001/* CopyOnWriteArrayList.java
002   Copyright (C) 2006 Free Software Foundation
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038package java.util.concurrent;
039
040import java.io.IOException;
041import java.io.ObjectInputStream;
042import java.io.ObjectOutputStream;
043import java.io.Serializable;
044
045import java.lang.reflect.Array;
046
047import java.util.AbstractList;
048import java.util.Arrays;
049import java.util.Collection;
050import java.util.ConcurrentModificationException;
051import java.util.Iterator;
052import java.util.List;
053import java.util.ListIterator;
054import java.util.NoSuchElementException;
055import java.util.RandomAccess;
056
057/**
058 * A thread-safe implementation of an ArrayList. A CopyOnWriteArrayList is
059 * as special ArrayList which performs copies of the underlying storage
060 * each time a write (<code>remove</code>, <code>add</code> etc..) operation
061 * is performed.<br />
062 * <br />
063 * The update operation in this class run usually in <code>O(n)</code> or worse,
064 * but traversal operations are fast and efficient, especially when running in
065 * a multi-thread environment without the need to design complex synchronize
066 * mechanisms.<br />
067 * <br />
068 * <code>Iterator</code>s in this class work on a snapshot of the backing store
069 * at the moment the iterator itself was created, hence the iterator will not
070 * reflect changes in the underlying storage. Thus, update operation on the
071 * <code>Iterator</code>s are not supported, but as interferences from other
072 * threads are impossible, no <code>ConcurrentModificationException</code>
073 * will be ever thrown from within the <code>Iterator</code>.
074 * <br /><br />
075 * This class is especially useful when used with event handling, like the
076 * following code demonstrates:<br />
077 * <code><pre>
078 *
079 * CopyOnWriteArrayList<EventListener> listeners =
080 *   new CopyOnWriteArrayList<EventListener>();
081 *
082 * [...]
083 *
084 * for (final EventListener listener : listeners)
085 *   {
086 *     Runnable dispatcher = new Runnable() {
087 *       public void run()
088 *       {
089 *         listener.preferenceChange(event);
090 *       }
091 *     };
092 *
093 *     Executor executor = Executors.newSingleThreadExecutor();
094 *     executor.execute(dispatcher);
095 *   }
096 * </pre></code>
097 *
098 * @since 1.5
099 */
100public class CopyOnWriteArrayList<E>
101  implements List<E>, RandomAccess, Cloneable, Serializable
102{
103  /**
104   *
105   */
106  private static final long serialVersionUID = 8673264195747942595L;
107
108  /**
109   * Where the data is stored.
110   */
111  private transient E[] data;
112
113  /**
114   * Construct a new ArrayList with the default capacity (16).
115   */
116  public CopyOnWriteArrayList()
117  {
118    data = (E[]) new Object[0];
119  }
120
121  /**
122   * Construct a new ArrayList, and initialize it with the elements in the
123   * supplied Collection. The initial capacity is 110% of the Collection's size.
124   *
125   * @param c
126   *          the collection whose elements will initialize this list
127   * @throws NullPointerException
128   *           if c is null
129   */
130  public CopyOnWriteArrayList(Collection< ? extends E> c)
131  {
132    // FIXME ... correct?  use c.toArray()
133    data = (E[]) new Object[c.size()];
134    int index = 0;
135    for (E value : c)
136      data[index++] = value;
137  }
138
139  /**
140   * Construct a new ArrayList, and initialize it with the elements in the
141   * supplied array.
142   *
143   * @param array
144   *          the array used to initialize this list
145   * @throws NullPointerException
146   *           if array is null
147   */
148  public CopyOnWriteArrayList(E[] array)
149  {
150    data = (E[]) array.clone();
151  }
152
153  /**
154   * Returns the number of elements in this list.
155   *
156   * @return the list size
157   */
158  public int size()
159  {
160    return data.length;
161  }
162
163  /**
164   * Checks if the list is empty.
165   *
166   * @return true if there are no elements
167   */
168  public boolean isEmpty()
169  {
170    return data.length == 0;
171  }
172
173  /**
174   * Returns true if element is in this ArrayList.
175   *
176   * @param e
177   *          the element whose inclusion in the List is being tested
178   * @return true if the list contains e
179   */
180  public boolean contains(Object e)
181  {
182    return indexOf(e) != -1;
183  }
184
185  /**
186   * Tests whether this collection contains all the elements in a given
187   * collection. This implementation iterates over the given collection,
188   * testing whether each element is contained in this collection. If any one
189   * is not, false is returned. Otherwise true is returned.
190   *
191   * @param c the collection to test against
192   * @return true if this collection contains all the elements in the given
193   *         collection
194   * @throws NullPointerException if the given collection is null
195   * @see #contains(Object)
196   */
197  public boolean containsAll(Collection<?> c)
198  {
199    Iterator<?> itr = c.iterator();
200    int pos = c.size();
201    while (--pos >= 0)
202      if (!contains(itr.next()))
203        return false;
204    return true;
205  }
206
207  /**
208   * Returns the lowest index at which element appears in this List, or -1 if it
209   * does not appear.
