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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
,
Cloneable
,
Iterable
<E>,
Collection
<E>,
Set
<E>
-
public class CopyOnWriteArraySet<E>
- extends AbstractSet<E>
- implements Serializable
Cloneable, 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.
-
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.
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.
The following code sketch uses a copy-on-write set to maintain a set of Handler objects that perform some action upon state updates.
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<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();
Iterator<Handler> it = handlers.iterator();
while (it.hasNext())
it.next().handle();
}
}
This class is a member of the
Java Collections Framework
.
-
Since:
-
1.5
-
See Also:
-
CopyOnWriteArrayList
,
Serialized Form
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
(
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. |
Iterator
<
E
> |
iterator
()
Returns an iterator over the elements contained in this collection. |
boolean |
remove
(
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). |
int |
size
()
Returns the number of elements in this collection. |
Object
[] |
toArray
()
Returns an array containing all of the elements in this collection. |
|
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
|
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
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
Collection
<
E
>
-
Specified by:
-
size
in interface
Set
<
E
>
-
Specified by:
-
size
in class
AbstractCollection
<
E
>
-
-
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
Collection
<
E
>
-
Specified by:
-
isEmpty
in interface
Set
<
E
>
-
Overrides:
-
isEmpty
in class
AbstractCollection
<
E
>
-
-
Returns:
-
true
if this collection contains no elements.
contains
public boolean contains(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
Collection
<
E
>
-
Specified by:
-
contains
in interface
Set
<
E
>
-
Overrides:
-
contains
in class
AbstractCollection
<
E
>
-
-
Parameters:
-
o - object to be checked for containment in this collection.
-
Returns:
-
true
if this collection contains the specified element.
toArray
public 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
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 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
Collection
<
E
>
-
Specified by:
-
toArray
in interface
Set
<
E
>
-
Overrides:
-
toArray
in class
AbstractCollection
<
E
>
-
-
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
Collection
<
E
>
-
Specified by:
-
clear
in interface
Set
<
E
>
-
Overrides:
-
clear
in class
AbstractCollection
<
E
>
-
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
Iterable
<
E
>
-
Specified by:
-
iterator
in interface
Collection
<
E
>
-
Specified by:
-
iterator
in interface
Set
<
E
>
-
Specified by:
-
iterator
in class
AbstractCollection
<
E
>
-
-
Returns:
-
an iterator over the elements contained in this collection.
remove
public boolean remove(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
Collection
<
E
>
-
Specified by:
-
remove
in interface
Set
<
E
>
-
Overrides:
-
remove
in class
AbstractCollection
<
E
>
-
-
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
Collection
<
E
>
-
Specified by:
-
add
in interface
Set
<
E
>
-
Overrides:
-
add
in class
AbstractCollection
<
E
>
-
-
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
Collection
<
E
>
-
Specified by:
-
containsAll
in interface
Set
<
E
>
-
Overrides:
-
containsAll
in class
AbstractCollection
<
E
>
-
-
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
Collection
<
E
>
-
Specified by:
-
addAll
in interface
Set
<
E
>
-
Overrides:
-
addAll
in class
AbstractCollection
<
E
>
-
-
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
Collection
<
E
>
-
Specified by:
-
removeAll
in interface
Set
<
E
>
-
Overrides:
-
removeAll
in class
AbstractSet
<
E
>
-
-
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
Collection
<
E
>
-
Specified by:
-
retainAll
in interface
Set
<
E
>
-
Overrides:
-
retainAll
in class
AbstractCollection
<
E
>
-
-
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)