sets, determining the current number of elements requires a traversal of the elements, and so may report inaccurate results if this collection is modified during traversal.

Bulk operations that add, remove, or examine multiple elements, such as AbstractCollection.addAll(java.util.Collection<? extends E>), Collection.removeIf(java.util.function.Predicate<? super E>) or Iterable.forEach(java.util.function.Consumer<? super T>), are not guaranteed to be performed atomically. For example, a forEach traversal concurrent with an addAll operation might observe only some of the added elements.

This class and its iterators implement all of the optional methods of the Set and Iterator interfaces. Like most other concurrent collection implementations, this class does not permit the use of null elements, because null arguments and return values cannot be reliably distinguished from the absence of elements.

This class is a member of the Java Collections Framework.

Since:
1.6
See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    ConcurrentSkipListSet()
    Constructs a new, empty set that orders its elements according to their natural ordering.
    ConcurrentSkipListSet(Collection<? extends E> c)
    Constructs a new set containing the elements in the specified collection, that orders its elements according to their natural ordering.
    ConcurrentSkipListSet(Comparator<? super E> comparator)
    Constructs a new, empty set that orders its elements according to the specified comparator.
    ConcurrentSkipListSet(SortedSet<E> s)
    Constructs a new set containing the same elements and using the same ordering as the specified sorted set.

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    add(E e)
    Adds the specified element to this set if it is not already present.
    E
    ceiling(E e)
    Returns the least element in this set greater than or equal to the given element, or null if there is no such element.
    void
    clear()
    Removes all of the elements from this set.
    ConcurrentSkipListSet<E>
    clone()
    Returns a shallow copy of this ConcurrentSkipListSet instance.
    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.
    boolean
    contains(Object o)
    Returns true if this set contains the specified element.
    Iterator<E>
    descendingIterator()
    Returns an iterator over the elements in this set in descending order.
    NavigableSet<E>
    descendingSet()
    Returns a reverse order view of the elements contained in this set.
    boolean
    equals(Object o)
    Compares the specified object with this set for equality.
    E
    first()
    Returns the first (lowest) element currently in this set.
    E
    floor(E e)
    Returns the greatest element in this set less than or equal to the given element, or null if there is no such element.
    NavigableSet<E>
    headSet(E toElement)
    Returns a view of the portion of this set whose elements are strictly less than toElement.
    NavigableSet<E>
    headSet(E toElement, boolean inclusive)
    Returns a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement.
    E
    higher(E e)
    Returns the least element in this set strictly greater than the given element, or null if there is no such element.
    boolean
    isEmpty()
    Returns true if this set contains no elements.
    Iterator<E>
    iterator()
    Returns an iterator over the elements in this set in ascending order.
    E
    last()
    Returns the last (highest) element currently in this set.
    E
    lower(E e)
    Returns the greatest element in this set strictly less than the given element, or null if there is no such element.
    E
    pollFirst()
    Retrieves and removes the first (lowest) element, or returns null if this set is empty.
    E
    pollLast()
    Retrieves and removes the last (highest) element, or returns null if this set is empty.
    boolean
    remove(Object o)
    Removes the specified element from this set if it is present.
    boolean
    removeAll(Collection<?> c)
    Removes from this set all of its elements that are contained in the specified collection.
    int
    size()
    Returns the number of elements in this set.
    Spliterator<E>
    spliterator()
    Returns a Spliterator over the elements in this set.
    NavigableSet<E>
    subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
    Returns a view of the portion of this set whose elements range from fromElement to toElement.
    NavigableSet<E>
    subSet(E fromElement, E toElement)
    Returns a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive.
    NavigableSet<E>
    tailSet(E fromElement)
    Returns a view of the portion of this set whose elements are greater than or equal to fromElement.
    NavigableSet<E>
    tailSet(E fromElement, boolean inclusive)
    Returns a view of the portion of this set whose elements are greater than (or equal to, if inclusive is true) fromElement.

    Methods declared in class java.util.AbstractSet

    hashCode

    Methods declared in class java.util.AbstractCollection

    addAll, containsAll, retainAll, toArray, toArray, toString

    Methods declared in class java.lang.Object

    finalize, getClass, notify, notifyAll, wait, wait, wait

    Methods declared in interface java.util.Collection

    parallelStream, removeIf, stream, toArray

    Methods declared in interface java.lang.Iterable

    forEach

    Methods declared in interface java.util.Set

    addAll, containsAll, hashCode, retainAll, toArray, toArray

  • Constructor Details

    • ConcurrentSkipListSet

      public ConcurrentSkipListSet()
      Constructs a new, empty set that orders its elements according to their natural ordering.