210   *
211   * @param e
212   *          the element whose inclusion in the List is being tested
213   * @return the index where e was found
214   */
215  public int indexOf(Object e)
216  {
217    E[] data = this.data;
218    for (int i = 0; i < data.length; i++)
219      if (equals(e, data[i]))
220        return i;
221    return -1;
222  }
223
224  /**
225   * Return the lowest index greater equal <code>index</code> at which
226   * <code>e</code> appears in this List, or -1 if it does not
227   * appear.
228   *
229   * @param e the element whose inclusion in the list is being tested
230   * @param index the index at which the search begins
231   * @return the index where <code>e</code> was found
232   */
233  public int indexOf(E e, int index)
234  {
235    E[] data = this.data;
236
237    for (int i = index; i < data.length; i++)
238      if (equals(e, data[i]))
239        return i;
240    return -1;
241  }
242
243  /**
244   * Returns the highest index at which element appears in this List, or -1 if
245   * it does not appear.
246   *
247   * @param e
248   *          the element whose inclusion in the List is being tested
249   * @return the index where e was found
250   */
251  public int lastIndexOf(Object e)
252  {
253    E[] data = this.data;
254    for (int i = data.length - 1; i >= 0; i--)
255      if (equals(e, data[i]))
256        return i;
257    return -1;
258  }
259
260  /**
261   * Returns the highest index lesser equal <code>index</code> at
262   * which <code>e</code> appears in this List, or -1 if it does not
263   * appear.
264   *
265   * @param e the element whose inclusion in the list is being tested
266   * @param index the index at which the search begins
267   * @return the index where <code>e</code> was found
268   */
269  public int lastIndexOf(E e, int index)
270  {
271    E[] data = this.data;
272
273    for (int i = index; i >= 0; i--)
274      if (equals(e, data[i]))
275        return i;
276    return -1;
277  }
278
279  /**
280   * Creates a shallow copy of this ArrayList (elements are not cloned).
281   *
282   * @return the cloned object
283   */
284  public Object clone()
285  {
286    CopyOnWriteArrayList<E> clone = null;
287    try
288      {
289        clone = (CopyOnWriteArrayList<E>) super.clone();
290      }
291    catch (CloneNotSupportedException e)
292      {
293        // Impossible to get here.
294      }
295    return clone;
296  }
297
298  /**
299   * Returns an Object array containing all of the elements in this ArrayList.
300   * The array is independent of this list.
301   *
302   * @return an array representation of this list
303   */
304  public Object[] toArray()
305  {
306    E[] data = this.data;
307    E[] array = (E[]) new Object[data.length];
308    System.arraycopy(data, 0, array, 0, data.length);
309    return array;
310  }
311
312  /**
313   * Returns an Array whose component type is the runtime component type of the
314   * passed-in Array. The returned Array is populated with all of the elements
315   * in this ArrayList. If the passed-in Array is not large enough to store all
316   * of the elements in this List, a new Array will be created and returned; if
317   * the passed-in Array is <i>larger</i> than the size of this List, then
318   * size() index will be set to null.
319   *
320   * @param a
321   *          the passed-in Array
322   * @return an array representation of this list
323   * @throws ArrayStoreException
324   *           if the runtime type of a does not allow an element in this list
325   * @throws NullPointerException
326   *           if a is null
327   */
328  public <T> T[] toArray(T[] a)
329  {
330    E[] data = this.data;
331    if (a.length < data.length)
332      a = (T[]) Array.newInstance(a.getClass().getComponentType(), data.length);
333    else if (a.length > data.length)
334      a[data.length] = null;
335    System.arraycopy(data, 0, a, 0, data.length);
336    return a;
337  }
338
339  /**
340   * Retrieves the element at the user-supplied index.
341   *
342   * @param index
343   *          the index of the element we are fetching
344   * @throws IndexOutOfBoundsException
345   *           if index &lt; 0 || index &gt;= size()
346   */
347  public E get(int index)
348  {
349    return data[index];
350  }
351
352  /**
353   * Sets the element at the specified index. The new element, e, can be an
354   * object of any type or null.
355   *
356   * @param index
357   *          the index at which the element is being set
358   * @param e
359   *          the element to be set
360   * @return the element previously at the specified index
361   * @throws IndexOutOfBoundsException
362   *           if index &lt; 0 || index &gt;= 0
363   */
364  public synchronized E set(int index, E e)
365  {
366    E result = data[index];
367    E[] newData = (E[]) data.clone();
368    newData[index] = e;
369    data = newData;
370    return result;
371  }
372
373  /**
374   * Appends the supplied element to the end of this list. The element, e, can
375   * be an object of any type or null.
376   *
377   * @param e
378   *          the element to be appended to this list
379   * @return true, the add will always succeed
380   */
381  public synchronized boolean add(E e)
382  {
383    E[] data = this.data;
384    E[] newData = (E[]) new Object[data.length + 1];
385    System.arraycopy(data, 0, newData, 0, data.length);
386    newData[data.length] = e;
387    this.data = newData;
388    return true;
389  }
390
391  /**
392   * Adds the supplied element at the specified index, shifting all elements
393   * currently at that index or higher one to the right. The element, e, can be
394   * an object of any type or null.
395   *
396   * @param index
397   *          the index at which the element is being added
398   * @param e
399   *          the item being added
400   * @throws IndexOutOfBoundsException
401   *           if index &lt; 0 || index &gt; size()
402   */
403  public synchronized void add(int index, E e)
404  {
405    E[] data = this.data;
406    E[] newData = (E[]) new Object[data.length + 1];
407    System.arraycopy(data, 0, newData, 0, index);
408    newData[index] = e;
409    System.arraycopy(data, index, newData, index + 1, data.length - index);
410    this.data = newData;
411  }
412
413  /**
414   * Removes the element at the user-supplied index.
