org.apache.wicket.util.concurrent
Class CopyOnWriteArrayList

java.lang.Object
  org.apache.wicket.util.concurrent.CopyOnWriteArrayList

All Implemented Interfaces:

java.io.Serializable, java.lang.Cloneable, java.lang.Iterable, java.util.Collection, java.util.List

public class CopyOnWriteArrayList
extends java.lang.Object
implements java.util.List, java.lang.Cloneable, java.io.Serializable

This class implements a 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 it becomes attractive when traversal operations vastly overwhelm mutations, and, especially, when you cannot or don't want to synchronize traversals, yet need to preclude interference among concurrent threads. The 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. (The iterator will not traverse elements added or changed since the iterator was created, but usually this is a desirable feature.)

As much code and documentation as possible was shamelessly copied from java.util.ArrayList (Thanks, Josh!), with the intent of preserving all semantics of ArrayList except for the copy-on-write property. (The java.util collection code could not be subclassed here since all of the existing collection classes assume elementwise mutability.)

Because of the copy-on-write policy, some one-by-one mutative operations in the java.util.Arrays and java.util.Collections classes are so time/space intensive as to never be worth calling (except perhaps as benchmarks for garbage collectors :-).

Three methods are supported in addition to those described in List and ArrayList. The addIfAbsent and addAllAbsent methods provide Set semantics for add, and are used in CopyOnWriteArraySet. However, they can also be used directly from this List version. The copyIn method (and a constructor that invokes it) allow you to copy in an initial array to use. This method can be useful when you first want to perform many operations on a plain array, and then make a copy available for use through the collection API.

Due to their strict read-only nature, element-changing operations on iterators (remove, set, and add) are not supported. These are the only methods throwing UnsupportedOperationException.

[ Introduction to this package. ]

See Also:

CopyOnWriteArraySet, Serialized Form

Nested Class Summary

protected static class	CopyOnWriteArrayList.COWIterator
protected static class	CopyOnWriteArrayList.COWSubList

Field Summary

protected java.lang.Object[]

array_ The held array.

Constructor Summary

CopyOnWriteArrayList()
Constructs an empty list

CopyOnWriteArrayList(java.util.Collection c)
Constructs an list containing the elements of the specified Collection, in the order they are returned by the Collection's iterator.

CopyOnWriteArrayList(java.lang.Object[] toCopyIn)
Create a new CopyOnWriteArrayList holding a copy of given array

