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 set
- 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.) 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(); } }
This class is a member of the Java Collections Framework.
- Since:
- 1.5
- See Also:
-
CopyOnWriteArrayList
, Serialized Form
Constructor Summary
Constructor | Description |
---|---|
CopyOnWriteArraySet() | Creates an empty set. |
CopyOnWriteArraySet(Collection<? extends E> c) | Creates a set containing all of the elements of the specified collection. |
Method Summary
Modifier and Type | Method | Description |
---|---|---|
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 |
boolean | containsAll(Collection<?> c) | Returns |
boolean | equals(Object o) | Compares the specified object with this set for equality. |
void | forEach(Consumer<? super E> action) | Performs the given action for each element of the |
boolean | isEmpty() | Returns |
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) | Removes all of the elements of this collection that satisfy the given predicate. |
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 |
Object[] | toArray() | Returns an array containing all of the 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. |
Methods declared in class java.util.AbstractSet
hashCode
Methods declared in class java.util.AbstractCollection
toString
Methods declared in class java.lang.Object
clone, finalize, getClass, notify, notifyAll, wait, wait, wait
Methods declared in interface java.util.Collection
parallelStream, stream, toArray
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.
- Parameters:
-
c
- the collection of elements to initially contain - Throws:
-
NullPointerException
- if the specified collection is null
Method Detail
size
public int size()
Returns the number of elements in this set.
- Specified by:
-
size
in interfaceCollection<E>
- Specified by:
-
size
in interfaceSet<E>
- Returns:
- the number of elements in this set
isEmpty
public boolean isEmpty()
Returns true
if this set contains no elements.
- Specified by:
-
isEmpty
in interfaceCollection<E>
- Specified by:
-
isEmpty
in interfaceSet<E>
- Overrides:
-
isEmpty
in classAbstractCollection<E>
- Returns:
-
true
if this set contains no elements
contains
public 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)
.
- Specified by:
-
contains
in interfaceCollection<E>
- Specified by:
-
contains
in interfaceSet<E>
- Overrides:
-
contains
in classAbstractCollection<E>
- Parameters:
-
o
- element whose presence in this set is to be tested - Returns:
-
true
if this set contains the specified element
toArray
public 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.
- Specified by:
-
toArray
in interfaceCollection<E>
- Specified by:
-
toArray
in interfaceSet<E>
- Overrides:
-
toArray
in classAbstractCollection<E>
- Returns:
- an array containing all the elements in this set
toArray
public <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. 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()
.- Specified by:
-
toArray
in interfaceCollection<E>
- Specified by:
-
toArray
in interfaceSet<E>
- Overrides:
-
toArray
in classAbstractCollection<E>
- Type Parameters:
-
T
- the component type of the array to contain the collection - 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
clear
public void clear()
Removes all of the elements from this set. The set will be empty after this call returns.
- Specified by:
-
clear
in interfaceCollection<E>
- Specified by:
-
clear
in interfaceSet<E>
- Overrides:
-
clear
in classAbstractCollection<E>
remove
public 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.)
- Specified by:
-
remove
in interfaceCollection<E>
- Specified by:
-
remove
in interfaceSet<E>
- Overrides:
-
remove
in classAbstractCollection<E>
- Parameters:
-
o
- object to be removed from this set, if present - Returns:
-
true
if this set contained the specified element
add
public 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
.
- Specified by:
-
add
in interfaceCollection<E>
- Specified by:
-
add
in interfaceSet<E>
- Overrides:
-
add
in classAbstractCollection<E>
- Parameters:
-
e
- element to be added to this set - Returns:
-
true
if this set did not already contain the specified element
containsAll
public 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.
- Specified by:
-
containsAll
in interfaceCollection<E>
- Specified by:
-
containsAll
in interfaceSet<E>
- Overrides:
-
containsAll
in classAbstractCollection<E>
- 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 - See Also:
contains(Object)
addAll
public 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.
- Specified by:
-
addAll
in interfaceCollection<E>
- Specified by:
-
addAll
in interfaceSet<E>
- Overrides:
-
addAll
in classAbstractCollection<E>
- 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 - See Also:
add(Object)
removeAll
public 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.
- Specified by:
-
removeAll
in interfaceCollection<E>
- Specified by:
-
removeAll
in interfaceSet<E>
- Overrides:
-
removeAll
in classAbstractSet<E>
- 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 - See Also:
remove(Object)
retainAll
public 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.
- Specified by:
-
retainAll
in interfaceCollection<E>
- Specified by:
-
retainAll
in interfaceSet<E>
- Overrides:
-
retainAll
in classAbstractCollection<E>
- 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 - See Also:
remove(Object)
iterator
public 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.
- Specified by:
-
iterator
in interfaceCollection<E>
- Specified by:
-
iterator
in interfaceIterable<E>
- Specified by:
-
iterator
in interfaceSet<E>
- Specified by:
-
iterator
in classAbstractCollection<E>
- Returns:
- an iterator over the elements in this set
equals
public 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)
.
- Specified by:
-
equals
in interfaceCollection<E>
- Specified by:
-
equals
in interfaceSet<E>
- Overrides:
-
equals
in classAbstractSet<E>
- Parameters:
-
o
- object to be compared for equality with this set - Returns:
-
true
if the specified object is equal to this set - See Also:
-
Object.hashCode()
,HashMap
removeIf
public boolean removeIf(Predicate<? super E> filter)
Description copied from interface: Collection
Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
- Specified by:
-
removeIf
in interfaceCollection<E>
- Parameters:
-
filter
- a predicate which returnstrue
for elements to be removed - Returns:
-
true
if any elements were removed - Throws:
-
NullPointerException
- if the specified filter is null
forEach
public void forEach(Consumer<? super E> action)
Description copied from interface: Iterable
Performs the given action for each element of the Iterable
until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.
The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.
- Specified by:
-
forEach
in interfaceIterable<E>
- Parameters:
-
action
- The action to be performed for each element - Throws:
-
NullPointerException
- if the specified action is null
spliterator
public Spliterator<E> spliterator()
Returns a Spliterator
over the elements in this set in the order in which these elements were added.
The Spliterator
reports Spliterator.IMMUTABLE
, Spliterator.DISTINCT
, Spliterator.SIZED
, and Spliterator.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.
- Specified by:
-
spliterator
in interfaceCollection<E>
- Specified by:
-
spliterator
in interfaceIterable<E>
- Specified by:
-
spliterator
in interfaceSet<E>
- Returns:
- a
Spliterator
over the elements in this set - Since:
- 1.8