1 /*
   2  * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.util;
  27 
  28 /**
  29  * Resizable-array implementation of the <tt>List</tt> interface.  Implements
  30  * all optional list operations, and permits all elements, including
  31  * <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface,
  32  * this class provides methods to manipulate the size of the array that is
  33  * used internally to store the list.  (This class is roughly equivalent to
  34  * <tt>Vector</tt>, except that it is unsynchronized.)
  35  *
  36  * <p>The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,
  37  * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant
  38  * time.  The <tt>add</tt> operation runs in <i>amortized constant time</i>,
  39  * that is, adding n elements requires O(n) time.  All of the other operations
  40  * run in linear time (roughly speaking).  The constant factor is low compared
  41  * to that for the <tt>LinkedList</tt> implementation.
  42  *
  43  * <p>Each <tt>ArrayList</tt> instance has a <i>capacity</i>.  The capacity is
  44  * the size of the array used to store the elements in the list.  It is always
  45  * at least as large as the list size.  As elements are added to an ArrayList,
  46  * its capacity grows automatically.  The details of the growth policy are not
  47  * specified beyond the fact that adding an element has constant amortized
  48  * time cost.
  49  *
  50  * <p>An application can increase the capacity of an <tt>ArrayList</tt> instance
  51  * before adding a large number of elements using the <tt>ensureCapacity</tt>
  52  * operation.  This may reduce the amount of incremental reallocation.
  53  *
  54  * <p><strong>Note that this implementation is not synchronized.</strong>
  55  * If multiple threads access an <tt>ArrayList</tt> instance concurrently,
  56  * and at least one of the threads modifies the list structurally, it
  57  * <i>must</i> be synchronized externally.  (A structural modification is
  58  * any operation that adds or deletes one or more elements, or explicitly
  59  * resizes the backing array; merely setting the value of an element is not
  60  * a structural modification.)  This is typically accomplished by
  61  * synchronizing on some object that naturally encapsulates the list.
  62  *
  63  * If no such object exists, the list should be "wrapped" using the
  64  * {@link Collections#synchronizedList Collections.synchronizedList}
  65  * method.  This is best done at creation time, to prevent accidental
  66  * unsynchronized access to the list:<pre>
  67  *   List list = Collections.synchronizedList(new ArrayList(...));</pre>
  68  *
  69  * <p><a name="fail-fast"/>
  70  * The iterators returned by this class's {@link #iterator() iterator} and
  71  * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:
  72  * if the list is structurally modified at any time after the iterator is
  73  * created, in any way except through the iterator's own
  74  * {@link ListIterator#remove() remove} or
  75  * {@link ListIterator#add(Object) add} methods, the iterator will throw a
  76  * {@link ConcurrentModificationException}.  Thus, in the face of
  77  * concurrent modification, the iterator fails quickly and cleanly, rather
  78  * than risking arbitrary, non-deterministic behavior at an undetermined
  79  * time in the future.
  80  *
  81  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
  82  * as it is, generally speaking, impossible to make any hard guarantees in the
  83  * presence of unsynchronized concurrent modification.  Fail-fast iterators
  84  * throw {@code ConcurrentModificationException} on a best-effort basis.
  85  * Therefore, it would be wrong to write a program that depended on this
  86  * exception for its correctness:  <i>the fail-fast behavior of iterators
  87  * should be used only to detect bugs.</i>
  88  *
  89  * <p>This class is a member of the
  90  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  91  * Java Collections Framework</a>.
  92  *
  93  * @author  Josh Bloch
  94  * @author  Neal Gafter
  95  * @see     Collection
  96  * @see     List
  97  * @see     LinkedList
  98  * @see     Vector
  99  * @since   1.2
 100  */
 101 
 102 public class ArrayList<E> extends AbstractList<E>
 103         implements List<E>, RandomAccess, Cloneable, java.io.Serializable
 104 {
 105     private static final long serialVersionUID = 8683452581122892189L;
 106 
 107     /**
 108      * The array buffer into which the elements of the ArrayList are stored.
 109      * The capacity of the ArrayList is the length of this array buffer.
 110      */
 111     private transient Object[] elementData;
 112 
 113     /**
 114      * The size of the ArrayList (the number of elements it contains).
 115      *
 116      * @serial
 117      */
 118     private int size;
 119 
 120     /**
 121      * Constructs an empty list with the specified initial capacity.
 122      *
 123      * @param  initialCapacity  the initial capacity of the list
 124      * @throws IllegalArgumentException if the specified initial capacity
 125      *         is negative
 126      */
 127     public ArrayList(int initialCapacity) {
 128         super();
 129         if (initialCapacity < 0)
 130             throw new IllegalArgumentException("Illegal Capacity: "+
 131                                                initialCapacity);
 132         this.elementData = new Object[initialCapacity];
 133     }
 134 
 135     /**
 136      * Constructs an empty list with an initial capacity of ten.
