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public
class
CopyOnWriteArrayList
extends Object
implements
List<E>,
RandomAccess,
Cloneable,
Serializable
java.lang.Object | |
↳ | java.util.concurrent.CopyOnWriteArrayList<E> |
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.
Public constructors | |
---|---|
CopyOnWriteArrayList()
Creates an empty list. |
|
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. |
|
CopyOnWriteArrayList(E[] toCopyIn)
Creates a list holding a copy of the given array. |
Public methods | |
---|---|
boolean
|
add(E e)
Appends the specified element to the end of this list. |
void
|
add(int index, E element)
Inserts the specified element at the specified position in this list. |
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. |
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. |
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)
|
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 |
int
|
indexOf(Object o)
Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. |
boolean
|
isEmpty()
Returns |
Iterator<E>
|
iterator()
Returns an iterator over the elements in this list in proper sequence. |
int
|
lastIndexOf(Object o)
Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. |
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(int index)
Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list. The returned iterator provides a snapshot of the state of the list when the iterator was constructed. |
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. |
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. |
void
|
replaceAll(UnaryOperator<E> operator)
Replaces each element of this list with the result of applying the operator to that element. |
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. |
void
|
sort(Comparator<? super E> c)
Sorts this list according to the order induced by the specified
|
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. |
Inherited methods | |
---|---|
public CopyOnWriteArrayList ()
Creates an empty list.
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 |
Collection : the collection of initially held elements |
Throws | |
---|---|
NullPointerException |
if the specified collection is null |
public CopyOnWriteArrayList (E[] toCopyIn)
Creates a list holding a copy of the given array.
Parameters | |
---|---|
toCopyIn |
E : the array (a copy of this array is used as the
internal array) |
Throws | |
---|---|
NullPointerException |
if the specified array is null |
public boolean add (E e)
Appends the specified element to the end of this list.
Parameters | |
---|---|
e |
E : element to be appended to this list |
Returns | |
---|---|
boolean |
true (as specified by Collection#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).
Parameters | |
---|---|
index |
int : index at which the specified element is to be inserted |
element |
E : element to be inserted |
Throws | |
---|---|
IndexOutOfBoundsException |
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.
Parameters | |
---|---|
c |
Collection : collection containing elements to be added to this list |
Returns | |
---|---|
boolean |
true if this list changed as a result of the call |
Throws | |
---|---|
NullPointerException |
if the specified collection is null |
See also:
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.
Parameters | |
---|---|
index |
int : index at which to insert the first element
from the specified collection |
c |
Collection : collection containing elements to be added to this list |
Returns | |
---|---|
boolean |
true if this list changed as a result of the call |
Throws | |
---|---|
IndexOutOfBoundsException |
|
NullPointerException |
if the specified collection is null |
See also:
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 : collection containing elements to be added to this list |
Returns | |
---|---|
int |
the number of elements added |
Throws | |
---|---|
NullPointerException |
if the specified collection is null |
See also:
public boolean addIfAbsent (E e)
Appends the element, if not present.
Parameters | |
---|---|
e |
E : element to be added to this list, if absent |
Returns | |
---|---|
boolean |
true if the element was added |
public void clear ()
Removes all of the elements from this list. The list will be empty after this call returns.
public Object clone ()
Returns a shallow copy of this list. (The elements themselves are not copied.)
Returns | |
---|---|
Object |
a clone of this list |
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)
.
Parameters | |
---|---|
o |
Object : element whose presence in this list is to be tested |
Returns | |
---|---|
boolean |
true if this list contains the specified element |
public boolean containsAll (Collection<?> c)
Returns true
if this list contains all of the elements of the
specified collection.
Parameters | |
---|---|
c |
Collection : collection to be checked for containment in this list |
Returns | |
---|---|
boolean |
true if this list contains all of the elements of the
specified collection |
Throws | |
---|---|
NullPointerException |
if the specified collection is null |
See also:
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 List#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)
.
Parameters | |
---|---|
o |
Object : the object to be compared for equality with this list |
Returns | |
---|---|
boolean |
true if the specified object is equal to this list |
public E get (int index)
Returns the element at the specified position in this list.
Parameters | |
---|---|
index |
int : index of the element to return |
Returns | |
---|---|
E |
the element at the specified position in this list |
Throws | |
---|---|
IndexOutOfBoundsException |
public int hashCode ()
Returns the hash code value for this list.
This implementation uses the definition in List#hashCode
.
Returns | |
---|---|
int |
the hash code value for this list |
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 |
E : element to search for |
index |
int : index to start searching from |
Returns | |
---|---|
int |
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 |
public int indexOf (Object o)
Returns the index of the first occurrence of the specified element
in this list, or -1 if this list does not contain the element.
