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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, SequencedCollection<E>

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:

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.
`void`	`addFirst(E e)`	Adds an element as the first element of this collection (optional operation).
`boolean`	`addIfAbsent(E e)`	Appends the element, if not present.
`void`	`addLast(E e)`	Adds an element as the last element of this collection (optional operation).
`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.
`E`	`getFirst()`	Gets the first element of this collection.
`E`	`getLast()`	Gets the last element of this collection.
`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.
`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 `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.
`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.
`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.
`E`	`removeFirst()`	Removes and returns the first element of this collection (optional operation).
`boolean`	`removeIf(Predicate<? super E> filter)`	Removes all of the elements of this collection that satisfy the given predicate.
`E`	`removeLast()`	Removes and returns the last element of this collection (optional operation).
`boolean`	`retainAll(Collection<?> c)`	Retains only the elements in this list that are contained in the specified collection.
`List<E>`	`reversed()`	Returns a reverse-ordered view of this 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

replaceAll, sort

Constructor Details

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 Details

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(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.

Specified by:: indexOf in interface List<E>
Parameters:: o - element to search for
Returns:: the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the 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(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.

Specified by:: lastIndexOf in interface List<E>
Parameters:: o - element to search for
Returns:: the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element

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:

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:

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())

getFirst

public E getFirst()

Gets the first element of this collection.

Specified by:: getFirst in interface List<E>
Specified by:: getFirst in interface SequencedCollection<E>
Returns:: the retrieved element
Throws:: NoSuchElementException - if this collection is empty
Since:: 21

getLast

public E getLast()

Gets the last element of this collection.

Specified by:: getLast in interface List<E>
Specified by:: getLast in interface SequencedCollection<E>
Returns:: the retrieved element
Throws:: NoSuchElementException - if this collection is empty
Since:: 21

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())

addFirst

public void addFirst(E e)

Adds an element as the first element of this collection (optional operation). After this operation completes normally, the given element will be a member of this collection, and it will be the first element in encounter order.

Specified by:: addFirst in interface List<E>
Specified by:: addFirst in interface SequencedCollection<E>
Parameters:: e - the element to be added
Since:: 21

addLast

public void addLast(E e)

Adds an element as the last element of this collection (optional operation). After this operation completes normally, the given element will be a member of this collection, and it will be the last element in encounter order.

Specified by:: addLast in interface List<E>
Specified by:: addLast in interface SequencedCollection<E>
Parameters:: e - the element to be added.
Since:: 21

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())

removeFirst

public E removeFirst()

Removes and returns the first element of this collection (optional operation).

Specified by:: removeFirst in interface List<E>
Specified by:: removeFirst in interface SequencedCollection<E>
Returns:: the removed element
Throws:: NoSuchElementException - if this collection is empty
Since:: 21

removeLast

public E removeLast()

Removes and returns the last element of this collection (optional operation).

Specified by:: removeLast in interface List<E>
Specified by:: removeLast in interface SequencedCollection<E>
Returns:: the removed element
Throws:: NoSuchElementException - if this collection is empty
Since:: 21

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:

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:

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:

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:

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:

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:

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:

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:

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).

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.

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)

reversed

public List<E> reversed()

Returns a reverse-ordered view of this collection. The encounter order of elements in the returned view is the inverse of the encounter order of elements in this collection. The reverse ordering affects all order-sensitive operations, including those on the view collections of the returned view. If the collection implementation permits modifications to this view, the modifications "write through" to the underlying collection. Changes to the underlying collection might or might not be visible in this reversed view, depending upon the implementation.

Modifications to the reversed view are permitted and will be propagated to this list. In addition, modifications to this list will be visible in the reversed view. Sublists and iterators of the reversed view have the same restrictions as those of this list.

Specified by:: reversed in interface List<E>
Specified by:: reversed in interface SequencedCollection<E>
Returns:: a reverse-ordered view of this collection, as a List
Since:: 21

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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/CopyOnWriteArrayList.html