 137      */
 138     public ArrayList() {
 139         this(10);
 140     }
 141 
 142     /**
 143      * Constructs a list containing the elements of the specified
 144      * collection, in the order they are returned by the collection's
 145      * iterator.
 146      *
 147      * @param c the collection whose elements are to be placed into this list
 148      * @throws NullPointerException if the specified collection is null
 149      */
 150     public ArrayList(Collection<? extends E> c) {
 151         elementData = c.toArray();
 152         size = elementData.length;
 153         // c.toArray might (incorrectly) not return Object[] (see 6260652)
 154         if (elementData.getClass() != Object[].class)
 155             elementData = Arrays.copyOf(elementData, size, Object[].class);
 156     }
 157 
 158     /**
 159      * Trims the capacity of this <tt>ArrayList</tt> instance to be the
 160      * list's current size.  An application can use this operation to minimize
 161      * the storage of an <tt>ArrayList</tt> instance.
 162      */
 163     public void trimToSize() {
 164         modCount++;
 165         int oldCapacity = elementData.length;
 166         if (size < oldCapacity) {
 167             elementData = Arrays.copyOf(elementData, size);
 168         }
 169     }
 170 
 171     /**
 172      * Increases the capacity of this <tt>ArrayList</tt> instance, if
 173      * necessary, to ensure that it can hold at least the number of elements
 174      * specified by the minimum capacity argument.
 175      *
 176      * @param   minCapacity   the desired minimum capacity
 177      */
 178     public void ensureCapacity(int minCapacity) {
 179         if (minCapacity > 0)
 180             ensureCapacityInternal(minCapacity);
 181     }
 182 
 183     private void ensureCapacityInternal(int minCapacity) {
 184         modCount++;
 185         // overflow-conscious code
 186         if (minCapacity - elementData.length > 0)
 187             grow(minCapacity);
 188     }
 189 
 190     /**
 191      * The maximum size of array to allocate.
 192      * Some VMs reserve some header words in an array.
 193      * Attempts to allocate larger arrays may result in
 194      * OutOfMemoryError: Requested array size exceeds VM limit
 195      */
 196     private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
 197 
 198     /**
 199      * Increases the capacity to ensure that it can hold at least the
 200      * number of elements specified by the minimum capacity argument.
 201      *
 202      * @param minCapacity the desired minimum capacity
 203      */
 204     private void grow(int minCapacity) {
 205         // overflow-conscious code
 206         int oldCapacity = elementData.length;
 207         int newCapacity = oldCapacity + (oldCapacity >> 1);
 208         if (newCapacity - minCapacity < 0)
 209             newCapacity = minCapacity;
 210         if (newCapacity - MAX_ARRAY_SIZE > 0)
 211             newCapacity = hugeCapacity(minCapacity);
 212         // minCapacity is usually close to size, so this is a win:
 213         elementData = Arrays.copyOf(elementData, newCapacity);
 214     }
 215 
 216     private static int hugeCapacity(int minCapacity) {
 217         if (minCapacity < 0) // overflow
 218             throw new OutOfMemoryError();
 219         return (minCapacity > MAX_ARRAY_SIZE) ?
 220             Integer.MAX_VALUE :
 221             MAX_ARRAY_SIZE;
 222     }
 223 
 224     /**
 225      * Returns the number of elements in this list.
 226      *
 227      * @return the number of elements in this list
 228      */
 229     public int size() {
 230         return size;
 231     }
 232 
 233     /**
 234      * Returns <tt>true</tt> if this list contains no elements.
 235      *
 236      * @return <tt>true</tt> if this list contains no elements
 237      */
 238     public boolean isEmpty() {
 239         return size == 0;
 240     }
 241 
 242     /**
 243      * Returns <tt>true</tt> if this list contains the specified element.
 244      * More formally, returns <tt>true</tt> if and only if this list contains
 245      * at least one element <tt>e</tt> such that
 246      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
 247      *
 248      * @param o element whose presence in this list is to be tested
 249      * @return <tt>true</tt> if this list contains the specified element
 250      */
 251     public boolean contains(Object o) {
 252         return indexOf(o) >= 0;
 253     }
 254 
 255     /**
 256      * Returns the index of the first occurrence of the specified element
 257      * in this list, or -1 if this list does not contain the element.
 258      * More formally, returns the lowest index <tt>i</tt> such that
 259      * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
 260      * or -1 if there is no such index.
 261      */
 262     public int indexOf(Object o) {
 263         if (o == null) {
 264             for (int i = 0; i < size; i++)
 265                 if (elementData[i]==null)
 266                     return i;
 267         } else {
 268             for (int i = 0; i < size; i++)
 269                 if (o.equals(elementData[i]))
 270                     return i;
 271         }
 272         return -1;
 273     }
 274 
 275     /**
 276      * Returns the index of the last occurrence of the specified element
 277      * in this list, or -1 if this list does not contain the element.