    • ConcurrentSkipListSet

      public ConcurrentSkipListSet(Comparator<? super E> comparator)
      Constructs a new, empty set that orders its elements according to the specified comparator.
      Parameters:
      comparator - the comparator that will be used to order this set. If null, the natural ordering of the elements will be used.

    • ConcurrentSkipListSet

      public ConcurrentSkipListSet(Collection<? extends E> c)
      Constructs a new set containing the elements in the specified collection, that orders its elements according to their natural ordering.
      Parameters:
      c - The elements that will comprise the new set
      Throws:
      ClassCastException - if the elements in c are not Comparable, or are not mutually comparable
      NullPointerException - if the specified collection or any of its elements are null

    • ConcurrentSkipListSet

      public ConcurrentSkipListSet(SortedSet<E> s)
      Constructs a new set containing the same elements and using the same ordering as the specified sorted set.
      Parameters:
      s - sorted set whose elements will comprise the new set
      Throws:
      NullPointerException - if the specified sorted set or any of its elements are null

  • Method Details

    • clone

      public ConcurrentSkipListSet<E> clone()
      Returns a shallow copy of this ConcurrentSkipListSet instance. (The elements themselves are not cloned.)
      Overrides:
      clone in class Object
      Returns:
      a shallow copy of this set
      See Also:

    • size

      public int size()
      Returns the number of elements in this set. If this set contains more than Integer.MAX_VALUE elements, it returns Integer.MAX_VALUE.

      Beware that, unlike in most collections, this method is NOT a constant-time operation. Because of the asynchronous nature of these sets, determining the current number of elements requires traversing them all to count them. Additionally, it is possible for the size to change during execution of this method, in which case the returned result will be inaccurate. Thus, this method is typically not very useful in concurrent applications.

      Specified by:
      size in interface Collection<E>
      Specified by:
      size in interface Set<E>
      Returns:
      the number of elements in this set

    • isEmpty

      public boolean isEmpty()
      Returns true if this set contains no elements.
      Specified by:
      isEmpty in interface Collection<E>
      Specified by:
      isEmpty in interface Set<E>
      Overrides:
      isEmpty in class AbstractCollection<E>
      Returns:
      true if this set contains no elements

    • contains

      public boolean contains(Object o)
      Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that o.equals(e).
      Specified by:
      contains in interface Collection<E>
      Specified by:
      contains in interface Set<E>
      Overrides:
      contains in class AbstractCollection<E>
      Parameters:
      o - object to be checked for containment in this set
      Returns:
      true if this set contains the specified element
      Throws:
      ClassCastException - if the specified element cannot be compared with the elements currently in this set
      NullPointerException - if the specified element is null

    • add

      public boolean add(E e)
      Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if the set contains no element e2 such that e.equals(e2). If this set already contains the element, the call leaves the set unchanged and returns false.
      Specified by:
      add in interface Collection<E>
      Specified by:
      add in interface Set<E>
      Overrides:
      add in class AbstractCollection<E>
      Parameters:
      e - element to be added to this set
      Returns:
      true if this set did not already contain the specified element
      Throws:
      ClassCastException - if e cannot be compared with the elements currently in this set
      NullPointerException - if the specified element is null

    • remove

      public boolean remove(Object o)
      Removes the specified element from this set if it is present. More formally, removes an element e such that o.equals(e), if this set contains such an element. Returns true if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)
      Specified by:
      remove in interface Collection<E>
      Specified by:
      remove in interface Set<E>
      Overrides:
      remove in class AbstractCollection<E>
      Parameters:
      o - object to be removed from this set, if present
      Returns:
      true if this set contained the specified element
      Throws:
      ClassCastException - if o cannot be compared with the elements currently in this set
      NullPointerException - if the specified element is null

    • clear

      public void clear()
      Removes all of the elements from this set.
      Specified by:
      clear in interface Collection<E>
      Specified by:
      clear in interface Set<E>
      Overrides:
      clear in class AbstractCollection<E>

