java.util.concurrent
Class CopyOnWriteArrayList<E>

java.lang.Object
  java.util.concurrent.CopyOnWriteArrayList<E>

Type Parameters:

E - the type of elements held in this collection

All Implemented Interfaces:

Serializable , Cloneable , Iterable <E>, Collection <E>, List <E>, RandomAccess

public class CopyOnWriteArrayList<E>
extends Object
implements List<E>, RandomAccess, Cloneable, Serializable

A thread-safe variant of ArrayList in which all mutative operations ( add , set , (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 ( Element-changing operations on iterators themselves (remove, set, and add) are not supported. These methods throw remove , set , and add ) are not supported. These methods throw UnsupportedOperationException .

All elements are permitted, including null .

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a CopyOnWriteArrayList happen-before actions subsequent to the access or removal of that element from the CopyOnWriteArrayList in another thread.

This class is a member of the Java Collections Framework .

Since:

1.5

See Also:

Serialized Form

Constructor Summary
CopyOnWriteArrayList ()           Creates an empty list.
CopyOnWriteArrayList ( Collection <? extends E           Creates a list containing the elements of the specified collection, Collection, in the order they are returned by the collection's Collection's iterator.
CopyOnWriteArrayList ( E           Creates           Create a list new CopyOnWriteArrayList holding a copy of the given array.

 

Method Summary
boolean	add ( 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           Appends all of the elements in the specified collection Collection to the end of this list, in the order that they are returned by the specified collection's iterator. Collection's Iterator.
boolean	addAll (int index, Collection <? extends E           Inserts all of the elements in the specified collection Collection into this list, starting at the specified position.
int	addAllAbsent ( Collection <? extends E           Appends all of the elements in the specified collection 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. Collection's Iterator.
boolean	addIfAbsent ( 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           Returns true if this list contains the specified element.
boolean	containsAll ( Collection           Returns true if this list Collection contains all of the elements of in the specified collection. Collection.
boolean	equals ( Object           Compares the specified object Object with this list List for equality.
E	get (int index)           Returns the element at the specified position in this list.
int	hashCode ()           Returns the hash code value for this list. List.
int	indexOf ( E           Returns           Searches for the index of the first occurrence of the specified element in this list, searching forwards from given argument, beginning the search at index , or returns -1 if and testing for equality using the element is not found. equals method.
int	indexOf ( Object           Returns           Searches for the index of the first occurrence of the specified element in this list, or -1 if this list does not contain given argument, testing for equality using the element. equals method.
boolean	isEmpty ()           Returns true           Tests if this list contains has no elements.
Iterator < E	iterator ()           Returns an iterator Iterator over the elements contained in this list in proper sequence. collection.
int	lastIndexOf ( E           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 ( E           Searches backwards for the specified object, starting from the specified index, and returns an index to it.
int	lastIndexOf ( Object           Returns the index of the last occurrence of the specified element object in this list, or -1 if this list does not contain the element. list.
ListIterator < E	listIterator ()           Returns a list iterator over an Iterator of the elements in this list List (in proper sequence).
ListIterator < E	listIterator (int index)           Returns a list iterator ListIterator of the elements in this list List (in proper sequence), starting at the specified position in this list. the List.
E	remove (int index)           Removes the element at the specified position in this list.
boolean	remove ( Object           Removes the first occurrence a single instance of the specified element from this list, if it is present. present (optional operation).
boolean	removeAll ( Collection           Removes from this list Collection all of its elements that are contained in the specified collection. Collection.
boolean	retainAll ( Collection           Retains only the elements in this list Collection that are contained in the specified collection. Collection (optional operation).
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 List between fromIndex , fromIndex, inclusive, and toIndex , toIndex, exclusive.
Object	toArray ()           Returns an array containing all of the elements in this list in proper sequence (from first to last element). the correct order.
<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. correct order.
String	toString ()           Returns a string representation of this list. Collection, containing the String representation of each element.

 

Methods inherited from class java.lang. Object
finalize , getClass , notify , notifyAll , wait , wait , wait

 

Constructor Detail

CopyOnWriteArrayList

public CopyOnWriteArrayList()

Creates an empty list.

