Class 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

Constructors

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.

Methods

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 true if this list contains the specified element.
boolean containsAll​(Collection<?> c)

Returns true if this list contains all of the elements of the specified collection.
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 Iterable until all elements have been processed or the action throws an exception.
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 index, or returns -1 if the element is not found.
boolean isEmpty()

Returns true if this list contains no elements.
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 index, or returns -1 if the element is not found.
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 Spliterator over the elements in this list.
List<E> subList​(int fromIndex, int toIndex)

Returns a view of the portion of this list between fromIndex, inclusive, and toIndex, exclusive.
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

Constructors

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

Methods

size

public int size()

Returns the number of elements in this list.

Specified by:
size in interface Collection<E>
Specified by:
size in interface List<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 interface Collection<E>
Specified by:
isEmpty in interface List<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 interface Collection<E>
Specified by:
contains in interface List<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.)

Overrides:
clone in class Object
Returns:
a clone of this list
See Also:
Cloneable

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 interface Collection<E>
Specified by:
toArray in interface List<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 interface Collection<E>
Specified by:
toArray in interface List<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 interface List<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 interface List<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 interface Collection<E>
Specified by:
add in interface List<E>
Parameters:
e - element to be appended to this list
Returns:
true (as specified by Collection.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 interface List<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 interface List<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 interface Collection<E>
Specified by:
remove in interface List<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 interface Collection<E>
Specified by:
containsAll in interface List<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 interface Collection<E>
Specified by:
removeAll in interface List<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 interface Collection<E>
Specified by:
retainAll in interface List<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.

Specified by:
clear in interface Collection<E>
Specified by:
clear in interface List<E>

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 interface Collection<E>
Specified by:
addAll in interface List<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 interface List<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 interface Iterable<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 interface Collection<E>
Parameters:
filter - a predicate which returns true 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).

Overrides:
toString in class Object
Returns:
a string representation of this list

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 interface Collection<E>
Specified by:
equals in interface List<E>
Overrides:
equals in class Object
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 interface Collection<E>
Specified by:
hashCode in interface List<E>
Overrides:
hashCode in class Object
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 interface Collection<E>
Specified by:
iterator in interface Iterable<E>
Specified by:
iterator in interface List<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 interface List<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 interface List<E>
Parameters:
index - index of the first element to be returned from the list iterator (by a call to next)
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 interface Collection<E>
Specified by:
spliterator in interface Iterable<E>
Specified by:
spliterator in interface List<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 interface List<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)

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
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https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/CopyOnWriteArrayList.html