    • iterator

      public Iterator<E> iterator()
      Returns an iterator over the elements in this set in ascending order.
      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Specified by:
      iterator in interface NavigableSet<E>
      Specified by:
      iterator in interface Set<E>
      Specified by:
      iterator in class AbstractCollection<E>
      Returns:
      an iterator over the elements in this set in ascending order

    • descendingIterator

      public Iterator<E> descendingIterator()
      Returns an iterator over the elements in this set in descending order.
      Specified by:
      descendingIterator in interface NavigableSet<E>
      Returns:
      an iterator over the elements in this set in descending order

    • equals

      public boolean equals(Object o)
      Compares the specified object with this set for equality. Returns true if the specified object is also a set, the two sets have the same size, and every member of the specified set is contained in this set (or equivalently, every member of this set is contained in the specified set). This definition ensures that the equals method works properly across different implementations of the set interface.
      Specified by:
      equals in interface Collection<E>
      Specified by:
      equals in interface Set<E>
      Overrides:
      equals in class AbstractSet<E>
      Parameters:
      o - the object to be compared for equality with this set
      Returns:
      true if the specified object is equal to this set
      See Also:

    • removeAll

      public boolean removeAll(Collection<?> c)
      Removes from this set all of its elements that are contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.
      Specified by:
      removeAll in interface Collection<E>
      Specified by:
      removeAll in interface Set<E>
      Overrides:
      removeAll in class AbstractSet<E>
      Parameters:
      c - collection containing elements to be removed from this set
      Returns:
      true if this set changed as a result of the call
      Throws:
      ClassCastException - if the class of an element of this set is incompatible with the specified collection (optional)
      NullPointerException - if the specified collection or any of its elements are null
      See Also:

    • lower

      public E lower(E e)
      Description copied from interface: NavigableSet
      Returns the greatest element in this set strictly less than the given element, or null if there is no such element.
      Specified by:
      lower in interface NavigableSet<E>
      Parameters:
      e - the value to match
      Returns:
      the greatest element less than e, or null if there is no such element
      Throws:
      ClassCastException - if the specified element cannot be compared with the elements currently in the set
      NullPointerException - if the specified element is null

    • floor

      public E floor(E e)
      Description copied from interface: NavigableSet
      Returns the greatest element in this set less than or equal to the given element, or null if there is no such element.
      Specified by:
      floor in interface NavigableSet<E>
      Parameters:
      e - the value to match
      Returns:
      the greatest element less than or equal to e, or null if there is no such element
      Throws:
      ClassCastException - if the specified element cannot be compared with the elements currently in the set
      NullPointerException - if the specified element is null

    • ceiling

      public E ceiling(E e)
      Description copied from interface: NavigableSet
      Returns the least element in this set greater than or equal to the given element, or null if there is no such element.
      Specified by:
      ceiling in interface NavigableSet<E>
      Parameters:
      e - the value to match
      Returns:
      the least element greater than or equal to e, or null if there is no such element
      Throws:
      ClassCastException - if the specified element cannot be compared with the elements currently in the set
      NullPointerException - if the specified element is null

    • higher

      public E higher(E e)
      Description copied from interface: NavigableSet
      Returns the least element in this set strictly greater than the given element, or null if there is no such element.
      Specified by:
      higher in interface NavigableSet<E>
      Parameters:
      e - the value to match
      Returns:
      the least element greater than e, or null if there is no such element
      Throws:
      ClassCastException - if the specified element cannot be compared with the elements currently in the set
      NullPointerException - if the specified element is null

    • pollFirst

      public E pollFirst()
      Description copied from interface: NavigableSet
      Retrieves and removes the first (lowest) element, or returns null if this set is empty.
      Specified by:
      pollFirst in interface NavigableSet<E>
      Returns:
      the first element, or null if this set is empty

    • pollLast

      public E pollLast()
      Description copied from interface: NavigableSet
      Retrieves and removes the last (highest) element, or returns null if this set is empty.
      Specified by:
      pollLast in interface NavigableSet<E>
      Returns:
      the last element, or null if this set is empty

    • comparator

      public Comparator<? super E> comparator()
      Description copied from interface: SortedSet
      Returns the comparator used to order the elements in this set, or null if this set uses the natural ordering of its elements.
      Specified by:
      comparator in interface SortedSet<E>
      Returns:
      the comparator used to order the elements in this set, or null if this set uses the natural ordering of its elements

