public class

CopyOnWriteArrayList

extends Object
implements Serializable Cloneable List<E> RandomAccess
java.lang.Object
   ↳ java.util.concurrent.CopyOnWriteArrayList<E>

Class Overview

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
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)
Append 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.
E get(int index)
Returns the element at the specified location 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)
Searches this List for the specified object and returns the index of the first occurrence.
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)
Searches this List for the specified object and returns the index of the last occurrence.
ListIterator<E> listIterator()
Returns a List iterator on the elements of this List.

The returned iterator provides a snapshot of the state of the list when the iterator was constructed.

ListIterator<E> listIterator(int index)
Returns a list iterator on the elements of this List.

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 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.
List<E> subList(int fromIndex, int toIndex)
Returns a view of the portion of this list between fromIndex, inclusive, and toIndex, exclusive.
<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.
Object[] toArray()
Returns an array containing all of the elements in this list in proper sequence (from first to last element).
String toString()
Returns a string representation of this list.
[Expand]
Inherited Methods
From class java.lang.Object
From interface java.lang.Iterable
From interface java.util.Collection
From interface java.util.List

Public Constructors

public CopyOnWriteArrayList ()

Since: API Level 1

Creates an empty list.

public CopyOnWriteArrayList (Collection<? extends E> c)

Since: API Level 1

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)

Since: API Level 1

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 void add (int index, E element)

Since: API Level 1

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 the index at which to insert.
element the object to add.
Throws
IndexOutOfBoundsException

public boolean add (E e)

Since: API Level 1

Appends the specified element to the end of this list.

Parameters
e element to be appended to this list
Returns
  • true (as specified by add(E))

public boolean addAll (int index, Collection<? extends E> c)

Since: API Level 1

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 boolean addAll (Collection<? extends E> c)

Since: API Level 1

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 int addAllAbsent (Collection<? extends E> c)

Since: API Level 1

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

public boolean addIfAbsent (E e)

Since: API Level 1

Append 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 ()

Since: API Level 1

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

public Object clone ()

Since: API Level 1

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

Returns
  • a clone of this list

public boolean contains (Object o)

Since: API Level 1

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 (o==null ? e==null : o.equals(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)

Since: API Level 1

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)

Since: API Level 1

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 (e1==null ? e2==null : e1.equals(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 E get (int index)

Since: API Level 1

Returns the element at the specified location in this List.

Parameters
index the index of the element to return.
Returns
  • the element at the specified location.
Throws
IndexOutOfBoundsException

public int hashCode ()

Since: API Level 1

Returns the hash code value for this list.

This implementation uses the definition in hashCode().

Returns
  • the hash code value for this list

public int indexOf (E e, int index)

Since: API Level 1

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 && (e==null ? get(i)==null : e.equals(get(i)))), 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)

Since: API Level 1

Searches this List for the specified object and returns the index of the first occurrence.

Parameters
o the object to search for.
Returns
  • the index of the first occurrence of the object or -1 if the object was not found.

public boolean isEmpty ()

Since: API Level 1

Returns true if this list contains no elements.

Returns
  • true if this list contains no elements

public Iterator<E> iterator ()

Since: API Level 1

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 (E e, int index)

Since: API Level 1

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 && (e==null ? get(i)==null : e.equals(get(i)))), 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 int lastIndexOf (Object o)

Since: API Level 1

Searches this List for the specified object and returns the index of the last occurrence.

Parameters
o the object to search for.
Returns
  • the index of the last occurrence of the object, or -1 if the object was not found.

public ListIterator<E> listIterator ()

Since: API Level 1

Returns a List iterator on the elements of this List. The elements are iterated in the same order that they occur in the List.

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 on the elements of this List

public ListIterator<E> listIterator (int index)

Since: API Level 1

Returns a list iterator on the elements of this List. The elements are iterated in the same order as they occur in the List. The iteration starts at the specified location.

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 the index at which to start the iteration.
Returns
  • a list iterator on the elements of this List.
Throws
IndexOutOfBoundsException

public E remove (int index)

Since: API Level 1

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 object to remove.
Returns
  • the removed object.
Throws
IndexOutOfBoundsException

public boolean remove (Object o)

Since: API Level 1

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 (o==null ? get(i)==null : o.equals(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)

Since: API Level 1

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 retainAll (Collection<?> c)

Since: API Level 1

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)

Since: API Level 1

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

Parameters
index the index at which to put the specified object.
element the object to insert.
Returns
  • the previous element at the index.
Throws
IndexOutOfBoundsException

public int size ()

Since: API Level 1

Returns the number of elements in this list.

Returns
  • the number of elements in this list

public List<E> subList (int fromIndex, int toIndex)

Since: API Level 1

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
Throws
IndexOutOfBoundsException

public T[] toArray (T[] a)

Since: API Level 1

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 Object[] toArray ()

Since: API Level 1

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 String toString ()

Since: API Level 1

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
  • a string representation of this list