415   *
416   * @param index
417   *          the index of the element to be removed
418   * @return the removed Object
419   * @throws IndexOutOfBoundsException
420   *           if index &lt; 0 || index &gt;= size()
421   */
422  public synchronized E remove(int index)
423  {
424    if (index < 0 || index >= this.size())
425      throw new IndexOutOfBoundsException("index = " +  index);
426
427    E[] snapshot = this.data;
428    E[] newData = (E[]) new Object[snapshot.length - 1];
429
430    E result = snapshot[index];
431
432    if (index > 0)
433      System.arraycopy(snapshot, 0, newData, 0, index);
434
435    System.arraycopy(snapshot, index + 1, newData, index,
436                     snapshot.length - index - 1);
437
438    this.data = newData;
439
440    return result;
441  }
442
443  /**
444   * Remove the first occurrence, if any, of the given object from this list,
445   * returning <code>true</code> if the object was removed, <code>false</code>
446   * otherwise.
447   *
448   * @param element the object to be removed.
449   * @return true if element was removed, false otherwise. false means also that
450   * the underlying storage was unchanged after this operation concluded.
451   */
452  public synchronized boolean remove(Object element)
453  {
454    E[] snapshot = this.data;
455    int len = snapshot.length;
456
457    if (len == 0)
458      return false;
459
460    E[] newData = (E[]) new Object[len - 1];
461
462    // search the element to remove while filling the backup array
463    // this way we can run this method in O(n)
464    int elementIndex = -1;
465    for (int i = 0; i < snapshot.length; i++)
466      {
467        if (equals(element, snapshot[i]))
468          {
469            elementIndex = i;
470            break;
471          }
472
473        if (i < newData.length)
474          newData[i] = snapshot[i];
475      }
476
477    if (elementIndex < 0)
478      return false;
479
480    System.arraycopy(snapshot, elementIndex + 1, newData, elementIndex,
481                     snapshot.length - elementIndex - 1);
482    this.data = newData;
483
484    return true;
485  }
486
487  /**
488   * Removes all the elements contained in the given collection.
489   * This method removes the elements that are contained in both
490   * this list and in the given collection.
491   *
492   * @param c the collection containing the elements to be removed from this
493   * list.
494   * @return true if at least one element was removed, indicating that
495   * the list internal storage changed as a result, false otherwise.
496   */
497  public synchronized boolean removeAll(Collection<?> c)
498  {
499    if (c.size() == 0)
500      return false;
501
502    E [] snapshot = this.data;
503    E [] storage = (E[]) new Object[this.data.length];
504    boolean changed = false;
505
506    int length = 0;
507    for (E element : snapshot)
508      {
509        // copy all the elements, including null values
510        // if the collection can hold it
511        // FIXME: slow operation
512        if (c.contains(element))
513          changed = true;
514        else
515          storage[length++] = element;
516      }
517
518    if (!changed)
519      return false;
520
521    E[] newData = (E[]) new Object[length];
522    System.arraycopy(storage, 0, newData, 0, length);
523
524    this.data = newData;
525
526    return true;
527  }
528
529  /**
530   * Removes all the elements that are not in the passed collection.
531   * If the collection is void, this method has the same effect of
532   * <code>clear()</code>.
533   * Please, note that this method is extremely slow (unless the argument has
534   * <code>size == 0</code>) and has bad performance is both space and time
535   * usage.
536   *
537   * @param c the collection containing the elements to be retained by this
538   * list.
539   * @return true the list internal storage changed as a result of this
540   * operation, false otherwise.
541   */
542  public synchronized boolean retainAll(Collection<?> c)
543  {
544    // if the given collection does not contain elements
545    // we remove all the elements from our storage
546    if (c.size() == 0)
547      {
548        this.clear();
549        return true;
550      }
551
552    E [] snapshot = this.data;
553    E [] storage = (E[]) new Object[this.data.length];
554
555    int length = 0;
556    for (E element : snapshot)
557      {
558        if (c.contains(element))
559          storage[length++] = element;
560      }
561
562    // means we retained all the elements previously in our storage
563    // we are running already slow here, but at least we avoid copying
564    // another array and changing the internal storage
565    if (length == snapshot.length)
566      return false;
567
568    E[] newData = (E[]) new Object[length];
569    System.arraycopy(storage, 0, newData, 0, length);
570
571    this.data = newData;
572
573    return true;
574  }
575
576  /**
577   * Removes all elements from this List
578   */
579  public synchronized void clear()
580  {
581    data = (E[]) new Object[0];
582  }
583
584  /**
585   * Add each element in the supplied Collection to this List. It is undefined
586   * what happens if you modify the list while this is taking place; for
587   * example, if the collection contains this list. c can contain objects of any
588   * type, as well as null values.
589   *
590   * @param c
591   *          a Collection containing elements to be added to this List
592   * @return true if the list was modified, in other words c is not empty
593   * @throws NullPointerException
594   *           if c is null
595   */
596  public synchronized boolean addAll(Collection< ? extends E> c)
597  {
598    return addAll(data.length, c);
599  }
600
601  /**
602   * Add all elements in the supplied collection, inserting them beginning at
603   * the specified index. c can contain objects of any type, as well as null
604   * values.
