Class CopyOnWriteArrayList<E>
- java.lang.Object
-
- java.util.concurrent.CopyOnWriteArrayList<E>
- Type Parameters:
-
E
- the type of elements held in this list
- All Implemented Interfaces:
-
Serializable
,Cloneable
,Iterable<E>
,Collection<E>
,List<E>
,RandomAccess
public class CopyOnWriteArrayList<E> extends Object implements List<E>, RandomAccess, Cloneable, Serializable
A thread-safe variant of ArrayList
in which all mutative operations (add
, set
, and so on) are implemented by making a fresh copy of the underlying array.
This is ordinarily too costly, but may be more efficient than alternatives when traversal operations vastly outnumber mutations, and is useful when you cannot or don't want to synchronize traversals, yet need to preclude interference among concurrent threads. The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException
. The iterator will not reflect additions, removals, or changes to the list since the iterator was created. Element-changing operations on iterators themselves (remove
, set
, and add
) are not supported. These methods throw UnsupportedOperationException
.
All elements are permitted, including null
.
Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a CopyOnWriteArrayList
happen-before actions subsequent to the access or removal of that element from the CopyOnWriteArrayList
in another thread.
This class is a member of the Java Collections Framework.
- Since:
- 1.5
- See Also:
- Serialized Form
Constructor Summary
Constructor | Description |
---|---|
CopyOnWriteArrayList() | Creates an empty list. |
CopyOnWriteArrayList(E[] toCopyIn) | Creates a list holding a copy of the given array. |
CopyOnWriteArrayList(Collection<? extends E> c) | Creates a list containing the elements of the specified collection, in the order they are returned by the collection's iterator. |
Method Summary
Modifier and Type | Method | Description |
---|---|---|
void | add(int index,
E element) | Inserts the specified element at the specified position in this list. |
boolean | add(E e) | Appends the specified element to the end of this list. |
boolean | addAll(int index,
Collection<? extends E> c) | Inserts all of the elements in the specified collection into this list, starting at the specified position. |
boolean | addAll(Collection<? extends E> c) | Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's iterator. |
int | addAllAbsent(Collection<? extends E> c) | Appends all of the elements in the specified collection that are not already contained in this list, to the end of this list, in the order that they are returned by the specified collection's iterator. |
boolean | addIfAbsent(E e) | Appends the element, if not present. |
void | clear() | Removes all of the elements from this list. |
Object | clone() | Returns a shallow copy of this list. |
boolean | contains(Object o) | Returns |
boolean | containsAll(Collection<?> c) | Returns |
boolean | equals(Object o) | Compares the specified object with this list for equality. |
void | forEach(Consumer<? super E> action) | Performs the given action for each element of the |
E | get(int index) | Returns the element at the specified position in this list. |
int | hashCode() | Returns the hash code value for this list. |
int | indexOf(E e,
int index) | Returns the index of the first occurrence of the specified element in this list, searching forwards from |
boolean | isEmpty() | Returns |
Iterator<E> | iterator() | Returns an iterator over the elements in this list in proper sequence. |
int | lastIndexOf(E e,
int index) | Returns the index of the last occurrence of the specified element in this list, searching backwards from |
ListIterator<E> | listIterator() | Returns a list iterator over the elements in this list (in proper sequence). |
ListIterator<E> | listIterator(int index) | Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list. |
E | remove(int index) | Removes the element at the specified position in this list. |
boolean | remove(Object o) | Removes the first occurrence of the specified element from this list, if it is present. |
boolean | removeAll(Collection<?> c) | Removes from this list 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 list that are contained in the specified collection. |
E | set(int index,
E element) | Replaces the element at the specified position in this list with the specified element. |
int | size() | Returns the number of elements in this list. |
Spliterator<E> | spliterator() | Returns a |
List<E> | subList(int fromIndex,
int toIndex) | Returns a view of the portion of this list between |
Object[] | toArray() | Returns an array containing all of the elements in this list in proper sequence (from first to last element). |
<T> T[] | toArray(T[] a) | Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. |
String | toString() | Returns a string representation of this list. |
Methods declared in class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
Methods declared in interface java.util.Collection
parallelStream, stream, toArray
Methods declared in interface java.util.List
indexOf, lastIndexOf, replaceAll, sort
Constructor Detail
CopyOnWriteArrayList
public CopyOnWriteArrayList()
Creates an empty list.
