java.util.concurrent
Class CopyOnWriteArraySet<E>

java.lang.Object
  java.util.AbstractCollection
      java.util.AbstractSet
          java.util.concurrent.CopyOnWriteArraySet

All Implemented Interfaces:

java.lang.Cloneable, Collection, java.lang.Iterable, java.io.Serializable, Set

public class CopyOnWriteArraySet<E>

extends AbstractSet<E>

implements java.lang.Cloneable, java.io.Serializable

A Set that uses 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.
• Mutative operations(add, set, remove, etc) are expensive since they usually entail copying the entire underlying array.
• Loops involving repeated element-by-element mutative operations are so expensive that they should generally be avoided.
• Iterators do not support the mutative remove operation
• Traversal via iterators is very fast and cannot ever encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

Sample Usage. Probably the main application of copy-on-write sets are classes that maintain sets of Handler objects that must be multicasted to upon an update command. This is a classic case where you do not want to be holding a lock while sending a message, and where traversals normally vastly overwhelm additions.

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

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

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

    public void update() {
       changeState();
       Iterator it = handlers.iterator();
       while (it.hasNext())
          ((Handler)(it.next()).handle();
    }
 }
 

Since:

1.5

Author:

Doug Lea

See Also:

CopyOnWriteArrayList, Serialized Form

Constructor Summary

CopyOnWriteArraySet()
Constructs an empty set.

CopyOnWriteArraySet(Collection<T> c)
Constructs a set containing all of the elements of the specified Collection.

Method Summary

boolean	add(E o) Ensures that this collection contains the specified element (optional operation).
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
void	clear() Removes all of the elements from this collection (optional operation).
boolean	contains(java.lang.Object o) Returns true if this collection contains the specified element.
boolean	containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection.
boolean	isEmpty() Returns true if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements contained in this collection.
boolean	remove(java.lang.Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
boolean	removeAll(Collection<?> c) Removes from this set all of its elements that are contained in the specified collection (optional operation).
boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
java.lang.Object[]	toArray() Returns an array containing all of the elements in this collection.
<T> T[]	toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.

Methods inherited from class java.util.AbstractSet

equals, 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()

Constructs an empty set.

CopyOnWriteArraySet

public CopyOnWriteArraySet(Collection<T> c)

Constructs a set containing all of the elements of the specified Collection.

Parameters:

c - the collection

Method Detail

size

public int size()

Description copied from class: AbstractCollection

Returns the number of elements in this collection. If the collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Specified by:

size in interface Set

Specified by:

size in class AbstractCollection

Returns:

the number of elements in this collection.

isEmpty

public boolean isEmpty()

Description copied from class: AbstractCollection

Returns true if this collection contains no elements.

This implementation returns size() == 0.

Specified by:

isEmpty in interface Set

Overrides:

isEmpty in class AbstractCollection

Returns:

true if this collection contains no elements.

contains

public boolean contains(java.lang.Object o)

Description copied from class: AbstractCollection

Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e)).

This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.

Specified by:

contains in interface Set

Overrides:

contains in class AbstractCollection

Parameters:

o - object to be checked for containment in this collection.

Returns:

true if this collection contains the specified element.

toArray

public java.lang.Object[] toArray()

Description copied from class: AbstractCollection

Returns an array containing all of the elements in this collection. If the collection 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 the collection. (In other words, this method must allocate a new array even if the collection is backed by an Array). The caller is thus free to modify the returned array.

This implementation allocates the array to be returned, and iterates over the elements in the collection, storing each object reference in the next consecutive element of the array, starting with element 0.

Specified by:

toArray in interface Set

Overrides:

toArray in class AbstractCollection

Returns:

an array containing all of the elements in this collection.

toArray

public <T> T[] toArray(T[] a)

Description copied from class: AbstractCollection

Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection 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 collection.

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

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

This implementation checks if the array is large enough to contain the collection; if not, it allocates a new array of the correct size and type (using reflection). Then, it iterates over the collection, storing each object reference in the next consecutive element of the array, starting with element 0. If the array is larger than the collection, a null is stored in the first location after the end of the collection.