605   *
606   * @param index
607   *          the index at which the elements will be inserted
608   * @param c
609   *          the Collection containing the elements to be inserted
610   * @throws IndexOutOfBoundsException
611   *           if index &lt; 0 || index &gt; 0
612   * @throws NullPointerException
613   *           if c is null
614   */
615  public synchronized boolean addAll(int index, Collection< ? extends E> c)
616  {
617    if (index < 0 || index > this.size())
618      throw new IndexOutOfBoundsException("index = " +  index);
619
620    int csize = c.size();
621    if (csize == 0)
622      return false;
623
624    E[] data = this.data;
625    Iterator<? extends E> itr = c.iterator();
626
627    E[] newData = (E[]) new Object[data.length + csize];
628
629    // avoid this call at all if we were asked to put the elements at the
630    // beginning of our storage
631    if (index != 0)
632      System.arraycopy(data, 0, newData, 0, index);
633
634    int itemsLeft = index;
635
636    for (E value : c)
637      newData[index++] = value;
638
639    // now copy the remaining elements
640    System.arraycopy(data, itemsLeft, newData, 0, data.length - itemsLeft);
641
642    this.data = newData;
643
644    return true;
645  }
646
647  /**
648   * Adds an element if the list does not contains it already.
649   *
650   * @param val the element to add to the list.
651   * @return true if the element was added, false otherwise.
652   */
653  public synchronized boolean addIfAbsent(E val)
654  {
655    if (contains(val))
656      return false;
657    add(val);
658    return true;
659  }
660
661  /**
662   * Adds all the element from the given collection that are not already
663   * in this list.
664   *
665   * @param c the Collection containing the elements to be inserted
666   * @return true the list internal storage changed as a result of this
667   * operation, false otherwise.
668   */
669  public synchronized int addAllAbsent(Collection<? extends E> c)
670  {
671    int size = c.size();
672    if (size == 0)
673      return 0;
674
675    E [] snapshot = this.data;
676    E [] storage = (E[]) new Object[size];
677
678    size = 0;
679    for (E val : c)
680      {
681        if (!this.contains(val))
682          storage[size++] = val;
683      }
684
685    if (size == 0)
686      return 0;
687
688    // append storage to data
689    E [] newData = (E[]) new Object[snapshot.length + size];
690
691    System.arraycopy(snapshot, 0, newData, 0, snapshot.length);
692    System.arraycopy(storage, 0, newData, snapshot.length, size);
693
694    this.data = newData;
695
696    return size;
697  }
698
699  public String toString()
700  {
701    return Arrays.toString(this.data);
702  }
703
704  public boolean equals(Object o)
705  {
706    if (o == null)
707      return false;
708
709    if (this == o)
710      return true;
711
712    // let's see if 'o' is a list, if so, we need to compare the elements
713    // as returned by the iterator
714    if (o instanceof List)
715      {
716        List<?> source = (List<?>) o;
717
718        if (source.size() != this.size())
719          return false;
720
721        Iterator<?> sourceIterator = source.iterator();
722        for (E element : this)
723          {
724            if (!element.equals(sourceIterator.next()))
725              return false;
726          }
727
728        return true;
729      }
730
731    return false;
732  }
733
734  public int hashCode()
735  {
736    // see http://java.sun.com/6/docs/api/java/util/List.html#hashcode()
737    int hashcode = 1;
738    for (E element : this)
739      {
740        hashcode = 31 * hashcode + (element == null ? 0 : element.hashCode());
741      }
742    return hashcode;
743  }
744
745  /**
746   * Return an Iterator containing the elements of this list.
747   * The Iterator uses a snapshot of the state of the internal storage
748   * at the moment this method is called and does <strong>not</strong> support
749   * update operations, so no synchronization is needed to traverse the
750   * iterator.
751   *
752   * @return an Iterator containing the elements of this list in sequence.
753   */
754  public Iterator<E> iterator()
755  {
756    return new Iterator<E>()
757    {
758      E [] iteratorData = CopyOnWriteArrayList.this.data;
759      int currentElement = 0;
760
761      public boolean hasNext()
762      {
763        return (currentElement < iteratorData.length);
764      }
765
766      public E next()
767      {
768        return iteratorData[currentElement++];
769      }
770
771      public void remove()
772      {
773        throw new UnsupportedOperationException("updating of elements in " +
774                                                "iterators is not supported " +
775                                                "by this class");
776      }
777    };
778  }
779
780  /**
781   * Return a ListIterator containing the elements of this list.
782   * The Iterator uses a snapshot of the state of the internal storage
783   * at the moment this method is called and does <strong>not</strong> support
784   * update operations, so no synchronization is needed to traverse the
785   * iterator.
786   *
787   * @return a ListIterator containing the elements of this list in sequence.
788   */
789  public ListIterator<E> listIterator()
790  {
791    return listIterator(0);
792  }
793
794  /**
795   * Return a ListIterator over the elements of this list starting at
796   * the specified index.  An initial call to {@code next()} will thus
797   * return the element at {@code index}, while an initial call to
798   * {@code previous()} will return the element at {@code index-1}.  The
799   * Iterator uses a snapshot of the state of the internal storage
800   * at the moment this method is called and does <strong>not</strong> support
801   * update operations, so no synchronization is needed to traverse the
802   * iterator.