CopyOnWriteArrayList
public CopyOnWriteArrayList(Collection<? extends E> c)
Creates a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.
- Parameters:
-
c
- the collection of initially held elements - Throws:
-
NullPointerException
- if the specified collection is null
CopyOnWriteArrayList
public CopyOnWriteArrayList(E[] toCopyIn)
Creates a list holding a copy of the given array.
- Parameters:
-
toCopyIn
- the array (a copy of this array is used as the internal array) - Throws:
-
NullPointerException
- if the specified array is null
Method Detail
size
public int size()
Returns the number of elements in this list.
- Specified by:
-
size
in interfaceCollection<E>
- Specified by:
-
size
in interfaceList<E>
- Returns:
- the number of elements in this list
isEmpty
public boolean isEmpty()
Returns true
if this list contains no elements.
- Specified by:
-
isEmpty
in interfaceCollection<E>
- Specified by:
-
isEmpty
in interfaceList<E>
- Returns:
-
true
if this list contains no elements
contains
public boolean contains(Object o)
Returns true
if this list contains the specified element. More formally, returns true
if and only if this list contains at least one element e
such that Objects.equals(o, e)
.
- Specified by:
-
contains
in interfaceCollection<E>
- Specified by:
-
contains
in interfaceList<E>
- Parameters:
-
o
- element whose presence in this list is to be tested - Returns:
-
true
if this list contains the specified element
indexOf
public int indexOf(E e, int index)
Returns the index of the first occurrence of the specified element in this list, searching forwards from index
, or returns -1 if the element is not found. More formally, returns the lowest index i
such that i >= index && Objects.equals(get(i), e)
, or -1 if there is no such index.
- Parameters:
-
e
- element to search for -
index
- index to start searching from - Returns:
- the index of the first occurrence of the element in this list at position
index
or later in the list;-1
if the element is not found. - Throws:
-
IndexOutOfBoundsException
- if the specified index is negative
lastIndexOf
public int lastIndexOf(E e, int index)
Returns the index of the last occurrence of the specified element in this list, searching backwards from index
, or returns -1 if the element is not found. More formally, returns the highest index i
such that i <= index && Objects.equals(get(i), e)
, or -1 if there is no such index.
- Parameters:
-
e
- element to search for -
index
- index to start searching backwards from - Returns:
- the index of the last occurrence of the element at position less than or equal to
index
in this list; -1 if the element is not found. - Throws:
-
IndexOutOfBoundsException
- if the specified index is greater than or equal to the current size of this list
clone
public Object clone()
Returns a shallow copy of this list. (The elements themselves are not copied.)
toArray
public Object[] toArray()
Returns an array containing all of the elements in this list in proper sequence (from first to last element).
The returned array will be "safe" in that no references to it are maintained by this list. (In other words, this method must allocate a new 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 interfaceList<E>
- Returns:
- an array containing all the elements in this list
- See Also:
Arrays.asList(Object[])
toArray
public <T> T[] toArray(T[] a)
Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the list 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 list.
If this list fits in the specified array with room to spare (i.e., the array has more elements than this list), the element in the array immediately following the end of the list is set to null
. (This is useful in determining the length of this list only if the caller knows that this list does not contain any null elements.)
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 list known to contain only strings. The following code can be used to dump the list 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 interfaceList<E>
- Type Parameters:
-
T
- the component type of the array to contain the collection - Parameters:
-
a
- the array into which the elements of the list 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 list
- Throws:
-
ArrayStoreException
- if the runtime type of the specified array is not a supertype of the runtime type of every element in this list -
NullPointerException
- if the specified array is null
get
public E get(int index)
Returns the element at the specified position in this list.
- Specified by:
-
get
in interfaceList<E>
- Parameters:
-
index
- index of the element to return - Returns:
- the element at the specified position in this list
- Throws:
-
IndexOutOfBoundsException
- if the index is out of range (index < 0 || index >= size()
)
set
public E set(int index, E element)
Replaces the element at the specified position in this list with the specified element.
