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

 

Method Summary
boolean	add ( E           Adds           Ensures that this collection contains the specified element to this set if it is not already present. (optional operation).
boolean	addAll ( Collection <? extends E           Adds all of the elements in the specified collection to this set if they're not already present. collection (optional operation).
void	clear ()           Removes all of the elements from this set. collection (optional operation).
boolean	contains ( Object           Returns true if this set collection contains the specified element.
boolean	containsAll ( Collection           Returns true if this set collection contains all of the elements of in the specified collection.
boolean	isEmpty ()           Returns true if this set collection 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. collection.
boolean	remove ( Object           Removes a single instance of the specified element from this set collection, if it is present. present (optional operation).
boolean	removeAll ( Collection           Removes from this set all of its elements that are contained in the specified collection. collection (optional operation).
boolean	retainAll ( Collection           Retains only the elements in this set collection that are contained in the specified collection. collection (optional operation).
int	size ()           Returns the number of elements in this set. collection.
Object	toArray ()           Returns an array containing all of the elements in this set. collection.
<T> T[]	toArray (T[] a)           Returns an array containing all of the elements in this set; 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()

Creates an empty set.

CopyOnWriteArraySet

public CopyOnWriteArraySet(Collection<? extends E> c)

Creates a set containing all of the elements of the specified collection. Collection.

Parameters:

c - the collection of elements to initially contain

Throws:

NullPointerException - if the specified collection is null c - the collection

size

public int size()

Returns the number of elements in this set. 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 Collection < E >

Specified by:

size in interface Set < E >

Specified by:

size in class AbstractCollection < E >

Returns:

the number of elements in this set collection.

isEmpty

public boolean isEmpty()

Description copied from class: AbstractCollection

Returns true if this set collection contains no elements.

This implementation returns size() == 0 .

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 collection contains no elements elements.

contains

public boolean contains(Object o)

Description copied from class: AbstractCollection

Returns true if this set collection contains the specified element. More formally, returns true if and only if this set collection contains an at least one element e such that (o==null ? e==null : o.equals(e)) (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 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 o - object to be checked for containment in this collection.

Returns:

true if this set collection contains the specified element element.

toArray

public Object[] toArray()

Description copied from class: AbstractCollection

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

This method acts as bridge between array-based and collection-based APIs. 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 Collection < E >

Specified by:

toArray in interface Set < E >

Overrides:

toArray in class AbstractCollection < E >

Returns:

an array containing all of the elements in this set collection.

toArray

public <T> T[] toArray(T[] a)

Description copied from class: AbstractCollection

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

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

If this set 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.

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 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 x null 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() . is stored in the first location after the end of the collection.

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 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 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 an array containing the elements of the collection.

clear

public void clear()

Description copied from class: AbstractCollection

Removes all of the elements from this set. collection (optional operation). The set collection will be empty after this call returns. 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 Collection < E >

Specified by:

clear in interface Set < E >

Overrides:

clear in class AbstractCollection < E >

remove iterator

public boolean 
public Iterator< E> remove iterator ( Object o) () 

Removes the specified element from this set if it is present. More formally, removes an element e such that (o==null ? e==null : 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.) Description copied from class: AbstractCollection

Returns an iterator over the elements contained in this collection.

Specified by:

remove iterator in interface Iterable < E >

Specified by:

iterator in interface Collection < E >

Specified by:

remove iterator in interface Set < E >

Overrides: Specified by:

remove iterator in class AbstractCollection < E >

Parameters:

o - object to be removed from this set, if present

Returns:

true if this set contained the specified element an iterator over the elements contained in this collection.

add remove

public boolean add remove ( EObject e)  o) 

Adds the specified element to this set if it is not already present. More formally, adds the specified element 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 to this set if the set contains no element such that e2 (o==null ? e==null : o.equals(e)) such that , if the collection contains one or more such elements. Returns (e==null ? e2==null : e.equals(e2)) true . If this set already contains the element, the call leaves the set unchanged and returns 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 false 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:

add remove in interface Collection < E >

Specified by:

add remove in interface Set < E >

Overrides:

add remove in class AbstractCollection < E >

Parameters:

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

Returns:

true if this set did not already contain the specified element if the collection contained the specified element.

containsAll add

public boolean containsAll add ( CollectionE<?> c)  o) 

Description copied from class: AbstractCollection

Ensures that this collection contains the specified element (optional operation). 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 if the collection changed as a result of the call. (Returns true false if it is a subset of this set. 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:

containsAll add in interface Collection < E >

Specified by:

containsAll add in interface Set < E >

Overrides:

containsAll add in class AbstractCollection < E >

Parameters:

c - collection to be checked for containment in this set o - element whose presence in this collection is to be ensured.

Returns:

true if this set contains all of the elements of the specified collection

Throws:

NullPointerException - if the specified collection is null

See Also:

contains(Object) if the collection changed as a result of the call.

addAll containsAll

public boolean addAll containsAll (Collection<? extends E> c) <?> 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 Description copied from class: AbstractCollection

Returns addAll true 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. 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:

addAll containsAll in interface Collection < E >

Specified by:

addAll containsAll in interface Set < E >

Overrides:

addAll containsAll in class AbstractCollection < E >

Parameters:

c - collection containing elements to be added to checked for containment in this set collection.

Returns:

true if this set changed as a result of the call if this collection contains all of the elements in the specified collection.

Throws:

NullPointerException - if the specified collection is null

See Also:

add(Object) AbstractCollection.contains(Object)

removeAll addAll

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

removeAll addAll in interface Collection < E >

Specified by:

removeAll addAll in interface Set < E >

Overrides:

removeAll addAll in class AbstractSet AbstractCollection < E >

Parameters:

c - collection containing whose elements are to be removed from added to this set collection.

Returns:

true if this set collection changed as a result of the call 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:

remove(Object) AbstractCollection.add(Object)

retainAll removeAll

public boolean retainAll removeAll (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. 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:

retainAll removeAll in interface Collection < E >

Specified by:

retainAll removeAll in interface Set < E >

Overrides:

retainAll removeAll in class AbstractCollection AbstractSet < E >

Parameters:

c - collection containing elements to be retained in removed from this set set.

Returns:

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

Throws: See Also:

ClassCastException AbstractCollection.remove(Object) - if the class of an element of this set is incompatible with the specified collection (optional)

NullPointerException , AbstractCollection.contains(Object) - 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:

remove(Object)

iterator retainAll

public Iterator< E> 
public boolean iterator retainAll () ( Collection<?> c) 

Returns an iterator over the elements contained in this set in the order in which these elements were added. 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.

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 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:

iterator retainAll in interface Iterable < E >

Specified by:

iterator in interface Collection < E >

Specified by:

iterator retainAll in interface Set < E >

Specified by: Overrides:

iterator retainAll in class AbstractCollection < E >

Parameters:

c - elements to be retained in this collection.

Returns:

an iterator over the elements in this set true if this collection changed as a result of the call.

See Also:

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