 278      * More formally, returns the highest index <tt>i</tt> such that
 279      * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
 280      * or -1 if there is no such index.
 281      */
 282     public int lastIndexOf(Object o) {
 283         if (o == null) {
 284             for (int i = size-1; i >= 0; i--)
 285                 if (elementData[i]==null)
 286                     return i;
 287         } else {
 288             for (int i = size-1; i >= 0; i--)
 289                 if (o.equals(elementData[i]))
 290                     return i;
 291         }
 292         return -1;
 293     }
 294 
 295     /**
 296      * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The
 297      * elements themselves are not copied.)
 298      *
 299      * @return a clone of this <tt>ArrayList</tt> instance
 300      */
 301     public Object clone() {
 302         try {
 303             @SuppressWarnings("unchecked")
 304                 ArrayList<E> v = (ArrayList<E>) super.clone();
 305             v.elementData = Arrays.copyOf(elementData, size);
 306             v.modCount = 0;
 307             return v;
 308         } catch (CloneNotSupportedException e) {
 309             // this shouldn't happen, since we are Cloneable
 310             throw new InternalError(e);
 311         }
 312     }
 313 
 314     /**
 315      * Returns an array containing all of the elements in this list
 316      * in proper sequence (from first to last element).
 317      *
 318      * <p>The returned array will be "safe" in that no references to it are
 319      * maintained by this list.  (In other words, this method must allocate
 320      * a new array).  The caller is thus free to modify the returned array.
 321      *
 322      * <p>This method acts as bridge between array-based and collection-based
 323      * APIs.
 324      *
 325      * @return an array containing all of the elements in this list in
 326      *         proper sequence
 327      */
 328     public Object[] toArray() {
 329         return Arrays.copyOf(elementData, size);
 330     }
 331 
 332     /**
 333      * Returns an array containing all of the elements in this list in proper
 334      * sequence (from first to last element); the runtime type of the returned
 335      * array is that of the specified array.  If the list fits in the
 336      * specified array, it is returned therein.  Otherwise, a new array is
 337      * allocated with the runtime type of the specified array and the size of
 338      * this list.
 339      *
 340      * <p>If the list fits in the specified array with room to spare
 341      * (i.e., the array has more elements than the list), the element in
 342      * the array immediately following the end of the collection is set to
 343      * <tt>null</tt>.  (This is useful in determining the length of the
 344      * list <i>only</i> if the caller knows that the list does not contain
 345      * any null elements.)
 346      *
 347      * @param a the array into which the elements of the list are to
 348      *          be stored, if it is big enough; otherwise, a new array of the
 349      *          same runtime type is allocated for this purpose.
 350      * @return an array containing the elements of the list
 351      * @throws ArrayStoreException if the runtime type of the specified array
 352      *         is not a supertype of the runtime type of every element in
 353      *         this list
 354      * @throws NullPointerException if the specified array is null
 355      */
 356     @SuppressWarnings("unchecked")
 357     public <T> T[] toArray(T[] a) {
 358         if (a.length < size)
 359             // Make a new array of a's runtime type, but my contents:
 360             return (T[]) Arrays.copyOf(elementData, size, a.getClass());
 361         System.arraycopy(elementData, 0, a, 0, size);
 362         if (a.length > size)
 363             a[size] = null;
 364         return a;
 365     }
 366 
 367     // Positional Access Operations
 368 
 369     @SuppressWarnings("unchecked")
 370     E elementData(int index) {
 371         return (E) elementData[index];
 372     }
 373 
 374     /**
 375      * Returns the element at the specified position in this list.
 376      *
 377      * @param  index index of the element to return
 378      * @return the element at the specified position in this list
 379      * @throws IndexOutOfBoundsException {@inheritDoc}
 380      */
 381     public E get(int index) {
 382         rangeCheck(index);
 383 
 384         return elementData(index);
 385     }
 386 
 387     /**
 388      * Replaces the element at the specified position in this list with
 389      * the specified element.
 390      *
 391      * @param index index of the element to replace
 392      * @param element element to be stored at the specified position
 393      * @return the element previously at the specified position
 394      * @throws IndexOutOfBoundsException {@inheritDoc}
 395      */
 396     public E set(int index, E element) {
 397         rangeCheck(index);
 398 
 399         E oldValue = elementData(index);
 400         elementData[index] = element;
 401         return oldValue;
 402     }
 403 
 404     /**
 405      * Appends the specified element to the end of this list.
 406      *
 407      * @param e element to be appended to this list
 408      * @return <tt>true</tt> (as specified by {@link Collection#add})
 409      */
 410     public boolean add(E e) {
 411         ensureCapacityInternal(size + 1);  // Increments modCount!!
 412         elementData[size++] = e;
 413         return true;
 414     }
 415 
 416     /**
 417      * Inserts the specified element at the specified position in this
 418      * list. Shifts the element currently at that position (if any) and
 419      * any subsequent elements to the right (adds one to their indices).