- Specified by:
-
set
in interfaceList<E>
- Parameters:
-
index
- index of the element to replace -
element
- element to be stored at the specified position - Returns:
- the element previously at the specified position
- Throws:
-
IndexOutOfBoundsException
- if the index is out of range (index < 0 || index >= size()
)
add
public boolean add(E e)
Appends the specified element to the end of this list.
- Specified by:
-
add
in interfaceCollection<E>
- Specified by:
-
add
in interfaceList<E>
- Parameters:
-
e
- element to be appended to this list - Returns:
-
true
(as specified byCollection.add(E)
)
add
public void add(int index, E element)
Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).
- Specified by:
-
add
in interfaceList<E>
- Parameters:
-
index
- index at which the specified element is to be inserted -
element
- element to be inserted - Throws:
-
IndexOutOfBoundsException
- if the index is out of range (index < 0 || index > size()
)
remove
public E remove(int index)
Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices). Returns the element that was removed from the list.
- Specified by:
-
remove
in interfaceList<E>
- Parameters:
-
index
- the index of the element to be removed - Returns:
- the element previously at the specified position
- Throws:
-
IndexOutOfBoundsException
- if the index is out of range (index < 0 || index >= size()
)
remove
public boolean remove(Object o)
Removes the first occurrence of the specified element from this list, if it is present. If this list does not contain the element, it is unchanged. More formally, removes the element with the lowest index i
such that Objects.equals(o, get(i))
(if such an element exists). Returns true
if this list contained the specified element (or equivalently, if this list changed as a result of the call).
- Specified by:
-
remove
in interfaceCollection<E>
- Specified by:
-
remove
in interfaceList<E>
- Parameters:
-
o
- element to be removed from this list, if present - Returns:
-
true
if this list contained the specified element
addIfAbsent
public boolean addIfAbsent(E e)
Appends the element, if not present.
- Parameters:
-
e
- element to be added to this list, if absent - Returns:
-
true
if the element was added
containsAll
public boolean containsAll(Collection<?> c)
Returns true
if this list contains all of the elements of the specified collection.
- Specified by:
-
containsAll
in interfaceCollection<E>
- Specified by:
-
containsAll
in interfaceList<E>
- Parameters:
-
c
- collection to be checked for containment in this list - Returns:
-
true
if this list contains all of the elements of the specified collection - Throws:
-
NullPointerException
- if the specified collection is null - See Also:
contains(Object)
removeAll
public boolean removeAll(Collection<?> c)
Removes from this list all of its elements that are contained in the specified collection. This is a particularly expensive operation in this class because of the need for an internal temporary array.
- Specified by:
-
removeAll
in interfaceCollection<E>
- Specified by:
-
removeAll
in interfaceList<E>
- Parameters:
-
c
- collection containing elements to be removed from this list - Returns:
-
true
if this list changed as a result of the call - Throws:
-
ClassCastException
- if the class of an element of this list is incompatible with the specified collection (optional) -
NullPointerException
- if this list 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 list that are contained in the specified collection. In other words, removes from this list all of its elements that are not contained in the specified collection.
- Specified by:
-
retainAll
in interfaceCollection<E>
- Specified by:
-
retainAll
in interfaceList<E>
- Parameters:
-
c
- collection containing elements to be retained in this list - Returns:
-
true
if this list changed as a result of the call - Throws:
-
ClassCastException
- if the class of an element of this list is incompatible with the specified collection (optional) -
NullPointerException
- if this list 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)
addAllAbsent
public int addAllAbsent(Collection<? extends E> c)
Appends all of the elements in the specified collection that are not already contained in this list, to the end of this list, in the order that they are returned by the specified collection's iterator.
- Parameters:
-
c
- collection containing elements to be added to this list - Returns:
- the number of elements added
- Throws:
-
NullPointerException
- if the specified collection is null - See Also:
addIfAbsent(Object)
clear
public void clear()
Removes all of the elements from this list. The list will be empty after this call returns.
addAll
public boolean addAll(Collection<? extends E> c)
Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's iterator.
- Specified by:
-
addAll
in interfaceCollection<E>
- Specified by:
-
addAll
in interfaceList<E>
- Parameters:
-
c
- collection containing elements to be added to this list - Returns:
-
true
if this list changed as a result of the call - Throws:
-
NullPointerException
- if the specified collection is null - See Also:
add(Object)
addAll
public boolean addAll(int index, Collection<? extends E> c)
Inserts all of the elements in the specified collection into this list, starting at the specified position. Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their indices). The new elements will appear in this list in the order that they are returned by the specified collection's iterator.