Specified by:

toArray in interface Set

Overrides:

toArray in class AbstractCollection

Parameters:

a - the array into which the elements of the collection 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 the elements of the collection.

clear

public void clear()

Description copied from class: AbstractCollection

Removes all of the elements from this collection (optional operation). The collection will be empty after this call returns (unless it throws an exception).

This implementation iterates over this collection, removing each element using the Iterator.remove operation. Most implementations will probably choose to override this method for efficiency.

Note that this implementation will throw an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection is non-empty.

Specified by:

clear in interface Set

Overrides:

clear in class AbstractCollection

iterator

public Iterator<E> iterator()

Description copied from class: AbstractCollection

Returns an iterator over the elements contained in this collection.

Specified by:

iterator in interface Set

Specified by:

iterator in class AbstractCollection

Returns:

an iterator over the elements contained in this collection.

remove

public boolean remove(java.lang.Object o)

Description copied from class: AbstractCollection

Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if the collection contains one or more such elements. Returns true if the collection contained the specified element (or equivalently, if the collection changed as a result of the call).

This implementation iterates over the collection looking for the specified element. If it finds the element, it removes the element from the collection using the iterator's remove method.

Note that this implementation throws an UnsupportedOperationException if the iterator returned by this collection's iterator method does not implement the remove method and this collection contains the specified object.

Specified by:

remove in interface Set

Overrides:

remove in class AbstractCollection

Parameters:

o - element to be removed from this collection, if present.

Returns:

true if the collection contained the specified element.

add

public boolean add(E o)

Description copied from class: AbstractCollection

Ensures that this collection contains the specified element (optional operation). Returns true if the collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.) Collections that support this operation may place limitations on what elements may be added to the collection. In particular, some collections will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added.

This implementation always throws an UnsupportedOperationException.

Specified by:

add in interface Set

Overrides:

add in class AbstractCollection

Parameters:

o - element whose presence in this collection is to be ensured.

Returns:

true if the collection changed as a result of the call.

containsAll

public boolean containsAll(Collection<?> c)

Description copied from class: AbstractCollection

Returns true if this collection contains all of the elements in the specified collection.

This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection. If all elements are so contained true is returned, otherwise false.

Specified by:

containsAll in interface Set

Overrides:

containsAll in class AbstractCollection

Parameters:

c - collection to be checked for containment in this collection.

Returns:

true if this collection contains all of the elements in the specified collection.

See Also:

AbstractCollection.contains(Object)

addAll

public boolean addAll(Collection<? extends E> c)

Description copied from class: AbstractCollection

Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.)

This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.

Note that this implementation will throw an UnsupportedOperationException unless add is overridden (assuming the specified collection is non-empty).

Specified by:

addAll in interface Set

Overrides:

addAll in class AbstractCollection

Parameters:

c - collection whose elements are to be added to this collection.

Returns:

true if this collection changed as a result of the call.

See Also:

AbstractCollection.add(Object)

removeAll

public boolean removeAll(Collection<?> c)

Description copied from class: AbstractSet

Removes from this set all of its elements that are contained in the specified collection (optional operation).

This implementation determines which is the smaller of this set and the specified collection, by invoking the size method on each. If this set has fewer elements, then the implementation iterates over this set, checking each element returned by the iterator in turn to see if it is contained in the specified collection. If it is so contained, it is removed from this set with the iterator's remove method. If the specified collection has fewer elements, then the implementation iterates over the specified collection, removing from this set each element returned by the iterator, using this set's remove method.

Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method.

Specified by:

removeAll in interface Set

Overrides:

removeAll in class AbstractSet

Parameters:

c - elements to be removed from this set.

Returns:

true if this set changed as a result of the call.

See Also:

AbstractCollection.remove(Object), AbstractCollection.contains(Object)

retainAll

public boolean retainAll(Collection<?> c)

Description copied from class: AbstractCollection

Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.

This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection. If it's not so contained, it's removed from this collection with the iterator's remove method.

Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method and this collection contains one or more elements not present in the specified collection.

Specified by:

retainAll in interface Set

Overrides:

retainAll in class AbstractCollection

Parameters:

c - elements to be retained in this collection.

Returns:

true if this collection changed as a result of the call.

See Also:

AbstractCollection.remove(Object), AbstractCollection.contains(Object)