CopyOnWriteArrayList

public class CopyOnWriteArrayList 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.

Public Constructor Summary

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 Method Summary

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 Method Summary

Public Constructors

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 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 the array (a copy of this array is used as the internal array)
Throws
NullPointerException if the specified array is null

Public Methods

public boolean add (E e)

Appends the specified element to the end of this list.

Parameters
e element to be appended to this list
Returns

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 index at which the specified element is to be inserted
element element to be inserted

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 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

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 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
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 containing elements to be added to this list
Returns
  • the number of elements added
Throws
NullPointerException if the specified collection is null

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

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
  • 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 element whose presence in this list is to be tested
Returns
  • 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 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

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 {@linkplain List#iterator() 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 the object to be compared for equality with this list
Returns
  • true if the specified object is equal to this list

public void forEach (Consumer<? super E> action)

Parameters
action

public E get (int index)

Returns the element at the specified position in this list.

Parameters
index index of the element to return
Returns
  • the element at the specified position in this list

public int hashCode ()

Returns the hash code value for this list.

This implementation uses the definition in List.hashCode().

Returns
  • 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 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

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 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

public boolean isEmpty ()

Returns true if this list contains no elements.

Returns
  • 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
  • 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 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

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

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.

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

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
  • 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 the index of the element to be removed
Returns
  • the element previously at the specified position

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 element to be removed from this list, if present
Returns
  • 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 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

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 a predicate which returns true for elements to be removed
Returns
  • 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 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 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

public E set (int index, E element)

Replaces the element at the specified position in this list with the specified element.

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

public int size ()

Returns the number of elements in this list.

Returns
  • 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' {@linkplain Comparable 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), 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 the Comparator used to compare list elements. A null value indicates that the elements' {@linkplain Comparable natural ordering} 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
  • 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 low endpoint (inclusive) of the subList
toIndex high endpoint (exclusive) of the subList
Returns
  • a view of the specified range within this list

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
  • 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 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

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
  • a string representation of this list