Method Summary

void	add(int index, java.lang.Object element) Inserts the specified element at the specified position in this list.
boolean	add(java.lang.Object element) Appends the specified element to the end of this list.
boolean	addAll(java.util.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.
boolean	addAll(int index, java.util.Collection c) Inserts all of the elements in the specified Collection into this list, starting at the specified position.
int	addAllAbsent(java.util.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.
boolean	addIfAbsent(java.lang.Object element) Append the element if not present.
protected java.lang.Object[]	array() Accessor to the array intended to be called from within unsynchronized read-only methods
void	clear() Removes all of the elements from this list.
java.lang.Object	clone() Returns a shallow copy of this list.
boolean	contains(java.lang.Object elem) Returns true if this list contains the specified element.
boolean	containsAll(java.util.Collection c) Returns true if this Collection contains all of the elements in the specified Collection.
void	copyIn(java.lang.Object[] toCopyIn, int first, int n) Replace the held array with a copy of the n elements of the provided array, starting at position first.
boolean	equals(java.lang.Object o) Compares the specified Object with this List for equality.
java.lang.Object	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(java.lang.Object elem) Searches for the first occurrence of the given argument, testing for equality using the equals method.
int	indexOf(java.lang.Object elem, int index) Searches for the first occurrence of the given argument, beginning the search at index, and testing for equality using the equals method.
protected static int	indexOf(java.lang.Object elem, java.lang.Object[] elementData, int len) static version allows repeated call without needed to grab lock for array each time
boolean	isEmpty() Tests if this list has no components.
java.util.Iterator	iterator() Returns an Iterator over the elements contained in this collection.
int	lastIndexOf(java.lang.Object elem) Returns the index of the last occurrence of the specified object in this list.
int	lastIndexOf(java.lang.Object elem, int index) Searches backwards for the specified object, starting from the specified index, and returns an index to it.
protected static int	lastIndexOf(java.lang.Object elem, java.lang.Object[] elementData, int len)
java.util.ListIterator	listIterator() Returns an Iterator of the elements in this List (in proper sequence).
java.util.ListIterator	listIterator(int index) Returns a ListIterator of the elements in this List (in proper sequence), starting at the specified position in the List.
protected void	rangeCheck(int index, int length) Check if the given index is in range.
java.lang.Object	remove(int index) Removes the element at the specified position in this list.
boolean	remove(java.lang.Object element) Removes a single instance of the specified element from this Collection, if it is present (optional operation).
boolean	removeAll(java.util.Collection c) Removes from this Collection all of its elements that are contained in the specified Collection.
void	removeRange(int fromIndex, int toIndex) Removes from this List all of the elements whose index is between fromIndex, inclusive and toIndex, exclusive.
boolean	retainAll(java.util.Collection c) Retains only the elements in this Collection that are contained in the specified Collection (optional operation).
java.lang.Object	set(int index, java.lang.Object element) Replaces the element at the specified position in this list with the specified element.
int	size() Returns the number of components in this list.
java.util.List	subList(int fromIndex, int toIndex) Returns a view of the portion of this List between fromIndex, inclusive, and toIndex, exclusive.
java.lang.Object[]	toArray() Returns an array containing all of the elements in this list in the correct order.
java.lang.Object[]	toArray(java.lang.Object[] a) Returns an array containing all of the elements in this list in the correct order.
java.lang.String	toString() Returns a string representation of this Collection, containing the String representation of each element.

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

array_

protected transient java.lang.Object[] array_

The held array. Directly access only within synchronized methods

Constructor Detail

CopyOnWriteArrayList

public CopyOnWriteArrayList()

Constructs an empty list

CopyOnWriteArrayList

public CopyOnWriteArrayList(java.util.Collection c)

Constructs an list containing the elements of the specified Collection, in the order they are returned by the Collection's iterator.

Parameters:

c - The collection to get the objects from.

CopyOnWriteArrayList

public CopyOnWriteArrayList(java.lang.Object[] toCopyIn)

Create a new CopyOnWriteArrayList holding a copy of given array

Parameters:

toCopyIn - the array. A copy of this array is used as the internal array.

Method Detail

array

protected java.lang.Object[] array()

Accessor to the array intended to be called from within unsynchronized read-only methods

Returns:

The internal array

copyIn

public void copyIn(java.lang.Object[] toCopyIn,
                   int first,
                   int n)

Replace the held array with a copy of the n elements of the provided array, starting at position first. To copy an entire array, call with arguments (array, 0, array.length).

Parameters:

toCopyIn - the array. A copy of the indicated elements of this array is used as the internal array.

first - The index of first position of the array to start copying from.

n - the number of elements to copy. This will be the new size of the list.

size

public int size()

Returns the number of components in this list.

Specified by:

size in interface java.util.Collection

Specified by:

size in interface java.util.List

Returns:

the number of components in this list.

isEmpty

public boolean isEmpty()

Tests if this list has no components.

Specified by:

isEmpty in interface java.util.Collection

Specified by:

isEmpty in interface java.util.List

Returns:

true if this list has no components; false otherwise.

contains

public boolean contains(java.lang.Object elem)

Returns true if this list contains the specified element.

Specified by:

contains in interface java.util.Collection

Specified by:

contains in interface java.util.List

Parameters:

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

indexOf

public int indexOf(java.lang.Object elem)

Searches for the first occurrence of the given argument, testing for equality using the equals method.

