java.util.concurrent.CopyOnWriteArrayList<E>

public class java.util.concurrent

CopyOnWriteArrayList<E>

Java SE 6

Implements: List, RandomAccess, Cloneable, Serializable

A thread-safe variant of java.util.ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.

This is ordinarily too costly, but may be more efficient than alternatives when traversal operations vastly outnumber mutations, and is useful when you cannot or don't want to synchronize traversals, yet need to preclude interference among concurrent threads. The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException. The iterator will not reflect additions, removals, or changes to the list since the iterator was created. Element-changing operations on iterators themselves (remove, set, and add) are not supported. These methods throw UnsupportedOperationException.

All elements are permitted, including null.

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

This class is a member of the Java Collections Framework.

since 1.5

E the type of elements held in this collection

Constructors

public

CopyOnWriteArrayList()
Creates an empty list.

public

CopyOnWriteArrayList(Collection c)
Creates a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

c	the collection of initially held elements
Throws	NullPointerException: if the specified collection is null

public

CopyOnWriteArrayList(Object[] toCopyIn)
Creates a list holding a copy of the given array.

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

Methods

public boolean

add(Object e)
Appends the specified element to the end of this list.

e	element to be appended to this list
return	`true` (as specified by `Collection#add`)

public void

add(int index, Object 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).

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

public boolean

addAll(Collection 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.

c	collection containing elements to be added to this list
return	`true` if this list changed as a result of the call
Throws	NullPointerException: if the specified collection is null
See also	add(Object)

public boolean

addAll(int index, Collection 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.

index	index at which to insert the first element from the specified collection
c	collection containing elements to be added to this list
return	`true` if this list changed as a result of the call
Throws	IndexOutOfBoundsException: if the index is out of range (`index < 0 \|\| index > size()`)
Throws	NullPointerException: if the specified collection is null
See also	add(int,Object)

public int

addAllAbsent(Collection 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.

c	collection containing elements to be added to this list
return	the number of elements added
Throws	NullPointerException: if the specified collection is null
See also	addIfAbsent(Object)

public boolean

addIfAbsent(Object e)
Append the element if not present.

e	element to be added to this list, if absent
return	`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.)

return

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 (o==null ? e==null : o.equals(e)).

o	element whose presence in this list is to be tested
return	`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.

c	collection to be checked for containment in this list
return	`true` if this list contains all of the elements of the specified collection
Throws	NullPointerException: if the specified collection is null
See also	contains(Object)

public boolean

equals(Object o)
Compares the specified object with this list for equality. Returns true if the specified object is the same object as this object, or if it is also a List and the sequence of elements returned by an iterator over the specified list is the same as the sequence returned by an iterator over this list. The two sequences are considered to be the same if they have the same length and corresponding elements at the same position in the sequence are equal. Two elements e1 and e2 are considered equal if (e1==null ? e2==null : e1.equals(e2)).

o	the object to be compared for equality with this list
return	`true` if the specified object is equal to this list

public Object

get(int index)
Returns the element at the specified position in this list.

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

public int

hashCode()
Returns the hash code value for this list.

This implementation uses the definition in List#hashCode.

return the hash code value for this list

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 (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

public int

indexOf(Object 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 && (e==null ? get(i)==null : e.equals(get(i)))), or -1 if there is no such index.

e	element to search for
index	index to start searching from
return	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 Iterator

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.

return 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 (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

public int

lastIndexOf(Object 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 && (e==null ? get(i)==null : e.equals(get(i)))), or -1 if there is no such index.

e	element to search for
index	index to start searching backwards from
return	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

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.

public ListIterator

listIterator(int index)
Returns a list iterator of the elements in this list (in proper sequence), starting at the specified position in this 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.

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

public Object

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.

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

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 (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).

o	element to be removed from this list, if present
return	`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.

c	collection containing elements to be removed from this list
return	`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)
Throws	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	remove(Object)

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.

c	collection containing elements to be retained in this list
return	`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)
Throws	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	remove(Object)

public Object

set(int index, Object element)
Replaces the element at the specified position in this list with the specified element.

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

public int

size()
Returns the number of elements in this list.

return

the number of elements in this list

public List

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, and vice-versa. While mutative operations are supported, they are probably not very useful for CopyOnWriteArrayLists.

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

fromIndex low endpoint (inclusive) of the subList

toIndex high endpoint (exclusive) of the subList

return a view of the specified range within this list

Throws IndexOutOfBoundsException: for an illegal endpoint index value (fromIndex < 0 || toIndex > size || fromIndex > toIndex)

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.

return an array containing all the elements in this list

public Object[]<T>

toArray(Object[] 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().

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.
return	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
Throws	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).

return

a string representation of this list

Properties

public boolean

isEmpty()
Returns true if this list contains no elements.

return true if this list contains no elements

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Java SE 6

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