 420      *
 421      * @param index index at which the specified element is to be inserted
 422      * @param element element to be inserted
 423      * @throws IndexOutOfBoundsException {@inheritDoc}
 424      */
 425     public void add(int index, E element) {
 426         rangeCheckForAdd(index);
 427 
 428         ensureCapacityInternal(size + 1);  // Increments modCount!!
 429         System.arraycopy(elementData, index, elementData, index + 1,
 430                          size - index);
 431         elementData[index] = element;
 432         size++;
 433     }
 434 
 435     /**
 436      * Removes the element at the specified position in this list.
 437      * Shifts any subsequent elements to the left (subtracts one from their
 438      * indices).
 439      *
 440      * @param index the index of the element to be removed
 441      * @return the element that was removed from the list
 442      * @throws IndexOutOfBoundsException {@inheritDoc}
 443      */
 444     public E remove(int index) {
 445         rangeCheck(index);
 446 
 447         modCount++;
 448         E oldValue = elementData(index);
 449 
 450         int numMoved = size - index - 1;
 451         if (numMoved > 0)
 452             System.arraycopy(elementData, index+1, elementData, index,
 453                              numMoved);
 454         elementData[--size] = null; // Let gc do its work
 455 
 456         return oldValue;
 457     }
 458 
 459     /**
 460      * Removes the first occurrence of the specified element from this list,
 461      * if it is present.  If the list does not contain the element, it is
 462      * unchanged.  More formally, removes the element with the lowest index
 463      * <tt>i</tt> such that
 464      * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
 465      * (if such an element exists).  Returns <tt>true</tt> if this list
 466      * contained the specified element (or equivalently, if this list
 467      * changed as a result of the call).
 468      *
 469      * @param o element to be removed from this list, if present
 470      * @return <tt>true</tt> if this list contained the specified element
 471      */
 472     public boolean remove(Object o) {
 473         if (o == null) {
 474             for (int index = 0; index < size; index++)
 475                 if (elementData[index] == null) {
 476                     fastRemove(index);
 477                     return true;
 478                 }
 479         } else {
 480             for (int index = 0; index < size; index++)
 481                 if (o.equals(elementData[index])) {
 482                     fastRemove(index);
 483                     return true;
 484                 }
 485         }
 486         return false;
 487     }
 488 
 489     /*
 490      * Private remove method that skips bounds checking and does not
 491      * return the value removed.
 492      */
 493     private void fastRemove(int index) {
 494         modCount++;
 495         int numMoved = size - index - 1;
 496         if (numMoved > 0)
 497             System.arraycopy(elementData, index+1, elementData, index,
 498                              numMoved);
 499         elementData[--size] = null; // Let gc do its work
 500     }
 501 
 502     /**
 503      * Removes all of the elements from this list.  The list will
 504      * be empty after this call returns.
 505      */
 506     public void clear() {
 507         modCount++;
 508 
 509         // Let gc do its work
 510         for (int i = 0; i < size; i++)
 511             elementData[i] = null;
 512 
 513         size = 0;
 514     }
 515 
 516     /**
 517      * Appends all of the elements in the specified collection to the end of
 518      * this list, in the order that they are returned by the
 519      * specified collection's Iterator.  The behavior of this operation is
 520      * undefined if the specified collection is modified while the operation
 521      * is in progress.  (This implies that the behavior of this call is
 522      * undefined if the specified collection is this list, and this
 523      * list is nonempty.)
 524      *
 525      * @param c collection containing elements to be added to this list
 526      * @return <tt>true</tt> if this list changed as a result of the call
 527      * @throws NullPointerException if the specified collection is null
 528      */
 529     public boolean addAll(Collection<? extends E> c) {
 530         Object[] a = c.toArray();
 531         int numNew = a.length;
 532         ensureCapacityInternal(size + numNew);  // Increments modCount
 533         System.arraycopy(a, 0, elementData, size, numNew);
 534         size += numNew;
 535         return numNew != 0;
 536     }
 537 
 538     /**
 539      * Inserts all of the elements in the specified collection into this
 540      * list, starting at the specified position.  Shifts the element
 541      * currently at that position (if any) and any subsequent elements to
 542      * the right (increases their indices).  The new elements will appear
 543      * in the list in the order that they are returned by the
 544      * specified collection's iterator.