    • first

      public E first()
      Description copied from interface: SortedSet
      Returns the first (lowest) element currently in this set.
      Specified by:
      first in interface SortedSet<E>
      Returns:
      the first (lowest) element currently in this set
      Throws:
      NoSuchElementException - if this set is empty

    • last

      public E last()
      Description copied from interface: SortedSet
      Returns the last (highest) element currently in this set.
      Specified by:
      last in interface SortedSet<E>
      Returns:
      the last (highest) element currently in this set
      Throws:
      NoSuchElementException - if this set is empty

    • subSet

      public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
      Description copied from interface: NavigableSet
      Returns a view of the portion of this set whose elements range from fromElement to toElement. If fromElement and toElement are equal, the returned set is empty unless fromInclusive and toInclusive are both true. 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.

      Specified by:
      subSet in interface NavigableSet<E>
      Parameters:
      fromElement - low endpoint of the returned set
      fromInclusive - true if the low endpoint is to be included in the returned view
      toElement - high endpoint of the returned set
      toInclusive - true if the high endpoint is to be included in the returned view
      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
      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

      public NavigableSet<E> headSet(E toElement, boolean inclusive)
      Description copied from interface: NavigableSet
      Returns a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) 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.

      Specified by:
      headSet in interface NavigableSet<E>
      Parameters:
      toElement - high endpoint of the returned set
      inclusive - true if the high endpoint is to be included in the returned view
      Returns:
      a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) 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
      IllegalArgumentException - if this set itself has a restricted range, and toElement lies outside the bounds of the range

    • tailSet

      public NavigableSet<E> tailSet(E fromElement, boolean inclusive)
      Description copied from interface: NavigableSet
      Returns a view of the portion of this set whose elements are greater than (or equal to, if inclusive is true) 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.

      Specified by:
      tailSet in interface NavigableSet<E>
      Parameters:
      fromElement - low endpoint of the returned set
      inclusive - true if the low endpoint is to be included in the returned view
      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
      IllegalArgumentException - if this set itself has a restricted range, and fromElement lies outside the bounds of the range

    • subSet

      public NavigableSet<E> subSet(E fromElement, E toElement)
      Description copied from interface: NavigableSet
      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.

      Equivalent to subSet(fromElement, true, toElement, false).

      Specified by:
      subSet in interface NavigableSet<E>
      Specified by:
      subSet in interface SortedSet<E>
      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
      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

      public NavigableSet<E> headSet(E toElement)
      Description copied from interface: NavigableSet
      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.

      Equivalent to headSet(toElement, false).

      Specified by:
      headSet in interface NavigableSet<E>
      Specified by:
      headSet in interface SortedSet<E>
      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
      IllegalArgumentException - if this set itself has a restricted range, and toElement lies outside the bounds of the range

    • tailSet

      public NavigableSet<E> tailSet(E fromElement)
      Description copied from interface: NavigableSet
      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.

      Equivalent to tailSet(fromElement, true).

      Specified by:
      tailSet in interface NavigableSet<E>
      Specified by:
      tailSet in interface SortedSet<E>
      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
      IllegalArgumentException - if this set itself has a restricted range, and fromElement lies outside the bounds of the range

    • descendingSet

      public NavigableSet<E> descendingSet()
      Returns a reverse order view of the elements contained in this set. The descending set is backed by this set, so changes to the set are reflected in the descending set, and vice-versa.

      The returned set has an ordering equivalent to Collections.reverseOrder(comparator()). The expression s.descendingSet().descendingSet() returns a view of s essentially equivalent to s.

      Specified by:
      descendingSet in interface NavigableSet<E>
      Returns:
      a reverse order view of this set

    • spliterator

      public Spliterator<E> spliterator()
      Returns a Spliterator over the elements in this set.

      The Spliterator reports Spliterator.CONCURRENT, Spliterator.NONNULL, Spliterator.DISTINCT, Spliterator.SORTED and Spliterator.ORDERED, with an encounter order that is ascending order. Overriding implementations should document the reporting of additional characteristic values.

      The spliterator's comparator is null if the set's comparator is null. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the set's comparator.

      Specified by:
      spliterator in interface Collection<E>
      Specified by:
      spliterator in interface Iterable<E>
      Specified by:
      spliterator in interface Set<E>
      Specified by:
      spliterator in interface SortedSet<E>
      Returns:
      a Spliterator over the elements in this set
      Since:
      1.8