Module java.base
Package java.util.concurrent

Class CopyOnWriteArraySet<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractSet<E>
java.util.concurrent.CopyOnWriteArraySet<E>
Type Parameters:
E - the type of elements held in this set
All Implemented Interfaces:
Serializable, Iterable<E>, Collection<E>, Set<E>
public class CopyOnWriteArraySet<E> extends AbstractSet<E> implements Serializable
A Set that uses an internal CopyOnWriteArrayList for all of its operations. Thus, it shares the same basic properties:
  • It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
  • It is thread-safe.
  • Mutative operations (add, set, remove, etc.) are expensive since they usually entail copying the entire underlying array.
  • Iterators do not support the mutative remove operation.
  • Traversal via iterators is fast and cannot encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

Sample Usage. The following code sketch uses a copy-on-write set to maintain a set of Handler objects that perform some action upon state updates. 

 
 class Handler { void handle() { ... } }

 class X {
   private final CopyOnWriteArraySet<Handler> handlers
     = new CopyOnWriteArraySet<>();
   public void addHandler(Handler h) { handlers.add(h); }

   private long internalState;
   private synchronized void changeState() { internalState = ...; }

   public void update() {
     changeState();
     for (Handler handler : handlers)
       handler.handle();
   }
 }

This class is a member of the Java Collections Framework.

Since:
1.5
Author:
Doug Lea
See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    CopyOnWriteArraySet()
    Creates an empty set.
    CopyOnWriteArraySet(Collection<? extends E> c)
    Creates a set containing all of the elements of the specified collection.

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    add(E e)
    Adds the specified element to this set if it is not already present.
    boolean
    addAll(Collection<? extends E> c)
    Adds all of the elements in the specified collection to this set if they're not already present.
    void
    clear()
    Removes all of the elements from this set.
    boolean
    contains(Object o)
    Returns true if this set contains the specified element.
    boolean
    containsAll(Collection<?> c)
    Returns true if this set contains all of the elements of the specified collection.
    boolean
    equals(Object o)
    Compares the specified object with this set for equality.
    void
    forEach(Consumer<? super E> action)
     
    boolean
    isEmpty()
    Returns true if this set contains no elements.
    Iterator<E>
    iterator()
    Returns an iterator over the elements contained in this set in the order in which these elements were added.
    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.
    boolean
    removeIf(Predicate<? super E> filter)
     
    boolean
    retainAll(Collection<?> c)
    Retains only the elements in this set 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 in the order in which these elements were added.
    Object[]
    toArray()
    Returns an array containing all of the elements in this set.
    <T> T[]
    toArray(T[] a)
    Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.

    Methods declared in class java.util.AbstractSet

    hashCode

    Methods declared in class java.util.AbstractCollection

    toString

    Methods declared in class java.lang.Object

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

    Methods declared in interface java.util.Collection

    parallelStream, stream, toArray

  • Constructor Details

    • CopyOnWriteArraySet

      public CopyOnWriteArraySet()
      Creates an empty set.

    • CopyOnWriteArraySet

      public CopyOnWriteArraySet(Collection<? extends E> c)
      Creates a set containing all of the elements of the specified collection.
      Parameters:
      c - the collection of elements to initially contain
      Throws:
      NullPointerException - if the specified collection is null

  • Method Details

    • size

      public int size()
      Returns the number of elements in this set.
      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

    • 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 Objects.equals(o, e).
      Specified by:
      contains in interface Collection<E>
      Specified by:
      contains in interface Set<E>
      Overrides:
      contains in class AbstractCollection<E>
      Parameters:
      o - element whose presence in this set is to be tested
      Returns:
      true if this set contains the specified element

    • toArray

      public Object[] toArray()
      Returns an array containing all of the elements in this set. If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

      The returned array will be "safe" in that no references to it are maintained by this set. (In other words, this method must allocate a new array even if this set is backed by an array). The caller is thus free to modify the returned array.

      This method acts as bridge between array-based and collection-based APIs.

      Specified by:
      toArray in interface Collection<E>
      Specified by:
      toArray in interface Set<E>
      Overrides:
      toArray in class AbstractCollection<E>
      Returns:
      an array containing all the elements in this set

    • toArray

      public <T> T[] toArray(T[] a)
      Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array. If the set fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this set.

      If this set fits in the specified array with room to spare (i.e., the array has more elements than this set), the element in the array immediately following the end of the set is set to null. (This is useful in determining the length of this set only if the caller knows that this set does not contain any null elements.)