 545      *
 546      * @param index index at which to insert the first element from the
 547      *              specified collection
 548      * @param c collection containing elements to be added to this list
 549      * @return <tt>true</tt> if this list changed as a result of the call
 550      * @throws IndexOutOfBoundsException {@inheritDoc}
 551      * @throws NullPointerException if the specified collection is null
 552      */
 553     public boolean addAll(int index, Collection<? extends E> c) {
 554         rangeCheckForAdd(index);
 555 
 556         Object[] a = c.toArray();
 557         int numNew = a.length;
 558         ensureCapacityInternal(size + numNew);  // Increments modCount
 559 
 560         int numMoved = size - index;
 561         if (numMoved > 0)
 562             System.arraycopy(elementData, index, elementData, index + numNew,
 563                              numMoved);
 564 
 565         System.arraycopy(a, 0, elementData, index, numNew);
 566         size += numNew;
 567         return numNew != 0;
 568     }
 569 
 570     /**
 571      * Removes from this list all of the elements whose index is between
 572      * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
 573      * Shifts any succeeding elements to the left (reduces their index).
 574      * This call shortens the list by {@code (toIndex - fromIndex)} elements.
 575      * (If {@code toIndex==fromIndex}, this operation has no effect.)
 576      *
 577      * @throws IndexOutOfBoundsException if {@code fromIndex} or
 578      *         {@code toIndex} is out of range
 579      *         ({@code fromIndex < 0 ||
 580      *          fromIndex >= size() ||
 581      *          toIndex > size() ||
 582      *          toIndex < fromIndex})
 583      */
 584     protected void removeRange(int fromIndex, int toIndex) {
 585         modCount++;
 586         int numMoved = size - toIndex;
 587         System.arraycopy(elementData, toIndex, elementData, fromIndex,
 588                          numMoved);
 589 
 590         // Let gc do its work
 591         int newSize = size - (toIndex-fromIndex);
 592         while (size != newSize)
 593             elementData[--size] = null;
 594     }
 595 
 596     /**
 597      * Checks if the given index is in range.  If not, throws an appropriate
 598      * runtime exception.  This method does *not* check if the index is
 599      * negative: It is always used immediately prior to an array access,
 600      * which throws an ArrayIndexOutOfBoundsException if index is negative.
 601      */
 602     private void rangeCheck(int index) {
 603         if (index >= size)
 604             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 605     }
 606 
 607     /**
 608      * A version of rangeCheck used by add and addAll.
 609      */
 610     private void rangeCheckForAdd(int index) {
 611         if (index > size || index < 0)
 612             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 613     }
 614 
 615     /**
 616      * Constructs an IndexOutOfBoundsException detail message.
 617      * Of the many possible refactorings of the error handling code,
 618      * this "outlining" performs best with both server and client VMs.
 619      */
 620     private String outOfBoundsMsg(int index) {
 621         return "Index: "+index+", Size: "+size;
 622     }
 623 
 624     /**
 625      * Removes from this list all of its elements that are contained in the
 626      * specified collection.
 627      *
 628      * @param c collection containing elements to be removed from this list
 629      * @return {@code true} if this list changed as a result of the call
 630      * @throws ClassCastException if the class of an element of this list
 631      *         is incompatible with the specified collection
 632      * (<a href="Collection.html#optional-restrictions">optional</a>)
 633      * @throws NullPointerException if this list contains a null element and the
 634      *         specified collection does not permit null elements
 635      * (<a href="Collection.html#optional-restrictions">optional</a>),
 636      *         or if the specified collection is null
 637      * @see Collection#contains(Object)
 638      */
 639     public boolean removeAll(Collection<?> c) {
 640         return batchRemove(c, false);
 641     }
 642 
 643     /**
 644      * Retains only the elements in this list that are contained in the
 645      * specified collection.  In other words, removes from this list all
 646      * of its elements that are not contained in the specified collection.
 647      *
 648      * @param c collection containing elements to be retained in this list
 649      * @return {@code true} if this list changed as a result of the call
 650      * @throws ClassCastException if the class of an element of this list
 651      *         is incompatible with the specified collection
 652      * (<a href="Collection.html#optional-restrictions">optional</a>)
 653      * @throws NullPointerException if this list contains a null element and the
 654      *         specified collection does not permit null elements
 655      * (<a href="Collection.html#optional-restrictions">optional</a>),
 656      *         or if the specified collection is null
 657      * @see Collection#contains(Object)
 658      */
 659     public boolean retainAll(Collection<?> c) {
 660         return batchRemove(c, true);
 661     }
 662 
 663     private boolean batchRemove(Collection<?> c, boolean complement) {
 664         final Object[] elementData = this.elementData;
 665         int r = 0, w = 0;
 666         boolean modified = false;
 667         try {
 668             for (; r < size; r++)
 669                 if (c.contains(elementData[r]) == complement)
 670                     elementData[w++] = elementData[r];
 671         } finally {
 672             // Preserve behavioral compatibility with AbstractCollection,
 673             // even if c.contains() throws.
 674             if (r != size) {
 675                 System.arraycopy(elementData, r,
 676                                  elementData, w,
 677                                  size - r);
 678                 w += size - r;
 679             }
 680             if (w != size) {
 681                 for (int i = w; i < size; i++)
 682                     elementData[i] = null;
 683                 modCount += size - w;
 684                 size = w;
 685                 modified = true;
 686             }
 687         }
 688         return modified;
 689     }
 690 
 691     /**
 692      * Save the state of the <tt>ArrayList</tt> instance to a stream (that
 693      * is, serialize it).
