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CopyOnWriteArrayList

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.

Summary

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 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)
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.

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(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 index, or returns -1 if the element is not found.

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 Comparator.

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.

Inherited methods

From class java.lang.Object
From interface java.util.List
From interface java.util.Collection
From interface java.lang.Iterable

Public constructors

CopyOnWriteArrayList

added in API level 1
CopyOnWriteArrayList ()

Creates an empty list.

CopyOnWriteArrayList

added in API level 1
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

CopyOnWriteArrayList

added in API level 1
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 methods

add

added in API level 1
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 add(E))

add

added in API level 1
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

addAll

added in API level 1
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:

addAll

added in API level 1
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:

addAllAbsent

added in API level 1
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

addIfAbsent

added in API level 1
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

clear

added in API level 1
void clear ()

Removes all of the elements from this list. The list will be empty after this call returns.

clone

added in API level 1
Object clone ()

Returns a shallow copy of this list. (The elements themselves are not copied.)

Returns
Object a clone of this list

contains

added in API level 1
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

containsAll

added in API level 1
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:

equals

added in API level 1
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).

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

forEach

added in API level 24
void forEach (Consumer<? super E> action)

Parameters
action Consumer

get

added in API level 1
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

hashCode

added in API level 1
int hashCode ()

Returns the hash code value for this list.

This implementation uses the definition in hashCode().

Returns
int the hash code value for this list

indexOf

added in API level 1
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

indexOf

added in API level 1
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 (o==null ? get(i)==null : o.equals(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

isEmpty

added in API level 1
boolean isEmpty ()

Returns true if this list contains no elements.

Returns
boolean true if this list contains no elements

iterator

added in API level 1
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

lastIndexOf

added in API level 1
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 (o==null ? get(i)==null : o.equals(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

lastIndexOf

added in API level 1
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

listIterator

added in API level 1
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.

Parameters
index int: index of the first element to be returned from the list iterator (by a call to 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

listIterator

added in API level 1
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)

remove

added in API level 1
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

remove

added in API level 1
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

removeAll

added in API level 1
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:

removeIf

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

replaceAll

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

retainAll

added in API level 1
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:

set

added in API level 1
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

size

added in API level 1
int size ()

Returns the number of elements in this list.

Returns
int the number of elements in this list

sort

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' natural ordering 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), sort(List) delegates to this method. Such apps must not call 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' natural ordering should be used

spliterator

Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this list.

The Spliterator reports IMMUTABLE, ORDERED, SIZED, and 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

subList

added in API level 1
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

toArray

added in API level 1
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

toArray

added in API level 1
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

toString

added in API level 1
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 valueOf(Object).

Returns
String a string representation of this list

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