CopyOnWriteArrayList

public CopyOnWriteArrayList(Collection<? extends E> c)

Creates a list containing the elements of the specified collection, Collection, in the order they are returned by the collection's 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 Create a list new CopyOnWriteArrayList 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

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 list.

isEmpty

public boolean isEmpty()

Returns true Tests if this list contains has no elements.

Specified by:

isEmpty in interface Collection < E >

Specified by:

isEmpty in interface List < E >

Returns:

true if this list contains has no elements elements; false otherwise.

contains

public boolean contains(Object o)  elem) 

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)) .

Specified by:

contains in interface Collection < E >

Specified by:

contains in interface List < E >

Parameters:

o elem - element whose presence in this list List is to be tested tested.

Returns:

true if this list contains the specified element is present; false otherwise.

indexOf

public int indexOf(Object o)  elem) 

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 Searches for the first occurrence of the given argument, testing for equality using the i equals such that (o==null ? get(i)==null : o.equals(get(i))) , or -1 if there is no such index. method.

Specified by:

indexOf in interface List < E >

Parameters:

o - element to search for elem - an object.

Returns:

the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element the index of the first occurrence of the argument in this list; returns -1 if the object is not found.

See Also:

Object.equals(Object)

indexOf

public int indexOf(E e,
  elem,
 int index) 

Returns the index of the first occurrence of the specified element in this list, searching forwards from Searches for the first occurrence of the given argument, beginning the search at index , or returns -1 if the element is not found. More formally, returns the lowest index , and testing for equality using the i equals such that (i >= index && (e==null ? get(i)==null : e.equals(get(i)))) , or -1 if there is no such index. method.

Parameters:

e - element to search for elem - an object.

index - the index to start searching from from.

Returns:

the index of the first occurrence of the element object argument in this list List at position index or later in the list; List; returns -1 if the element object is not found.

Throws: See Also:

IndexOutOfBoundsException Object.equals(Object) - if the specified index is negative

lastIndexOf

public int lastIndexOf(Object o)  elem) 

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. Returns the index of the last occurrence of the specified object in this list.

Specified by:

lastIndexOf in interface List < E >

Parameters:

o - element to search for elem - the desired element.

Returns:

the index of the last occurrence of the specified element object in this list, or list; returns -1 if this list does the object is not contain the element found.

lastIndexOf

public int lastIndexOf(E e,
  elem,
 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 && (e==null ? get(i)==null : e.equals(get(i)))) , or -1 if there is no such index. Searches backwards for the specified object, starting from the specified index, and returns an index to it.

Parameters:

e - element to search for elem - the desired element.

index - the index to start searching backwards from 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 the index of the last occurrence of the specified object in this List at position less than index in the List; -1 if the object is not found.

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 list.

See Also:

Cloneable

toArray

public Object[] toArray()

Returns an array containing all of the elements in this list in proper sequence (from first to last element). the correct order.

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 of the elements in this list in the correct order.

See Also:

Arrays.asList(Object[])

toArray

public <T> T[] toArray(T[] a)

Returns an array containing all of the elements in this list in proper sequence (from first to last element); the correct order. 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 the list fits in the specified array with room to spare (i.e., the array has more elements than this the list), the element in the array immediately following the end of the list collection is set to null . (This null. This is useful in determining the length of this the list only if the caller knows that this the list does not contain any null elements.) 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 >

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 of the list.

Throws:

ArrayStoreException - if the runtime type of the specified array a is not a supertype of the runtime type of every element in this list

NullPointerException - if the specified array is null list.

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 return.

Returns:

the element at the specified position in this list list.

Throws:

IndexOutOfBoundsException - if the index is out of range ( index (index < 0 || index >= size() size()) ) .

set

public E set(int index,
             E element)

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

Specified by:

set in interface List < E >

Parameters:

index - index of the element to replace replace.

element - element to be stored at the specified position position.

Returns:

the element previously at the specified position position.