803   *
804   * @param index the index at which to start iterating.
805   * @return a ListIterator containing the elements of this list in sequence.
806   */
807  public ListIterator<E> listIterator(final int index)
808  {
809    if (index < 0 || index > size())
810      throw new IndexOutOfBoundsException("Index: " + index + ", Size:"
811                                          + size());
812
813    return new ListIterator<E>()
814    {
815      E [] iteratorData = CopyOnWriteArrayList.this.data;
816      int currentElement = index;
817
818      public void add(E o)
819      {
820        throw new UnsupportedOperationException("updating of elements in " +
821                                                "iterators is not supported " +
822                                                "by this class");
823      }
824
825      public boolean hasNext()
826      {
827        return (currentElement < iteratorData.length);
828      }
829
830      public boolean hasPrevious()
831      {
832        return (currentElement > 0);
833      }
834
835      public E next()
836      {
837        if (hasNext() == false)
838          throw new java.util.NoSuchElementException();
839
840        return iteratorData[currentElement++];
841      }
842
843      public int nextIndex()
844      {
845        return (currentElement + 1);
846      }
847
848      public E previous()
849      {
850        if (hasPrevious() == false)
851          throw new java.util.NoSuchElementException();
852
853        return iteratorData[--currentElement];
854      }
855
856      public int previousIndex()
857      {
858        return (currentElement - 1);
859      }
860
861      public void remove()
862      {
863        throw new UnsupportedOperationException("updating of elements in " +
864                                                "iterators is not supported " +
865                                                "by this class");
866      }
867
868      public void set(E o)
869      {
870        throw new UnsupportedOperationException("updating of elements in " +
871                                                "iterators is not supported " +
872                                                "by this class");
873      }
874
875    };
876  }
877
878  /**
879   * Obtain a List view of a subsection of this list, from fromIndex
880   * (inclusive) to toIndex (exclusive). If the two indices are equal, the
881   * sublist is empty. The returned list should be modifiable if and only
882   * if this list is modifiable. Changes to the returned list should be
883   * reflected in this list. If this list is structurally modified in
884   * any way other than through the returned list, the result of any subsequent
885   * operations on the returned list is undefined.
886   * <p>
887   *
888   * This implementation returns a subclass of AbstractList. It stores, in
889   * private fields, the offset and size of the sublist, and the expected
890   * modCount of the backing list. If the backing list implements RandomAccess,
891   * the sublist will also.
892   * <p>
893   *
894   * The subclass's <code>set(int, Object)</code>, <code>get(int)</code>,
895   * <code>add(int, Object)</code>, <code>remove(int)</code>,
896   * <code>addAll(int, Collection)</code> and
897   * <code>removeRange(int, int)</code> methods all delegate to the
898   * corresponding methods on the backing abstract list, after
899   * bounds-checking the index and adjusting for the offset. The
900   * <code>addAll(Collection c)</code> method merely returns addAll(size, c).
901   * The <code>listIterator(int)</code> method returns a "wrapper object"
902   * over a list iterator on the backing list, which is created with the
903   * corresponding method on the backing list. The <code>iterator()</code>
904   * method merely returns listIterator(), and the <code>size()</code> method
905   * merely returns the subclass's size field.
906   * <p>
907   *
908   * All methods first check to see if the actual modCount of the backing
909   * list is equal to its expected value, and throw a
910   * ConcurrentModificationException if it is not.
911   *
912   * @param fromIndex the index that the returned list should start from
913   *        (inclusive)
914   * @param toIndex the index that the returned list should go to (exclusive)
915   * @return a List backed by a subsection of this list
916   * @throws IndexOutOfBoundsException if fromIndex &lt; 0
917   *         || toIndex &gt; size()
918   * @throws IndexOutOfBoundsException if fromIndex &gt; toIndex
919   * @see ConcurrentModificationException
920   * @see RandomAccess
921   */
922  public synchronized List<E> subList(int fromIndex, int toIndex)
923  {
924    // This follows the specification of AbstractList, but is inconsistent
925    // with the one in List. Don't you love Sun's inconsistencies?
926    if (fromIndex > toIndex)
927      throw new IndexOutOfBoundsException(fromIndex + " > " + toIndex);
928    if (fromIndex < 0 || toIndex > size())
929      throw new IndexOutOfBoundsException();
930
931    if (this instanceof RandomAccess)
932      return new RandomAccessSubList<E>(this, fromIndex, toIndex);
933    return new SubList<E>(this, fromIndex, toIndex);
934  }
935
936  /**
937   * This class follows the implementation requirements set forth in
938   * {@link AbstractList#subList(int, int)}. It matches Sun's implementation
939   * by using a non-public top-level class in the same package.
940   *
941   * @author Original author unknown
942   * @author Eric Blake (ebb9@email.byu.edu)
943   */
944  private static class SubList<E>
945    extends AbstractList<E>
946  {
947    // Package visible, for use by iterator.
948    /** The original list. */
949    final CopyOnWriteArrayList<E> backingList;
950    /** The index of the first element of the sublist. */
951    final int offset;
952    /** The size of the sublist. */
953    int size;
954    /** The backing data */
955    E[] data;
956
957    /**
958     * Construct the sublist.
