1 /*
2 * Copyright (c) 1997, 2012, 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
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20 *
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23 * questions.
24 */
25
26 package java.util;
27
28 /**
29 * This class provides a skeletal implementation of the {@link List}
30 * interface to minimize the effort required to implement this interface
31 * backed by a "random access" data store (such as an array). For sequential
32 * access data (such as a linked list), {@link AbstractSequentialList} should
33 * be used in preference to this class.
34 *
35 * <p>To implement an unmodifiable list, the programmer needs only to extend
36 * this class and provide implementations for the {@link #get(int)} and
37 * {@link List#size() size()} methods.
38 *
39 * <p>To implement a modifiable list, the programmer must additionally
40 * override the {@link #set(int, Object) set(int, E)} method (which otherwise
41 * throws an {@code UnsupportedOperationException}). If the list is
42 * variable-size the programmer must additionally override the
43 * {@link #add(int, Object) add(int, E)} and {@link #remove(int)} methods.
44 *
45 * <p>The programmer should generally provide a void (no argument) and collection
46 * constructor, as per the recommendation in the {@link Collection} interface
47 * specification.
48 *
49 * <p>Unlike the other abstract collection implementations, the programmer does
50 * <i>not</i> have to provide an iterator implementation; the iterator and
51 * list iterator are implemented by this class, on top of the "random access"
52 * methods:
53 * {@link #get(int)},
54 * {@link #set(int, Object) set(int, E)},
55 * {@link #add(int, Object) add(int, E)} and
56 * {@link #remove(int)}.
57 *
58 * <p>The documentation for each non-abstract method in this class describes its
59 * implementation in detail. Each of these methods may be overridden if the
60 * collection being implemented admits a more efficient implementation.
61 *
62 * <p>This class is a member of the
63 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
64 * Java Collections Framework</a>.
65 *
66 * @author Josh Bloch
67 * @author Neal Gafter
68 * @since 1.2
69 */
70
71 public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
72 /**
73 * Sole constructor. (For invocation by subclass constructors, typically
74 * implicit.)
75 */
76 protected AbstractList() {
77 }
78
79 /**
80 * Appends the specified element to the end of this list (optional
81 * operation).
82 *
83 * <p>Lists that support this operation may place limitations on what
84 * elements may be added to this list. In particular, some
85 * lists will refuse to add null elements, and others will impose
86 * restrictions on the type of elements that may be added. List
87 * classes should clearly specify in their documentation any restrictions
88 * on what elements may be added.
89 *
90 * @implSpec
91 * This implementation calls {@code add(size(), e)}.
92 *
93 * <p>Note that this implementation throws an
94 * {@code UnsupportedOperationException} unless
95 * {@link #add(int, Object) add(int, E)} is overridden.
96 *
97 * @param e element to be appended to this list
98 * @return {@code true} (as specified by {@link Collection#add})
99 * @throws UnsupportedOperationException if the {@code add} operation
100 * is not supported by this list
101 * @throws ClassCastException if the class of the specified element
102 * prevents it from being added to this list
103 * @throws NullPointerException if the specified element is null and this
104 * list does not permit null elements
105 * @throws IllegalArgumentException if some property of this element
106 * prevents it from being added to this list
107 */
108 public boolean add(E e) {
109 add(size(), e);
110 return true;
111 }
112
113 /**
114 * {@inheritDoc}
115 *
116 * @throws IndexOutOfBoundsException {@inheritDoc}
117 */
118 abstract public E get(int index);
119
120 /**
121 * {@inheritDoc}
122 *
123 * @implSpec
124 * This implementation always throws an
125 * {@code UnsupportedOperationException}.
126 *
127 * @throws UnsupportedOperationException {@inheritDoc}
128 * @throws ClassCastException {@inheritDoc}
129 * @throws NullPointerException {@inheritDoc}
130 * @throws IllegalArgumentException {@inheritDoc}
131 * @throws IndexOutOfBoundsException {@inheritDoc}
132 */
133 public E set(int index, E element) {
134 throw new UnsupportedOperationException();
135 }
136
137 /**
138 * {@inheritDoc}
139 *
140 * @implSpec
141 * This implementation always throws an
142 * {@code UnsupportedOperationException}.
143 *
144 * @throws UnsupportedOperationException {@inheritDoc}
145 * @throws ClassCastException {@inheritDoc}
146 * @throws NullPointerException {@inheritDoc}
147 * @throws IllegalArgumentException {@inheritDoc}
148 * @throws IndexOutOfBoundsException {@inheritDoc}
149 */
150 public void add(int index, E element) {
151 throw new UnsupportedOperationException();
152 }
153
154 /**
155 * {@inheritDoc}
156 *
157 * @implSpec
158 * This implementation always throws an
159 * {@code UnsupportedOperationException}.