 694      *
 695      * @serialData The length of the array backing the <tt>ArrayList</tt>
 696      *             instance is emitted (int), followed by all of its elements
 697      *             (each an <tt>Object</tt>) in the proper order.
 698      */
 699     private void writeObject(java.io.ObjectOutputStream s)
 700         throws java.io.IOException{
 701         // Write out element count, and any hidden stuff
 702         int expectedModCount = modCount;
 703         s.defaultWriteObject();
 704 
 705         // Write out array length
 706         s.writeInt(elementData.length);
 707 
 708         // Write out all elements in the proper order.
 709         for (int i=0; i<size; i++)
 710             s.writeObject(elementData[i]);
 711 
 712         if (modCount != expectedModCount) {
 713             throw new ConcurrentModificationException();
 714         }
 715 
 716     }
 717 
 718     /**
 719      * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
 720      * deserialize it).
 721      */
 722     private void readObject(java.io.ObjectInputStream s)
 723         throws java.io.IOException, ClassNotFoundException {
 724         // Read in size, and any hidden stuff
 725         s.defaultReadObject();
 726 
 727         // Read in array length and allocate array
 728         int arrayLength = s.readInt();
 729         Object[] a = elementData = new Object[arrayLength];
 730 
 731         // Read in all elements in the proper order.
 732         for (int i=0; i<size; i++)
 733             a[i] = s.readObject();
 734     }
 735 
 736     /**
 737      * Returns a list iterator over the elements in this list (in proper
 738      * sequence), starting at the specified position in the list.
 739      * The specified index indicates the first element that would be
 740      * returned by an initial call to {@link ListIterator#next next}.
 741      * An initial call to {@link ListIterator#previous previous} would
 742      * return the element with the specified index minus one.
 743      *
 744      * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
 745      *
 746      * @throws IndexOutOfBoundsException {@inheritDoc}
 747      */
 748     public ListIterator<E> listIterator(int index) {
 749         if (index < 0 || index > size)
 750             throw new IndexOutOfBoundsException("Index: "+index);
 751         return new ListItr(index);
 752     }
 753 
 754     /**
 755      * Returns a list iterator over the elements in this list (in proper
 756      * sequence).
 757      *
 758      * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
 759      *
 760      * @see #listIterator(int)
 761      */
 762     public ListIterator<E> listIterator() {
 763         return new ListItr(0);
 764     }
 765 
 766     /**
 767      * Returns an iterator over the elements in this list in proper sequence.
 768      *
 769      * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
 770      *
 771      * @return an iterator over the elements in this list in proper sequence
 772      */
 773     public Iterator<E> iterator() {
 774         return new Itr();
 775     }
 776 
 777     /**
 778      * An optimized version of AbstractList.Itr
 779      */
 780     private class Itr implements Iterator<E> {
 781         int cursor;       // index of next element to return
 782         int lastRet = -1; // index of last element returned; -1 if no such
 783         int expectedModCount = modCount;
 784 
 785         public boolean hasNext() {
 786             return cursor != size;
 787         }
 788 
 789         @SuppressWarnings("unchecked")
 790         public E next() {
 791             checkForComodification();
 792             int i = cursor;
 793             if (i >= size)
 794                 throw new NoSuchElementException();
 795             Object[] elementData = ArrayList.this.elementData;
 796             if (i >= elementData.length)
 797                 throw new ConcurrentModificationException();
 798             cursor = i + 1;
 799             return (E) elementData[lastRet = i];
 800         }
 801 
 802         public void remove() {
 803             if (lastRet < 0)
 804                 throw new IllegalStateException();
 805             checkForComodification();
 806 
 807             try {
 808                 ArrayList.this.remove(lastRet);
 809                 cursor = lastRet;
 810                 lastRet = -1;
 811                 expectedModCount = modCount;
 812             } catch (IndexOutOfBoundsException ex) {
 813                 throw new ConcurrentModificationException();
 814             }
 815         }
 816 
 817         final void checkForComodification() {
 818             if (modCount != expectedModCount)
 819                 throw new ConcurrentModificationException();
 820         }
 821     }
 822 
 823     /**
 824      * An optimized version of AbstractList.ListItr
 825      */
 826     private class ListItr extends Itr implements ListIterator<E> {
 827         ListItr(int index) {
 828             super();
 829             cursor = index;
 830         }
 831 
 832         public boolean hasPrevious() {
 833             return cursor != 0;
 834         }
 835 
 836         public int nextIndex() {
 837             return cursor;
 838         }
 839 
 840         public int previousIndex() {
 841             return cursor - 1;
 842         }
 843 
 844         @SuppressWarnings("unchecked")
 845         public E previous() {
 846             checkForComodification();
 847             int i = cursor - 1;
 848             if (i < 0)
 849                 throw new NoSuchElementException();
 850             Object[] elementData = ArrayList.this.elementData;
 851             if (i >= elementData.length)
 852                 throw new ConcurrentModificationException();
 853             cursor = i;
 854             return (E) elementData[lastRet = i];
 855         }
 856 
 857         public void set(E e) {
 858             if (lastRet < 0)
 859                 throw new IllegalStateException();
 860             checkForComodification();
 861 
 862             try {
 863                 ArrayList.this.set(lastRet, e);
 864             } catch (IndexOutOfBoundsException ex) {
 865                 throw new ConcurrentModificationException();
 866             }
 867         }
 868 
 869         public void add(E e) {
 870             checkForComodification();
 871 
 872             try {
 873                 int i = cursor;
 874                 ArrayList.this.add(i, e);
 875                 cursor = i + 1;
 876                 lastRet = -1;
 877                 expectedModCount = modCount;
 878             } catch (IndexOutOfBoundsException ex) {
 879                 throw new ConcurrentModificationException();
 880             }
 881         }
 882     }
 883 
 884     /**
 885      * Returns a view of the portion of this list between the specified
 886      * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  (If
 887      * {@code fromIndex} and {@code toIndex} are equal, the returned list is
 888      * empty.)  The returned list is backed by this list, so non-structural
 889      * changes in the returned list are reflected in this list, and vice-versa.