- Specified by:
-
addAll
in interfaceList<E>
- Parameters:
-
index
- index at which to insert the first element from the specified collection -
c
- collection containing elements to be added to this list - Returns:
-
true
if this list changed as a result of the call - Throws:
-
IndexOutOfBoundsException
- if the index is out of range (index < 0 || index > size()
) -
NullPointerException
- if the specified collection is null - See Also:
add(int,Object)
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
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
toString
public String toString()
Returns a string representation of this list. The string representation consists of the string representations of the list's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"
). Adjacent elements are separated by the characters ", "
(comma and space). Elements are converted to strings as by String.valueOf(Object)
.
equals
public boolean equals(Object o)
Compares the specified object with this list for equality. Returns true
if the specified object is the same object as this object, or if it is also a List
and the sequence of elements returned by an iterator over the specified list is the same as the sequence returned by an iterator over this list. The two sequences are considered to be the same if they have the same length and corresponding elements at the same position in the sequence are equal. Two elements e1
and e2
are considered equal if Objects.equals(e1, e2)
.
- Specified by:
-
equals
in interfaceCollection<E>
- Specified by:
-
equals
in interfaceList<E>
- Overrides:
-
equals
in classObject
- Parameters:
-
o
- the object to be compared for equality with this list - Returns:
-
true
if the specified object is equal to this list - See Also:
-
Object.hashCode()
,HashMap
hashCode
public int hashCode()
Returns the hash code value for this list.
This implementation uses the definition in List.hashCode()
.
- Specified by:
-
hashCode
in interfaceCollection<E>
- Specified by:
-
hashCode
in interfaceList<E>
- Overrides:
-
hashCode
in classObject
- Returns:
- the hash code value for this list
- See Also:
-
Object.equals(java.lang.Object)
,System.identityHashCode(java.lang.Object)
iterator
public Iterator<E> iterator()
Returns an iterator over the elements in this list in proper sequence.
The returned iterator provides a snapshot of the state of the list 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 interfaceList<E>
- Returns:
- an iterator over the elements in this list in proper sequence
listIterator
public ListIterator<E> listIterator()
Returns a list iterator over the elements in this list (in proper sequence).
The returned iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove
, set
or add
methods.
- Specified by:
-
listIterator
in interfaceList<E>
- Returns:
- a list iterator over the elements in this list (in proper sequence)
listIterator
public ListIterator<E> listIterator(int index)
Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list. The specified index indicates the first element that would be returned by an initial call to next
. An initial call to previous
would return the element with the specified index minus one.
The returned iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove
, set
or add
methods.
- Specified by:
-
listIterator
in interfaceList<E>
- Parameters:
-
index
- index of the first element to be returned from the list iterator (by a call tonext
) - Returns:
- a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list
- Throws:
-
IndexOutOfBoundsException
- if the index is out of range (index < 0 || index > size()
)
spliterator
public Spliterator<E> spliterator()
Returns a Spliterator
over the elements in this list.
The Spliterator
reports Spliterator.IMMUTABLE
, Spliterator.ORDERED
, Spliterator.SIZED
, and Spliterator.SUBSIZED
.
The spliterator provides a snapshot of the state of the list 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 interfaceList<E>
- Returns:
- a
Spliterator
over the elements in this list - Since:
- 1.8
subList
public List<E> subList(int fromIndex, int toIndex)
Returns a view of the portion of this list between fromIndex
, inclusive, and toIndex
, exclusive. The returned list is backed by this list, so changes in the returned list are reflected in this list.
The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is modified in any way other than via the returned list.
- Specified by:
-
subList
in interfaceList<E>
- Parameters:
-
fromIndex
- low endpoint (inclusive) of the subList -
toIndex
- high endpoint (exclusive) of the subList - Returns:
- a view of the specified range within this list
- Throws:
-
IndexOutOfBoundsException
- for an illegal endpoint index value (fromIndex < 0 || toIndex > size || fromIndex > toIndex
)