Module java.base
Package java.util.concurrent

Class 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
    See Also:
    CopyOnWriteArrayList, Serialized Form

    • Method Summary

      All Methods Instance Methods Concrete Methods 
      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)
      Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
      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)
      Removes all of the elements of this collection that satisfy the given predicate.
      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.

    • Constructor Detail

      • 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 Detail

      • 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