959     *
960     * @param backing the list this comes from
961     * @param fromIndex the lower bound, inclusive
962     * @param toIndex the upper bound, exclusive
963     */
964    SubList(CopyOnWriteArrayList<E> backing, int fromIndex, int toIndex)
965    {
966      backingList = backing;
967      data = backing.data;
968      offset = fromIndex;
969      size = toIndex - fromIndex;
970    }
971
972    /**
973     * This method checks the two modCount fields to ensure that there has
974     * not been a concurrent modification, returning if all is okay.
975     *
976     * @throws ConcurrentModificationException if the backing list has been
977     *         modified externally to this sublist
978     */
979    // This can be inlined. Package visible, for use by iterator.
980    void checkMod()
981    {
982      if (data != backingList.data)
983        throw new ConcurrentModificationException();
984    }
985
986    /**
987     * This method checks that a value is between 0 and size (inclusive). If
988     * it is not, an exception is thrown.
989     *
990     * @param index the value to check
991     * @throws IndexOutOfBoundsException if index &lt; 0 || index &gt; size()
992     */
993    // This will get inlined, since it is private.
994    private void checkBoundsInclusive(int index)
995    {
996      if (index < 0 || index > size)
997        throw new IndexOutOfBoundsException("Index: " + index +
998                                            ", Size:" + size);
999    }
1000
1001    /**
1002     * This method checks that a value is between 0 (inclusive) and size
1003     * (exclusive). If it is not, an exception is thrown.
1004     *
1005     * @param index the value to check
1006     * @throws IndexOutOfBoundsException if index &lt; 0 || index &gt;= size()
1007     */
1008    // This will get inlined, since it is private.
1009    private void checkBoundsExclusive(int index)
1010    {
1011      if (index < 0 || index >= size)
1012        throw new IndexOutOfBoundsException("Index: " + index +
1013                                            ", Size:" + size);
1014    }
1015
1016    /**
1017     * Specified by AbstractList.subList to return the private field size.
1018     *
1019     * @return the sublist size
1020     * @throws ConcurrentModificationException if the backing list has been
1021     *         modified externally to this sublist
1022     */
1023    public int size()
1024    {
1025      synchronized (backingList)
1026        {
1027          checkMod();
1028          return size;
1029        }
1030    }
1031
1032    public void clear()
1033    {
1034      synchronized (backingList)
1035        {
1036          E[] snapshot = backingList.data;
1037          E[] newData = (E[]) new Object[snapshot.length - size];
1038
1039          int toIndex = size + offset;
1040
1041          System.arraycopy(snapshot, 0, newData, 0, offset);
1042          System.arraycopy(snapshot, toIndex, newData, offset,
1043                           snapshot.length - toIndex);
1044
1045          backingList.data = newData;
1046          this.data = backingList.data;
1047          this.size = 0;
1048        }
1049    }
1050
1051    /**
1052     * Specified by AbstractList.subList to delegate to the backing list.
1053     *
1054     * @param index the location to modify
1055     * @param o the new value
1056     * @return the old value
1057     * @throws ConcurrentModificationException if the backing list has been
1058     *         modified externally to this sublist
1059     * @throws UnsupportedOperationException if the backing list does not
1060     *         support the set operation
1061     * @throws IndexOutOfBoundsException if index &lt; 0 || index &gt;= size()
1062     * @throws ClassCastException if o cannot be added to the backing list due
1063     *         to its type
1064     * @throws IllegalArgumentException if o cannot be added to the backing list
1065     *         for some other reason
1066     */
1067    public E set(int index, E o)
1068    {
1069      synchronized (backingList)
1070        {
1071          checkMod();
1072          checkBoundsExclusive(index);
1073
1074          E el =  backingList.set(index + offset, o);
1075          this.data = backingList.data;
1076
1077          return el;
1078        }
1079    }
1080
1081    /**
1082     * Specified by AbstractList.subList to delegate to the backing list.
1083     *
1084     * @param index the location to get from
1085     * @return the object at that location
1086     * @throws ConcurrentModificationException if the backing list has been
1087     *         modified externally to this sublist
1088     * @throws IndexOutOfBoundsException if index &lt; 0 || index &gt;= size()
1089     */
1090    public E get(int index)
1091    {
1092      synchronized (backingList)
1093      {
1094        checkMod();
1095        checkBoundsExclusive(index);
1096
1097        return backingList.get(index + offset);
1098      }
1099    }
1100
1101    /**
1102     * Specified by AbstractList.subList to delegate to the backing list.
1103     *
1104     * @param index the index to insert at
1105     * @param o the object to add
1106     * @throws ConcurrentModificationException if the backing list has been
1107     *         modified externally to this sublist
1108     * @throws IndexOutOfBoundsException if index &lt; 0 || index &gt; size()
1109     * @throws UnsupportedOperationException if the backing list does not
1110     *         support the add operation.
1111     * @throws ClassCastException if o cannot be added to the backing list due
1112     *         to its type.
1113     * @throws IllegalArgumentException if o cannot be added to the backing
1114     *         list for some other reason.
1115     */
1116    public void add(int index, E o)
1117    {
1118      synchronized (backingList)
1119      {
1120        checkMod();
1121        checkBoundsInclusive(index);
1122
1123        backingList.add(index + offset, o);
1124
1125        this.data = backingList.data;
1126        size++;
1127      }
1128    }
1129
1130    /**
1131     * Specified by AbstractList.subList to delegate to the backing list.