      If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

      Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

      Suppose x is a set known to contain only strings. The following code can be used to dump the set into a newly allocated array of String: 

       String[] y = x.toArray(new String[0]);
      Note that toArray(new Object[0]) is identical in function to toArray().
      Specified by:
      toArray in interface Collection<E>
      Specified by:
      toArray in interface Set<E>
      Overrides:
      toArray in class AbstractCollection<E>
      Parameters:
      a - the array into which the elements of this set are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
      Returns:
      an array containing all the elements in this set
      Throws:
      ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this set
      NullPointerException - if the specified array is null

    • clear

      public void clear()
      Removes all of the elements from this set. The set will be empty after this call returns.
      Specified by:
      clear in interface Collection<E>
      Specified by:
      clear in interface Set<E>
      Overrides:
      clear in class AbstractCollection<E>

    • 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 Objects.equals(o, 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

    • 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 Objects.equals(e, 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

    • containsAll

      public boolean containsAll(Collection<?> c)
      Returns true if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns true if it is a subset of this set.
      Specified by:
      containsAll in interface Collection<E>
      Specified by:
      containsAll in interface Set<E>
      Overrides:
      containsAll in class AbstractCollection<E>
      Parameters:
      c - collection to be checked for containment in this set
      Returns:
      true if this set contains all of the elements of the specified collection
      Throws:
      NullPointerException - if the specified collection is null
      See Also:

    • addAll

      public boolean addAll(Collection<? extends E> c)
      Adds all of the elements in the specified collection to this set if they're not already present. If the specified collection is also a set, the addAll operation effectively modifies this set so that its value is the union of the two sets. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
      Specified by:
      addAll in interface Collection<E>
      Specified by:
      addAll in interface Set<E>
      Overrides:
      addAll in class AbstractCollection<E>
      Parameters:
      c - collection containing elements to be added to this set
      Returns:
      true if this set changed as a result of the call
      Throws:
      NullPointerException - if the specified collection is null
      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 this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:

    • retainAll

      public boolean retainAll(Collection<?> c)
      Retains only the elements in this set that are contained in the specified collection. In other words, removes from this set all of its elements that are not 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 intersection of the two sets.
      Specified by:
      retainAll in interface Collection<E>
      Specified by:
      retainAll in interface Set<E>
      Overrides:
      retainAll in class AbstractCollection<E>
      Parameters:
      c - collection containing elements to be retained in 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 this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
      See Also:

    • iterator

      public Iterator<E> iterator()
      Returns an iterator over the elements contained in this set in the order in which these elements were added.

      The returned iterator provides a snapshot of the state of the set when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove method.

      Specified by:
      iterator in interface Collection<E>
      Specified by:
      iterator in interface Iterable<E>
      Specified by:
      iterator in interface Set<E>
      Specified by:
      iterator in class AbstractCollection<E>
      Returns:
      an iterator over the elements in this set

    • equals

      public boolean equals(Object o)
      Compares the specified object with this set for equality. Returns true if the specified object is the same object as this object, or if it is also a Set and the elements returned by an iterator over the specified set are the same as the elements returned by an iterator over this set. More formally, the two iterators are considered to return the same elements if they return the same number of elements and for every element e1 returned by the iterator over the specified set, there is an element e2 returned by the iterator over this set such that Objects.equals(e1, e2).
      Specified by:
      equals in interface Collection<E>
      Specified by:
      equals in interface Set<E>
      Overrides:
      equals in class AbstractSet<E>
      Parameters:
      o - object to be compared for equality with this set
      Returns:
      true if the specified object is equal to this set

    • removeIf

      public boolean removeIf(Predicate<? super E> filter)
      Specified by:
      removeIf in interface Collection<E>
      Throws:
      NullPointerException

    • forEach

      public void forEach(Consumer<? super E> action)
      Specified by:
      forEach in interface Iterable<E>
      Throws:
      NullPointerException

    • spliterator

      public Spliterator<E> spliterator()
      Returns a Spliterator over the elements in this set in the order in which these elements were added.

      The Spliterator reports Spliterator.IMMUTABLE, Spliterator.DISTINCT, Spliterator.SIZED, and Spliterator.SUBSIZED.

      The spliterator provides a snapshot of the state of the set when the spliterator was constructed. No synchronization is needed while operating on the spliterator.

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