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CopyOnWriteArraySet

public class CopyOnWriteArraySet
extends AbstractSet<E> implements Serializable

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


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();
   }
 }

See also:

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.

void forEach(Consumer<? super E> action)
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)
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.

<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.

Inherited methods

From class java.util.AbstractSet
From class java.util.AbstractCollection
From class java.lang.Object
From interface java.util.Set
From interface java.util.Collection
From interface java.lang.Iterable

Public constructors

CopyOnWriteArraySet

Added in API level 1
CopyOnWriteArraySet ()

Creates an empty set.

CopyOnWriteArraySet

Added in API level 1
CopyOnWriteArraySet (Collection<? extends E> c)

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

Parameters
c Collection: the collection of elements to initially contain
Throws
NullPointerException if the specified collection is null

Public methods

add

Added in API level 1
boolean add (E e)

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

Parameters
e E: element to be added to this set
Returns
boolean true if this set did not already contain the specified element

addAll

Added in API level 1
boolean addAll (Collection<? extends E> 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 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: collection containing elements to be added to this set
Returns
boolean true if this set changed as a result of the call
Throws
NullPointerException if the specified collection is null

See also:

clear

Added in API level 1
void clear ()

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

contains

Added in API level 1
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 Objects.equals(o, e).

Parameters
o Object: element whose presence in this set is to be tested
Returns
boolean true if this set contains the specified element

containsAll

Added in API level 1
boolean containsAll (Collection<?> c)

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: collection to be checked for containment in this set
Returns
boolean true if this set contains all of the elements of the specified collection
Throws
NullPointerException if the specified collection is null

See also:

equals

Added in API level 1
boolean equals (Object o)

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 Objects.equals(e1, e2).

Parameters
o Object: object to be compared for equality with this set
Returns
boolean true if the specified object is equal to this set

forEach

Added in API level 24
void forEach (Consumer<? super E> action)

Parameters
action Consumer

isEmpty

Added in API level 1
boolean isEmpty ()

Returns true if this set contains no elements.

Returns
boolean true if this set contains no elements

iterator

Added in API level 1
Iterator<E> iterator ()

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
Iterator<E> an iterator over the elements in this set

remove

Added in API level 1
boolean remove (Object o)

Removes the specified element from this set if it is present. More formally, removes an element e such that Objects.equals(o, 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: object to be removed from this set, if present
Returns
boolean true if this set contained the specified element

removeAll

Added in API level 1
boolean removeAll (Collection<?> 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.

Parameters
c Collection: collection containing elements to be removed from this set
Returns
boolean 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

See also:

removeIf

Added in API level 24
boolean removeIf (Predicate<? super E> filter)

Parameters
filter Predicate
Returns
boolean

retainAll

Added in API level 1
boolean retainAll (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.

Parameters
c Collection: collection containing elements to be retained in this set
Returns
boolean 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

See also:

size

Added in API level 1
int size ()

Returns the number of elements in this set.

Returns
int the number of elements in this set

spliterator

Added in API level 24
Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this set in the order in which these elements were added.

The Spliterator reports IMMUTABLE, DISTINCT, SIZED, and SUBSIZED.

The spliterator provides a snapshot of the state of the set when the spliterator was constructed. No synchronization is needed while operating on the spliterator.

Returns
Spliterator<E> a Spliterator over the elements in this set

toArray

Added in API level 1
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. 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 T: 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
T[] 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

toArray

Added in API level 1
Object[] toArray ()

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
Object[] an array containing all the elements in this set
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