Specified by:

indexOf in interface java.util.List

Parameters:

elem - an object.

Returns:

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

protected static int indexOf(java.lang.Object elem,
                             java.lang.Object[] elementData,
                             int len)

static version allows repeated call without needed to grab lock for array each time

Parameters:

elem -

elementData -

len -

Returns:

The index that is found. -1 if not found

indexOf

public int indexOf(java.lang.Object elem,
                   int index)

Searches for the first occurrence of the given argument, beginning the search at index, and testing for equality using the equals method.

Parameters:

elem - an object.

index - the index to start searching from.

Returns:

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

See Also:

Object.equals(Object)

lastIndexOf

public int lastIndexOf(java.lang.Object elem)

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

Specified by:

lastIndexOf in interface java.util.List

Parameters:

elem - the desired component.

Returns:

the index of the last occurrence of the specified object in this list; returns -1 if the object is not found.

lastIndexOf

protected static int lastIndexOf(java.lang.Object elem,
                                 java.lang.Object[] elementData,
                                 int len)

lastIndexOf

public int lastIndexOf(java.lang.Object elem,
                       int index)

Searches backwards for the specified object, starting from the specified index, and returns an index to it.

Parameters:

elem - the desired component.

index - the index to start searching from.

Returns:

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 java.lang.Object clone()

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

Overrides:

clone in class java.lang.Object

Returns:

a clone of this list.

toArray

public java.lang.Object[] toArray()

Returns an array containing all of the elements in this list in the correct order.

Specified by:

toArray in interface java.util.Collection

Specified by:

toArray in interface java.util.List

toArray

public java.lang.Object[] toArray(java.lang.Object[] a)

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

Specified by:

toArray in interface java.util.Collection

Specified by:

toArray in interface java.util.List

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 the elements of the list.

Throws:

java.lang.ArrayStoreException - the runtime type of a is not a supertype of the runtime type of every element in this list.

get

public java.lang.Object get(int index)

Returns the element at the specified position in this list.

Specified by:

get in interface java.util.List

Parameters:

index - index of element to return.

Throws:

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

set

public java.lang.Object set(int index,
                            java.lang.Object element)

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

Specified by:

set in interface java.util.List

Parameters:

index - index of element to replace.

element - element to be stored at the specified position.

Returns:

the element previously at the specified position.

Throws:

java.lang.IndexOutOfBoundsException - index out of range (index < 0 || index >= size()).

add

public boolean add(java.lang.Object element)

Appends the specified element to the end of this list.

Specified by:

add in interface java.util.Collection

Specified by:

add in interface java.util.List

Parameters:

element - element to be appended to this list.

Returns:

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

add

public void add(int index,
                java.lang.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).

Specified by:

add in interface java.util.List

Parameters:

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

element - element to be inserted.

Throws:

java.lang.IndexOutOfBoundsException - index is out of range (index < 0 || index > size()).

remove

public java.lang.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.

Specified by:

remove in interface java.util.List

Parameters:

index - the index of the element to removed.

Throws:

java.lang.IndexOutOfBoundsException - index out of range (index < 0 || index >= size()).

remove

public boolean remove(java.lang.Object element)

Removes a single instance of the specified element from this Collection, if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if the Collection contains one or more such elements. Returns true if the Collection contained the specified element (or equivalently, if the Collection changed as a result of the call).

Specified by:

remove in interface java.util.Collection

Specified by:

remove in interface java.util.List

Parameters:

element - element to be removed from this Collection, if present.

Returns:

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

removeRange

public void removeRange(int fromIndex,
                        int toIndex)

Removes from this List all of the elements whose index is between fromIndex, inclusive and toIndex, exclusive. Shifts any succeeding elements to the left (reduces their index). This call shortens the List by (toIndex - fromIndex) elements. (If toIndex==fromIndex, this operation has no effect.)

Parameters:

fromIndex - index of first element to be removed.

toIndex - index after last element to be removed.