 890      * The returned list supports all of the optional list operations.
 891      *
 892      * <p>This method eliminates the need for explicit range operations (of
 893      * the sort that commonly exist for arrays).  Any operation that expects
 894      * a list can be used as a range operation by passing a subList view
 895      * instead of a whole list.  For example, the following idiom
 896      * removes a range of elements from a list:
 897      * <pre>
 898      *      list.subList(from, to).clear();
 899      * </pre>
 900      * Similar idioms may be constructed for {@link #indexOf(Object)} and
 901      * {@link #lastIndexOf(Object)}, and all of the algorithms in the
 902      * {@link Collections} class can be applied to a subList.
 903      *
 904      * <p>The semantics of the list returned by this method become undefined if
 905      * the backing list (i.e., this list) is <i>structurally modified</i> in
 906      * any way other than via the returned list.  (Structural modifications are
 907      * those that change the size of this list, or otherwise perturb it in such
 908      * a fashion that iterations in progress may yield incorrect results.)
 909      *
 910      * @throws IndexOutOfBoundsException {@inheritDoc}
 911      * @throws IllegalArgumentException {@inheritDoc}
 912      */
 913     public List<E> subList(int fromIndex, int toIndex) {
 914         subListRangeCheck(fromIndex, toIndex, size);
 915         return new SubList(this, 0, fromIndex, toIndex);
 916     }
 917 
 918     static void subListRangeCheck(int fromIndex, int toIndex, int size) {
 919         if (fromIndex < 0)
 920             throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
 921         if (toIndex > size)
 922             throw new IndexOutOfBoundsException("toIndex = " + toIndex);
 923         if (fromIndex > toIndex)
 924             throw new IllegalArgumentException("fromIndex(" + fromIndex +
 925                                                ") > toIndex(" + toIndex + ")");
 926     }
 927 
 928     private class SubList extends AbstractList<E> implements RandomAccess {
 929         private final AbstractList<E> parent;
 930         private final int parentOffset;
 931         private final int offset;
 932         int size;
 933 
 934         SubList(AbstractList<E> parent,
 935                 int offset, int fromIndex, int toIndex) {
 936             this.parent = parent;
 937             this.parentOffset = fromIndex;
 938             this.offset = offset + fromIndex;
 939             this.size = toIndex - fromIndex;
 940             this.modCount = ArrayList.this.modCount;
 941         }
 942 
 943         public E set(int index, E e) {
 944             rangeCheck(index);
 945             checkForComodification();
 946             E oldValue = ArrayList.this.elementData(offset + index);
 947             ArrayList.this.elementData[offset + index] = e;
 948             return oldValue;
 949         }
 950 
 951         public E get(int index) {
 952             rangeCheck(index);
 953             checkForComodification();
 954             return ArrayList.this.elementData(offset + index);
 955         }
 956 
 957         public int size() {
 958             checkForComodification();
 959             return this.size;
 960         }
 961 
 962         public void add(int index, E e) {
 963             rangeCheckForAdd(index);
 964             checkForComodification();
 965             parent.add(parentOffset + index, e);
 966             this.modCount = parent.modCount;
 967             this.size++;
 968         }
 969 
 970         public E remove(int index) {
 971             rangeCheck(index);
 972             checkForComodification();
 973             E result = parent.remove(parentOffset + index);
 974             this.modCount = parent.modCount;
 975             this.size--;
 976             return result;
 977         }
 978 
 979         protected void removeRange(int fromIndex, int toIndex) {
 980             checkForComodification();
 981             parent.removeRange(parentOffset + fromIndex,
 982                                parentOffset + toIndex);
 983             this.modCount = parent.modCount;
 984             this.size -= toIndex - fromIndex;
 985         }
 986 
 987         public boolean addAll(Collection<? extends E> c) {
 988             return addAll(this.size, c);
 989         }
 990 
 991         public boolean addAll(int index, Collection<? extends E> c) {
 992             rangeCheckForAdd(index);
 993             int cSize = c.size();
 994             if (cSize==0)
 995                 return false;
 996 
 997             checkForComodification();
 998             parent.addAll(parentOffset + index, c);
 999             this.modCount = parent.modCount;
1000             this.size += cSize;
1001             return true;
1002         }
1003 
1004         public Iterator<E> iterator() {
1005             return listIterator();
1006         }
1007 
1008         public ListIterator<E> listIterator(final int index) {
1009             checkForComodification();