1132     *
1133     * @param index the index to remove
1134     * @return the removed object
1135     * @throws ConcurrentModificationException if the backing list has been
1136     *         modified externally to this sublist
1137     * @throws IndexOutOfBoundsException if index &lt; 0 || index &gt;= size()
1138     * @throws UnsupportedOperationException if the backing list does not
1139     *         support the remove operation
1140     */
1141    public E remove(int index)
1142    {
1143      synchronized (backingList)
1144      {
1145        checkMod();
1146        checkBoundsExclusive(index);
1147        E o = backingList.remove(index + offset);
1148
1149        this.data = backingList.data;
1150        size--;
1151
1152        return o;
1153      }
1154    }
1155
1156    /**
1157     * Specified by AbstractList.subList to delegate to the backing list.
1158     *
1159     * @param index the location to insert at
1160     * @param c the collection to insert
1161     * @return true if this list was modified, in other words, c is non-empty
1162     * @throws ConcurrentModificationException if the backing list has been
1163     *         modified externally to this sublist
1164     * @throws IndexOutOfBoundsException if index &lt; 0 || index &gt; size()
1165     * @throws UnsupportedOperationException if this list does not support the
1166     *         addAll operation
1167     * @throws ClassCastException if some element of c cannot be added to this
1168     *         list due to its type
1169     * @throws IllegalArgumentException if some element of c cannot be added
1170     *         to this list for some other reason
1171     * @throws NullPointerException if the specified collection is null
1172     */
1173    public boolean addAll(int index, Collection<? extends E> c)
1174    {
1175      synchronized (backingList)
1176      {
1177        checkMod();
1178        checkBoundsInclusive(index);
1179        int csize = c.size();
1180        boolean result = backingList.addAll(offset + index, c);
1181
1182        this.data = backingList.data;
1183        size += csize;
1184
1185        return result;
1186      }
1187    }
1188
1189    /**
1190     * Specified by AbstractList.subList to return addAll(size, c).
1191     *
1192     * @param c the collection to insert
1193     * @return true if this list was modified, in other words, c is non-empty
1194     * @throws ConcurrentModificationException if the backing list has been
1195     *         modified externally to this sublist
1196     * @throws UnsupportedOperationException if this list does not support the
1197     *         addAll operation
1198     * @throws ClassCastException if some element of c cannot be added to this
1199     *         list due to its type
1200     * @throws IllegalArgumentException if some element of c cannot be added
1201     *         to this list for some other reason
1202     * @throws NullPointerException if the specified collection is null
1203     */
1204    public boolean addAll(Collection<? extends E> c)
1205    {
1206      synchronized (backingList)
1207      {
1208        return addAll(size, c);
1209      }
1210    }
1211
1212    /**
1213     * Specified by AbstractList.subList to return listIterator().
1214     *
1215     * @return an iterator over the sublist
1216     */
1217    public Iterator<E> iterator()
1218    {
1219      return listIterator();
1220    }
1221
1222    /**
1223     * Specified by AbstractList.subList to return a wrapper around the
1224     * backing list's iterator.
1225     *
1226     * @param index the start location of the iterator
1227     * @return a list iterator over the sublist
1228     * @throws ConcurrentModificationException if the backing list has been
1229     *         modified externally to this sublist
1230     * @throws IndexOutOfBoundsException if the value is out of range
1231     */
1232    public ListIterator<E> listIterator(final int index)
1233    {
1234      checkMod();
1235      checkBoundsInclusive(index);
1236
1237      return new ListIterator<E>()
1238      {
1239        private final ListIterator<E> i =
1240          backingList.listIterator(index + offset);
1241        private int position = index;
1242
1243        /**
1244         * Tests to see if there are any more objects to
1245         * return.
1246         *
1247         * @return True if the end of the list has not yet been
1248         *         reached.
1249         */
1250        public boolean hasNext()
1251        {
1252          return position < size;
1253        }
1254
1255        /**
1256         * Tests to see if there are objects prior to the
1257         * current position in the list.
1258         *
1259         * @return True if objects exist prior to the current
1260         *         position of the iterator.
1261         */
1262        public boolean hasPrevious()
1263        {
1264          return position > 0;
1265        }
1266
1267        /**
1268         * Retrieves the next object from the list.
1269         *
1270         * @return The next object.
1271         * @throws NoSuchElementException if there are no
1272         *         more objects to retrieve.
1273         * @throws ConcurrentModificationException if the
1274         *         list has been modified elsewhere.
1275         */
1276        public E next()
1277        {
1278          if (position == size)
1279            throw new NoSuchElementException();
1280          position++;
1281          return i.next();
1282        }
1283
1284        /**
1285         * Retrieves the previous object from the list.
1286         *
1287         * @return The next object.
1288         * @throws NoSuchElementException if there are no
1289         *         previous objects to retrieve.
1290         * @throws ConcurrentModificationException if the
1291         *         list has been modified elsewhere.
1292         */
1293        public E previous()
1294        {
1295          if (position == 0)
1296            throw new NoSuchElementException();
1297          position--;
1298          return i.previous();
1299        }
1300
1301        /**
1302         * Returns the index of the next element in the
1303         * list, which will be retrieved by <code>next()</code>
1304         *
1305         * @return The index of the next element.