More formally, returns the lowest index i
such that
Objects.equals(o, get(i))
,
or -1 if there is no such index.
Parameters | |
---|---|
o |
Object : element to search for |
Returns | |
---|---|
int |
the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element |
public boolean isEmpty ()
Returns true
if this list contains no elements.
Returns | |
---|---|
boolean |
true if this list contains no elements |
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.
Returns | |
---|---|
Iterator<E> |
an iterator over the elements in this list in proper sequence |
public int lastIndexOf (Object o)
Returns the index of the last occurrence of the specified element
in this list, or -1 if this list does not contain the element.
More formally, returns the highest index i
such that
Objects.equals(o, get(i))
,
or -1 if there is no such index.
Parameters | |
---|---|
o |
Object : element to search for |
Returns | |
---|---|
int |
the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element |
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 |
E : element to search for |
index |
int : index to start searching backwards from |
Returns | |
---|---|
int |
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 |
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 ListIterator#next
.
An initial call to ListIterator#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.
Parameters | |
---|---|
index |
int : index of the first element to be returned from the
list iterator (by a call to ListIterator#next ) |
Returns | |
---|---|
ListIterator<E> |
a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list |
Throws | |
---|---|
IndexOutOfBoundsException |
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.
Returns | |
---|---|
ListIterator<E> |
a list iterator over the elements in this list (in proper sequence) |
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.
Parameters | |
---|---|
index |
int : the index of the element to be removed |
Returns | |
---|---|
E |
the element previously at the specified position |
Throws | |
---|---|
IndexOutOfBoundsException |
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).
Parameters | |
---|---|
o |
Object : element to be removed from this list, if present |
Returns | |
---|---|
boolean |
true if this list contained the specified element |
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.
Parameters | |
---|---|
c |
Collection : collection containing elements to be removed from this list |
Returns | |
---|---|
boolean |
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:
public boolean removeIf (Predicate<? super E> filter)
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.
Parameters | |
---|---|
filter |
Predicate : a predicate which returns true for elements to be
removed |
Returns | |
---|---|
boolean |
true if any elements were removed |
public void replaceAll (UnaryOperator<E> operator)
Replaces each element of this list with the result of applying the operator to that element. Errors or runtime exceptions thrown by the operator are relayed to the caller.
Parameters | |
---|---|
operator |
UnaryOperator : the operator to apply to each element |
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.
Parameters | |
---|---|
c |
Collection : collection containing elements to be retained in this list |
Returns | |
---|---|
boolean |
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:
public E set (int index, E element)
Replaces the element at the specified position in this list with the specified element.
Parameters | |
---|---|
index |
int : index of the element to replace |
element |
E : element to be stored at the specified position |
Returns | |
---|---|
E |
the element previously at the specified position |
Throws | |
---|---|
IndexOutOfBoundsException |
public int size ()
Returns the number of elements in this list.
Returns | |
---|---|
int |
the number of elements in this list |
public void sort (Comparator<? super E> c)
Sorts this list according to the order induced by the specified
Comparator
.
All elements in this list must be mutually comparable using the
specified comparator (that is, c.compare(e1, e2)
must not throw
a ClassCastException
for any elements e1
and e2
in the list).
If the specified comparator is null
then all elements in this
list must implement the Comparable
interface and the elements'
Comparable should be used.
This list must be modifiable, but need not be resizable.
For apps running on and targeting Android versions greater than
Nougat (API level > 25
), Collections#sort(List)
delegates to this method. Such apps must not call
Collections#sort(List)
from this method. Instead, prefer
not overriding this method at all. If you must override it, consider
this implementation:
@Override public void sort(Comparator<? super E> c) { Object[] elements = toArray(); Arrays.sort(elements, c); ListIterator<E> iterator = (ListIterator<Object>) listIterator(); for (Object element : elements) { iterator.next(); iterator.set((E) element); } }
Parameters | |
---|---|
c |
Comparator : the Comparator used to compare list elements.
A null value indicates that the elements'
Comparable should be used |
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.
Returns | |
---|---|
Spliterator<E> |
a Spliterator over the elements in this list |
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.
Parameters | |
---|---|
fromIndex |
int : low endpoint (inclusive) of the subList |
toIndex |
int : high endpoint (exclusive) of the subList |
Returns | |
---|---|
List<E> |
a view of the specified range within this list |
Throws | |
---|---|
IndexOutOfBoundsException |
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.
Returns | |
---|---|
Object[] |
an array containing all the elements in this list |
public 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()
.
Parameters | |
---|---|
a |
T : 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 | |
---|---|
T[] |
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 |
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)
.
Returns | |
---|---|
String |
a string representation of this list |