1010             rangeCheckForAdd(index);
1011             final int offset = this.offset;
1012 
1013             return new ListIterator<E>() {
1014                 int cursor = index;
1015                 int lastRet = -1;
1016                 int expectedModCount = ArrayList.this.modCount;
1017 
1018                 public boolean hasNext() {
1019                     return cursor != SubList.this.size;
1020                 }
1021 
1022                 @SuppressWarnings("unchecked")
1023                 public E next() {
1024                     checkForComodification();
1025                     int i = cursor;
1026                     if (i >= SubList.this.size)
1027                         throw new NoSuchElementException();
1028                     Object[] elementData = ArrayList.this.elementData;
1029                     if (offset + i >= elementData.length)
1030                         throw new ConcurrentModificationException();
1031                     cursor = i + 1;
1032                     return (E) elementData[offset + (lastRet = i)];
1033                 }
1034 
1035                 public boolean hasPrevious() {
1036                     return cursor != 0;
1037                 }
1038 
1039                 @SuppressWarnings("unchecked")
1040                 public E previous() {
1041                     checkForComodification();
1042                     int i = cursor - 1;
1043                     if (i < 0)
1044                         throw new NoSuchElementException();
1045                     Object[] elementData = ArrayList.this.elementData;
1046                     if (offset + i >= elementData.length)
1047                         throw new ConcurrentModificationException();
1048                     cursor = i;
1049                     return (E) elementData[offset + (lastRet = i)];
1050                 }
1051 
1052                 public int nextIndex() {
1053                     return cursor;
1054                 }
1055 
1056                 public int previousIndex() {
1057                     return cursor - 1;
1058                 }
1059 
1060                 public void remove() {
1061                     if (lastRet < 0)
1062                         throw new IllegalStateException();
1063                     checkForComodification();
1064 
1065                     try {
1066                         SubList.this.remove(lastRet);
1067                         cursor = lastRet;
1068                         lastRet = -1;
1069                         expectedModCount = ArrayList.this.modCount;
1070                     } catch (IndexOutOfBoundsException ex) {
1071                         throw new ConcurrentModificationException();
1072                     }
1073                 }
1074 
1075                 public void set(E e) {
1076                     if (lastRet < 0)
1077                         throw new IllegalStateException();
1078                     checkForComodification();
1079 
1080                     try {
1081                         ArrayList.this.set(offset + lastRet, e);
1082                     } catch (IndexOutOfBoundsException ex) {
1083                         throw new ConcurrentModificationException();
1084                     }
1085                 }
1086 
1087                 public void add(E e) {
1088                     checkForComodification();
1089 
1090                     try {
1091                         int i = cursor;
1092                         SubList.this.add(i, e);
1093                         cursor = i + 1;
1094                         lastRet = -1;
1095                         expectedModCount = ArrayList.this.modCount;
1096                     } catch (IndexOutOfBoundsException ex) {
1097                         throw new ConcurrentModificationException();
1098                     }
1099                 }
1100 
1101                 final void checkForComodification() {
1102                     if (expectedModCount != ArrayList.this.modCount)
1103                         throw new ConcurrentModificationException();
1104                 }
1105             };
1106         }
1107 
1108         public List<E> subList(int fromIndex, int toIndex) {
1109             subListRangeCheck(fromIndex, toIndex, size);
1110             return new SubList(this, offset, fromIndex, toIndex);
1111         }
1112 
1113         private void rangeCheck(int index) {
1114             if (index < 0 || index >= this.size)
1115                 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
1116         }
1117 
1118         private void rangeCheckForAdd(int index) {
1119             if (index < 0 || index > this.size)
1120                 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
1121         }
1122 
1123         private String outOfBoundsMsg(int index) {
1124             return "Index: "+index+", Size: "+this.size;
1125         }
1126 
1127         private void checkForComodification() {
1128             if (ArrayList.this.modCount != this.modCount)
1129                 throw new ConcurrentModificationException();
1130         }
1131     }
1132 }