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 collection

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<Handler>();
   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

Constructor Summary

Constructors

Constructor and 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

Methods

Modifier and Type	Method and 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.
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	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.
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 inherited from class java.util.AbstractSet

hashCode

Methods inherited from class java.util.AbstractCollection

toString

Methods inherited from class java.lang.Object

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

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 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==null ? e==null : o.equals(e)).

Specified by:

contains in interface Collection<E>

Specified by:

contains in interface