Throws:

java.lang.IndexOutOfBoundsException - fromIndex or toIndex out of range (fromIndex < 0 || fromIndex >= size() || toIndex > size() || toIndex < fromIndex).

addIfAbsent

public boolean addIfAbsent(java.lang.Object element)

Append the element if not present. This operation can be used to obtain Set semantics for lists.

Parameters:

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

Returns:

true if added

containsAll

public boolean containsAll(java.util.Collection c)

Returns true if this Collection contains all of the elements in the specified 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 java.util.Collection

Specified by:

containsAll in interface java.util.List

removeAll

public boolean removeAll(java.util.Collection c)

Removes from this Collection 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.

Specified by:

removeAll in interface java.util.Collection

Specified by:

removeAll in interface java.util.List

Returns:

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

retainAll

public boolean retainAll(java.util.Collection c)

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

Specified by:

retainAll in interface java.util.Collection

Specified by:

retainAll in interface java.util.List

Returns:

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

addAllAbsent

public int addAllAbsent(java.util.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.

Parameters:

c - elements to be added into this list.

Returns:

the number of elements added

clear

public void clear()

Removes all of the elements from this list.

Specified by:

clear in interface java.util.Collection

Specified by:

clear in interface java.util.List

addAll

public boolean addAll(java.util.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.

Specified by:

addAll in interface java.util.Collection

Specified by:

addAll in interface java.util.List

Parameters:

c - elements to be inserted into this list.

addAll

public boolean addAll(int index,
                      java.util.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 the list in the order that they are returned by the specified Collection's iterator.

Specified by:

addAll in interface java.util.List

Parameters:

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

c - elements to be inserted into this list.

Throws:

java.lang.IndexOutOfBoundsException - index out of range (index < 0 || index > size()).

rangeCheck

protected void rangeCheck(int index,
                          int length)

Check if the given index is in range. If not, throw an appropriate runtime exception.

Parameters:

index -

length -

toString

public java.lang.String toString()

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

Overrides:

toString in class java.lang.Object

equals

public boolean equals(java.lang.Object o)

Compares the specified Object with this List for equality. Returns true if and only if the specified Object is also a List, both Lists have the same size, and all corresponding pairs of elements in the two Lists are equal. (Two elements e1 and e2 are equal if (e1==null ? e2==null : e1.equals(e2)).) In other words, two 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 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 java.util.Collection

Specified by:

equals in interface java.util.List

Overrides:

equals in class java.lang.Object

Parameters:

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

Returns:

true if the specified Object is equal to this List.

hashCode

public int hashCode()

Returns the hash code value for this List.

This implementation uses exactly the code that is used to define the List hash function in the documentation for List.hashCode.

Specified by:

hashCode in interface java.util.Collection

Specified by:

hashCode in interface java.util.List

Overrides:

hashCode in class java.lang.Object

iterator

public java.util.Iterator iterator()

Returns an Iterator over the elements contained in this collection. The 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 java.lang.Iterable

Specified by:

iterator in interface java.util.Collection

Specified by:

iterator in interface java.util.List

listIterator

public java.util.ListIterator listIterator()

Returns an Iterator of the elements in this List (in proper sequence). The 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 java.util.List

listIterator

public java.util.ListIterator listIterator(int index)

Returns a ListIterator of 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 nextElement. An initial call to 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.

Specified by:

listIterator in interface java.util.List

Parameters:

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

Throws:

java.lang.IndexOutOfBoundsException - index is out of range (index < 0 || index > size()).

subList

public java.util.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 CopyOnWriteArrays.

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.)

Specified by:

subList in interface java.util.List

Parameters:

fromIndex - low endpoint (inclusive) of the subList.

toKey - high endpoint (exclusive) of the subList.

Returns:

a view of the specified range within this List.

Throws:

java.lang.IndexOutOfBoundsException - Illegal endpoint index value (fromIndex < 0 || toIndex > size || fromIndex > toIndex).