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Summary: Ctors | Methods | Inherited Methods | [Expand All]

Added in API level 1

public class

CopyOnWriteArraySet

extends AbstractSet<E>
implements Serializable

java.lang.Object
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractSet<E>
		↳	java.util.concurrent.CopyOnWriteArraySet<E>

Class Overview

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 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();
   }
 }}

Summary

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

Public Methods
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.
<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.
Object[]	toArray() Returns an array containing all of the elements in this set.

[Expand]

Inherited Methods

From class java.util.AbstractSet

From class java.util.AbstractCollection

boolean	add(E object) Attempts to add `object` to the contents of this `Collection` (optional).
boolean	addAll(Collection<? extends E> collection) Attempts to add all of the objects contained in `collection` to the contents of this `Collection` (optional).
void	clear() Removes all elements from this `Collection`, leaving it empty (optional).
boolean	contains(Object object) Tests whether this `Collection` contains the specified object.
boolean	containsAll(Collection<?> collection) Tests whether this `Collection` contains all objects contained in the specified `Collection`.
boolean	isEmpty() Returns if this `Collection` contains no elements.
abstract Iterator<E>	iterator() Returns an instance of `Iterator` that may be used to access the objects contained by this `Collection`.
boolean	remove(Object object) Removes one instance of the specified object from this `Collection` if one is contained (optional).
boolean	removeAll(Collection<?> collection) Removes all occurrences in this `Collection` of each object in the specified `Collection` (optional).
boolean	retainAll(Collection<?> collection) Removes all objects from this `Collection` that are not also found in the `Collection` passed (optional).
abstract int	size() Returns a count of how many objects this `Collection` contains.
<T> T[]	toArray(T[] contents) Returns an array containing all elements contained in this `Collection`.
Object[]	toArray() Returns a new array containing all elements contained in this `Collection`.
String	toString() Returns the string representation of this `Collection`.

From class java.lang.Object

Object	clone() Creates and returns a copy of this `Object`.
boolean	equals(Object o) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of `Class` that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object.
final void	wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait(long millis) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.

From interface java.lang.Iterable

From interface java.util.Collection

abstract boolean	add(E object) Attempts to add `object` to the contents of this `Collection` (optional).
abstract boolean	addAll(Collection<? extends E> collection) Attempts to add all of the objects contained in `Collection` to the contents of this `Collection` (optional).
abstract void	clear() Removes all elements from this `Collection`, leaving it empty (optional).
abstract boolean	contains(Object object) Tests whether this `Collection` contains the specified object.
abstract boolean	containsAll(Collection<?> collection) Tests whether this `Collection` contains all objects contained in the specified `Collection`.
abstract boolean	equals(Object object) Compares the argument to the receiver, and returns true if they represent the same object using a class specific comparison.
abstract int	hashCode() Returns an integer hash code for the receiver.
abstract boolean	isEmpty() Returns if this `Collection` contains no elements.
abstract Iterator<E>	iterator() Returns an instance of `Iterator` that may be used to access the objects contained by this `Collection`.
abstract boolean	remove(Object object) Removes one instance of the specified object from this `Collection` if one is contained (optional).
abstract boolean	removeAll(Collection<?> collection) Removes all occurrences in this `Collection` of each object in the specified `Collection` (optional).
abstract boolean	retainAll(Collection<?> collection) Removes all objects from this `Collection` that are not also found in the `Collection` passed (optional).
abstract int	size() Returns a count of how many objects this `Collection` contains.
abstract <T> T[]	toArray(T[] array) Returns an array containing all elements contained in this `Collection`.
abstract Object[]	toArray() Returns a new array containing all elements contained in this `Collection`.

From interface java.util.Set

abstract boolean	add(E object) Adds the specified object to this set.
abstract boolean	addAll(Collection<? extends E> collection) Adds the objects in the specified collection which do not exist yet in this set.
abstract void	clear() Removes all elements from this set, leaving it empty.
abstract boolean	contains(Object object) Searches this set for the specified object.
abstract boolean	containsAll(Collection<?> collection) Searches this set for all objects in the specified collection.
abstract boolean	equals(Object object) Compares the specified object to this set, and returns true if they represent the same object using a class specific comparison.
abstract int	hashCode() Returns the hash code for this set.
abstract boolean	isEmpty() Returns true if this set has no elements.
abstract Iterator<E>	iterator() Returns an iterator on the elements of this set.
abstract boolean	remove(Object object) Removes the specified object from this set.
abstract boolean	removeAll(Collection<?> collection) Removes all objects in the specified collection from this set.
abstract boolean	retainAll(Collection<?> collection) Removes all objects from this set that are not contained in the specified collection.
abstract int	size() Returns the number of elements in this set.
abstract <T> T[]	toArray(T[] array) Returns an array containing all elements contained in this set.
abstract Object[]	toArray() Returns an array containing all elements contained in this set.

Public Constructors

public CopyOnWriteArraySet ()

Added in API level 1

Creates an empty set.

public CopyOnWriteArraySet (Collection<? extends E> c)

Added in API level 1

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

Public Methods

public boolean add (E e)

Added in API level 1

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 (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

Parameters

e	element to be added to this set

Returns

true if this set did not already contain the specified element

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

Added in API level 1

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.

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

public void clear ()

Added in API level 1

Removes all of the elements from this set. The set will be empty after this call returns.

public boolean contains (Object o)

Added in API level 1

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)).

Parameters

o	element whose presence in this set is to be tested

Returns

true if this set contains the specified element

public boolean containsAll (Collection<?> c)

Added in API level 1

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.

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

public boolean equals (Object o)

Added in API level 1

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 (e1==null ? e2==null : e1.equals(e2)).

Parameters

o	object to be compared for equality with this set

Returns

true if the specified object is equal to this set

public boolean isEmpty ()

Added in API level 1

Returns true if this set contains no elements.

Returns

true if this set contains no elements

public Iterator<E> iterator ()

Added in API level 1

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.

Returns

an iterator over the elements in this set

public boolean remove (Object o)

Added in API level 1

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.)

Parameters

o	object to be removed from this set, if present

Returns

true if this set contained the specified element

public boolean removeAll (Collection<?> c)

Added in API level 1

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.

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

public boolean retainAll (Collection<?> c)

Added in API level 1

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.

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

public int size ()

Added in API level 1

Returns the number of elements in this set.

Returns

the number of elements in this set

public T[] toArray (T[] a)

Added in API level 1

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().

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

public Object[] toArray ()

Added in API level 1

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.

Returns

an array containing all the elements in this set

Results

Interfaces

Classes

Enums

Exceptions

CopyOnWriteArraySet

Class Overview

See Also

Summary

Public Constructors

public CopyOnWriteArraySet ()

public CopyOnWriteArraySet (Collection<? extends E> c)

Parameters

Throws

Public Methods

public boolean add (E e)

Parameters

Returns

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

Parameters

Returns

Throws

See Also

public void clear ()

public boolean contains (Object o)

Parameters

Returns

public boolean containsAll (Collection<?> c)

Parameters

Returns

Throws

See Also

public boolean equals (Object o)

Parameters

Returns

public boolean isEmpty ()

Returns

public Iterator<E> iterator ()

Returns

public boolean remove (Object o)

Parameters

Returns

public boolean removeAll (Collection<?> c)

Parameters

Returns

Throws

See Also

public boolean retainAll (Collection<?> c)

Parameters

Returns

Throws

See Also

public int size ()

Returns

public T[] toArray (T[] a)

Parameters

Returns

Throws

public Object[] toArray ()

Returns