Throws:

IndexOutOfBoundsException - if the index is out of range ( index (index < 0 || index >= size() size()) ) .

add

public boolean add(E e)  element) 

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 - element to be appended to this list list.

Returns:

true (as specified by Collection.add(E) ) true (as per the general contract of Collection.add).

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 inserted.

element - element to be inserted inserted.

Throws:

IndexOutOfBoundsException - if the index is out of range ( index (index < 0 || index > size() size()) ) .

remove

public E remove(int index)

Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices). Returns the element that was removed from the list.

Specified by:

remove in interface List < E >

Parameters:

index - the index of the element to be removed removed.

Returns:

the element previously at the specified position the element that was removed from the list.

Throws:

IndexOutOfBoundsException - if the index is out of range ( index (index < 0 || index >= size() size()) ) .

remove

public boolean remove(Object o)

Removes the first occurrence a single instance of the specified element from this list, if it is present. If this list does not contain the element, it is unchanged. present (optional operation). More formally, removes the an element with the lowest index i e such that (o==null ? get(i)==null : o.equals(get(i))) (o==null ? e==null : o.equals(e)) (if such an element exists). , if the list contains one or more such elements. Returns true if this the list contained the specified element (or equivalently, if this the 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 present.

Returns:

true if this the list contained the specified element element.

addIfAbsent

public boolean addIfAbsent(E e)  element) 

Append the element if not present.

Parameters:

e element - element to be added to this list, Collection, if absent absent.

Returns:

true if the element was added

containsAll

public boolean containsAll(Collection<?> c)

Returns true if this list Collection contains all of the elements of in the specified collection. Collection.

This implementation iterates over the specified Collection, checking each element returned by the Iterator in turn to see if it's contained in this Collection. If all elements are so contained true is returned, otherwise false.

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 c - the collection

Returns:

true if this list contains all of the elements of the specified collection true if all elements are contained

Throws:

NullPointerException - if the specified collection is null

See Also:

contains(Object) Collection.contains(Object)

removeAll

public boolean removeAll(Collection<?> c)

Removes from this list Collection all of its elements that are contained in the specified collection. 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 c - the collection

Returns:

true if this list Collection changed as a result of the call call.

Throws: See Also:

ClassCastException Collection.remove(Object) - if the class of an element of this list is incompatible with the specified collection (optional)

NullPointerException , Collection.contains(Object) - if this list contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null

See Also:

remove(Object)

retainAll

public boolean retainAll(Collection<?> c)

Retains only the elements in this list Collection that are contained in the specified collection. Collection (optional operation). In other words, removes from this list Collection all of its elements that are not contained in the specified collection. 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 c - the collection

Returns:

true if this list Collection changed as a result of the call call.

Throws: See Also:

ClassCastException Collection.remove(Object) - if the class of an element of this list is incompatible with the specified collection (optional)

NullPointerException , Collection.contains(Object) - if this list contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null

See Also:

remove(Object)

addAllAbsent

public int addAllAbsent(Collection<? extends E> c)

Appends all of the elements in the specified collection 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. Collection's Iterator.

Parameters:

c - collection containing elements to be added to into this list list.

Returns:

the number of elements added

Throws:

NullPointerException - if the specified collection is null

See Also:

addIfAbsent(Object)

clear

public void clear()

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

Specified by:

clear in interface Collection < E >

Specified by:

clear in interface List < E >

addAll

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

Appends all of the elements in the specified collection Collection to the end of this list, in the order that they are returned by the specified collection's iterator. 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 inserted into this list list.

Returns:

true if this list changed as a result of the call true if any elements are added

Throws:

NullPointerException - if the specified collection is null

See Also:

add(Object) Collection.add(Object)

addAll

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

Inserts all of the elements in the specified collection 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 the list in the order that they are returned by the specified collection's Collection's iterator.

Specified by:

addAll in interface List < E >

Parameters:

index - index at which to insert the first element from the specified collection collection.

c - collection containing elements to be added to inserted into this list list.