1306         */
1307        public int nextIndex()
1308        {
1309          return i.nextIndex() - offset;
1310        }
1311
1312        /**
1313         * Returns the index of the previous element in the
1314         * list, which will be retrieved by <code>previous()</code>
1315         *
1316         * @return The index of the previous element.
1317         */
1318        public int previousIndex()
1319        {
1320          return i.previousIndex() - offset;
1321        }
1322
1323        /**
1324         * Removes the last object retrieved by <code>next()</code>
1325         * from the list, if the list supports object removal.
1326         *
1327         * @throws IllegalStateException if the iterator is positioned
1328         *         before the start of the list or the last object has already
1329         *         been removed.
1330         * @throws UnsupportedOperationException if the list does
1331         *         not support removing elements.
1332         */
1333        public void remove()
1334        {
1335          throw new UnsupportedOperationException("Modification not supported " +
1336              "on CopyOnWriteArrayList iterators");
1337        }
1338
1339        /**
1340         * Replaces the last object retrieved by <code>next()</code>
1341         * or <code>previous</code> with o, if the list supports object
1342         * replacement and an add or remove operation has not already
1343         * been performed.
1344         *
1345         * @throws IllegalStateException if the iterator is positioned
1346         *         before the start of the list or the last object has already
1347         *         been removed.
1348         * @throws UnsupportedOperationException if the list doesn't support
1349         *         the addition or removal of elements.
1350         * @throws ClassCastException if the type of o is not a valid type
1351         *         for this list.
1352         * @throws IllegalArgumentException if something else related to o
1353         *         prevents its addition.
1354         * @throws ConcurrentModificationException if the list
1355         *         has been modified elsewhere.
1356         */
1357        public void set(E o)
1358        {
1359          throw new UnsupportedOperationException("Modification not supported " +
1360              "on CopyOnWriteArrayList iterators");
1361        }
1362
1363        /**
1364         * Adds the supplied object before the element that would be returned
1365         * by a call to <code>next()</code>, if the list supports addition.
1366         *
1367         * @param o The object to add to the list.
1368         * @throws UnsupportedOperationException if the list doesn't support
1369         *         the addition of new elements.
1370         * @throws ClassCastException if the type of o is not a valid type
1371         *         for this list.
1372         * @throws IllegalArgumentException if something else related to o
1373         *         prevents its addition.
1374         * @throws ConcurrentModificationException if the list
1375         *         has been modified elsewhere.
1376         */
1377        public void add(E o)
1378        {
1379          throw new UnsupportedOperationException("Modification not supported " +
1380              "on CopyOnWriteArrayList iterators");
1381        }
1382      };
1383    }
1384  } // class SubList
1385
1386  /**
1387   * This class is a RandomAccess version of SubList, as required by
1388   * {@link AbstractList#subList(int, int)}.
1389   *
1390   * @author Eric Blake (ebb9@email.byu.edu)
1391   */
1392  private static final class RandomAccessSubList<E> extends SubList<E>
1393    implements RandomAccess
1394  {
1395    /**
1396     * Construct the sublist.
1397     *
1398     * @param backing the list this comes from
1399     * @param fromIndex the lower bound, inclusive
1400     * @param toIndex the upper bound, exclusive
1401     */
1402    RandomAccessSubList(CopyOnWriteArrayList<E> backing, int fromIndex, int toIndex)
1403    {
1404      super(backing, fromIndex, toIndex);
1405    }
1406  } // class RandomAccessSubList
1407
1408  /**
1409   * Serializes this object to the given stream.
1410   *
1411   * @param s
1412   *          the stream to write to
1413   * @throws IOException
1414   *           if the underlying stream fails
1415   * @serialData the size field (int), the length of the backing array (int),
1416   *             followed by its elements (Objects) in proper order.
1417   */
1418  private void writeObject(ObjectOutputStream s) throws IOException
1419  {
1420    // The 'size' field.
1421    s.defaultWriteObject();
1422    // We serialize unused list entries to preserve capacity.
1423    int len = data.length;
1424    s.writeInt(len);
1425    // it would be more efficient to just write "size" items,
1426    // this need readObject read "size" items too.
1427    for (int i = 0; i < data.length; i++)
1428      s.writeObject(data[i]);
1429  }
1430
1431  /**
1432   * Deserializes this object from the given stream.
1433   *
1434   * @param s
1435   *          the stream to read from
1436   * @throws ClassNotFoundException
1437   *           if the underlying stream fails
1438   * @throws IOException
1439   *           if the underlying stream fails
1440   * @serialData the size field (int), the length of the backing array (int),
1441   *             followed by its elements (Objects) in proper order.
1442   */
1443  private void readObject(ObjectInputStream s) throws IOException,
1444      ClassNotFoundException
1445  {
1446    // the `size' field.
1447    s.defaultReadObject();
1448    int capacity = s.readInt();
1449    data = (E[]) new Object[capacity];
1450    for (int i = 0; i < capacity; i++)
1451      data[i] = (E) s.readObject();
1452  }
1453
1454  static final boolean equals(Object o1, Object o2)
1455  {
1456    return o1 == null ? o2 == null : o1.equals(o2);
1457  }
1458
1459  Object[] getArray()
1460  {
1461    return data;
1462  }
1463}