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ConcurrentSkipListSet


java.util.concurrent

Class 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 are weakly consistent, returning elements reflecting the state of the set at some point at or since the creation of the iterator. They do not throw ConcurrentModificationException, and may proceed concurrently with other operations. 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. Additionally, the bulk operations addAll, removeAll, retainAll, containsAll, equals, and toArray are not guaranteed to be performed atomically. For example, an iterator operating concurrently with an addAll operation might view 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.

    Since:
    1.6
    See Also:
    Serialized Form
    • Method Summary

      Methods
      Modifier and Type Method and 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 compare elements in this SortedSet.
      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 element in this SortedSet.
      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 SortedSet of the specified portion of this SortedSet which contains elements less than the end element.
      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 element in this SortedSet.
      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.
      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 SortedSet of the specified portion of this SortedSet which contains elements greater or equal to the start element but less than the end element.
      NavigableSet<E> tailSet(E fromElement)
      Returns a SortedSet of the specified portion of this SortedSet which contains elements greater or equal to the start element.
      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.
    • Constructor Detail

      • 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 Detail

      • 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
      • 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>
        Specified by:
        size in class AbstractCollection<E>
        Returns:
        the number of elements in this set
      • 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
      • 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:
        AbstractSet.hashCode()
      • 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 types of one or more elements in this set are incompatible with the specified collection
        NullPointerException - if the specified collection or any of its elements are null
      • 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 compare elements in this SortedSet.
        Specified by:
        comparator in interface SortedSet<E>
        Returns:
        a comparator or null if the natural ordering is used.
      • first

        public E first()
        Description copied from interface: SortedSet
        Returns the first element in this SortedSet. The first element is the lowest element.
        Specified by:
        first in interface SortedSet<E>
        Returns:
        the first element.
        Throws:
        NoSuchElementException - when this SortedSet is empty.
      • last

        public E last()
        Description copied from interface: SortedSet
        Returns the last element in this SortedSet. The last element is the highest element.
        Specified by:
        last in interface SortedSet<E>
        Returns:
        the last element.
        Throws:
        NoSuchElementException - when this SortedSet 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 fromExclusive and toExclusive 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 SortedSet of the specified portion of this SortedSet which contains elements greater or equal to the start element but less than the end element. The returned SortedSet is backed by this SortedMap so changes to one set are reflected by the other.

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

        Specified by:
        subSet in interface NavigableSet<E>
        Specified by:
        subSet in interface SortedSet<E>
        Parameters:
        fromElement - the start element.
        toElement - the end element.
        Returns:
        a subset where the elements are greater or equal to start and less than end.
        Throws:
        ClassCastException - when the class of the start or end element is inappropriate for this SubSet.
        NullPointerException - if fromElement or toElement is null
        IllegalArgumentException - when the start element is greater than the end element.
      • headSet

        public NavigableSet<E> headSet(E toElement)
        Description copied from interface: NavigableSet
        Returns a SortedSet of the specified portion of this SortedSet which contains elements less than the end element. The returned SortedSet is backed by this SortedSet so changes to one set are reflected by the other.

        Equivalent to headSet(toElement, false).

        Specified by:
        headSet in interface NavigableSet<E>
        Specified by:
        headSet in interface SortedSet<E>
        Parameters:
        toElement - the end element.
        Returns:
        a subset where the elements are less than end.
        Throws:
        ClassCastException - when the class of the end element is inappropriate for this SubSet.
        NullPointerException - if toElement is null
        IllegalArgumentException - na
      • tailSet

        public NavigableSet<E> tailSet(E fromElement)
        Description copied from interface: NavigableSet
        Returns a SortedSet of the specified portion of this SortedSet which contains elements greater or equal to the start element. The returned SortedSet is backed by this SortedSet so changes to one set are reflected by the other.

        Equivalent to tailSet(fromElement, true).

        Specified by:
        tailSet in interface NavigableSet<E>
        Specified by:
        tailSet in interface SortedSet<E>
        Parameters:
        fromElement - the start element.
        Returns:
        a subset where the elements are greater or equal to start .
        Throws:
        ClassCastException - when the class of the start element is inappropriate for this SubSet.
        NullPointerException - if fromElement is null
        IllegalArgumentException
      • 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


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