Returns:

true if this list changed as a result of the call true if any elements are added

Throws:

IndexOutOfBoundsException - if the index is out of range ( index < 0 || index > size() )

NullPointerException - if the specified collection is null

See Also:

add(int,Object) - index out of range (index < 0 || index > size()).

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) . Returns a string representation of this Collection, containing the String representation of each element.

Overrides:

toString in class Object

Returns:

a string representation of this list the object.

equals

public boolean equals(Object o)

Compares the specified object Object with this list List for equality. Returns true if and only if the specified object Object is also a List , List, both lists Lists have the same size, and all corresponding pairs of elements in the two lists Lists are equal. (Two elements e1 and e2 are equal if (e1==null ? e2==null : e1.equals(e2)) .) In other words, two lists Lists are defined to be equal if they contain the same elements in the same order.

This implementation first checks if the specified object is this List. If so, it returns true; if not, it checks if the specified object is a List. If not, it returns false; if so, it iterates over both lists, comparing corresponding pairs of elements. If any comparison returns false, this method returns false. If either Iterator runs out of elements before the other it returns false (as the Lists are of unequal length); otherwise it returns true when the iterations complete.

Specified by:

equals in interface Collection < E >

Specified by:

equals in interface List < E >

Overrides:

equals in class Object

Parameters:

o - the object Object to be compared for equality with this list List.

Returns:

true if the specified object Object is equal to this list List.

See Also:

Object.hashCode() , Hashtable

hashCode

public int hashCode()

Returns the hash code value for this list. List.

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

Specified by:

hashCode in interface Collection < E >

Specified by:

hashCode in interface List < E >

Overrides:

hashCode in class Object

Returns:

the hash code value for this list

See Also:

Object.equals(java.lang.Object) , Hashtable

iterator

public Iterator<E> iterator()

Returns an iterator Iterator over the elements contained in this list in proper sequence.

collection. 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 Iterable < E >

Specified by:

iterator in interface Collection < E >

Specified by:

iterator in interface List < E >

Returns:

an iterator over the elements in this list in proper sequence the iterator

listIterator

public ListIterator<E> listIterator()

Returns a list iterator over an Iterator of the elements in this list List (in proper sequence).

The returned iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove , set , or add methods.

Specified by:

listIterator in interface List < E >

Returns:

a list iterator over the elements in this list (in proper sequence) the iterator

listIterator

public ListIterator<E> listIterator(int index)

Returns a list iterator ListIterator of the elements in this list List (in proper sequence), starting at the specified position in this list. the List. The specified index indicates the first element that would be returned by an initial call to next . nextElement. An initial call to previous previousElement would return the element with the specified index minus one. The ListIterator returned by this implementation will throw an UnsupportedOperationException in its remove, set and add methods.

The returned iterator provides a snapshot of the state of the list when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove , set or add methods.

Specified by:

listIterator in interface List < E >

Parameters:

index - index of first element to be returned from the list iterator ListIterator (by a call to the next method) getNext).

Returns:

a list iterator of the elements in this list (in proper sequence), starting at the specified position in this list the iterator

Throws:

IndexOutOfBoundsException - if the index is out of range ( index (index < 0 || index > size() ) size()).

subList

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

Returns a view of the portion of this list List between fromIndex , fromIndex, inclusive, and toIndex , toIndex, exclusive. The returned list List is backed by this list, List, so changes in the returned list List are reflected in this list, List, and vice-versa. While mutative operations are supported, they are probably not very useful for CopyOnWriteArrayLists.

The semantics of the list List returned by this method become undefined if the backing list (i.e., this list) List) is structurally modified in any way other than via the returned list. List. (Structural modifications are those that change the size of the list, List, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

Specified by:

subList in interface List < E >

Parameters:

fromIndex - low endpoint (inclusive) of the subList subList.

toIndex - high endpoint (exclusive) of the subList subList.

Returns:

a view of the specified range within this list List.

Throws:

IndexOutOfBoundsException - for an illegal Illegal endpoint index value ( fromIndex (fromIndex < 0 || toIndex > size || fromIndex > toIndex ) toIndex).