160 *
161 * @throws UnsupportedOperationException {@inheritDoc}
162 * @throws IndexOutOfBoundsException {@inheritDoc}
163 */
164 public E remove(int index) {
165 throw new UnsupportedOperationException();
166 }
167
168
169 // Search Operations
170
171 /**
172 * {@inheritDoc}
173 *
174 * @implSpec
175 * This implementation first gets a list iterator (with
176 * {@code listIterator()}). Then, it iterates over the list until the
177 * specified element is found or the end of the list is reached.
178 *
179 * @throws ClassCastException {@inheritDoc}
180 * @throws NullPointerException {@inheritDoc}
181 */
182 public int indexOf(Object o) {
183 ListIterator<E> it = listIterator();
184 if (o==null) {
185 while (it.hasNext())
186 if (it.next()==null)
187 return it.previousIndex();
188 } else {
189 while (it.hasNext())
190 if (o.equals(it.next()))
191 return it.previousIndex();
192 }
193 return -1;
194 }
195
196 /**
197 * {@inheritDoc}
198 *
199 * @implSpec
200 * This implementation first gets a list iterator that points to the end
201 * of the list (with {@code listIterator(size())}). Then, it iterates
202 * backwards over the list until the specified element is found, or the
203 * beginning of the list is reached.
204 *
205 * @throws ClassCastException {@inheritDoc}
206 * @throws NullPointerException {@inheritDoc}
207 */
208 public int lastIndexOf(Object o) {
209 ListIterator<E> it = listIterator(size());
210 if (o==null) {
211 while (it.hasPrevious())
212 if (it.previous()==null)
213 return it.nextIndex();
214 } else {
215 while (it.hasPrevious())
216 if (o.equals(it.previous()))
217 return it.nextIndex();
218 }
219 return -1;
220 }
221
222
223 // Bulk Operations
224
225 /**
226 * Removes all of the elements from this list (optional operation).
227 * The list will be empty after this call returns.
228 *
229 * @implSpec
230 * This implementation calls {@code removeRange(0, size())}.
231 *
232 * <p>Note that this implementation throws an
233 * {@code UnsupportedOperationException} unless {@code remove(int
234 * index)} or {@code removeRange(int fromIndex, int toIndex)} is
235 * overridden.
236 *
237 * @throws UnsupportedOperationException if the {@code clear} operation
238 * is not supported by this list
239 */
240 public void clear() {
241 removeRange(0, size());
242 }
243
244 /**
245 * {@inheritDoc}
246 *
247 * @implSpec
248 * This implementation gets an iterator over the specified collection
249 * and iterates over it, inserting the elements obtained from the
250 * iterator into this list at the appropriate position, one at a time,
251 * using {@code add(int, E)}.
252 * Many implementations will override this method for efficiency.
253 *
254 * <p>Note that this implementation throws an
255 * {@code UnsupportedOperationException} unless
256 * {@link #add(int, Object) add(int, E)} is overridden.
257 *
258 * @throws UnsupportedOperationException {@inheritDoc}
259 * @throws ClassCastException {@inheritDoc}
260 * @throws NullPointerException {@inheritDoc}
261 * @throws IllegalArgumentException {@inheritDoc}
262 * @throws IndexOutOfBoundsException {@inheritDoc}
263 */
264 public boolean addAll(int index, Collection<? extends E> c) {
265 rangeCheckForAdd(index);
266 boolean modified = false;
267 for (E e : c) {
268 add(index++, e);
269 modified = true;
270 }
271 return modified;
272 }
273
274
275 // Iterators
276
277 /**
278 * Returns an iterator over the elements in this list in proper sequence.
279 *
280 * @implSpec
281 * This implementation returns a straightforward implementation of the
282 * iterator interface, relying on the backing list's {@code size()},
283 * {@code get(int)}, and {@code remove(int)} methods.
284 *
285 * <p>Note that the iterator returned by this method will throw an
286 * {@link UnsupportedOperationException} in response to its
287 * {@code remove} method unless the list's {@code remove(int)} method is
288 * overridden.
289 *
290 * <p>This implementation can be made to throw runtime exceptions in the
291 * face of concurrent modification, as described in the specification
292 * for the (protected) {@link #modCount} field.
293 *
294 * @return an iterator over the elements in this list in proper sequence
295 */
296 public Iterator<E> iterator() {
297 return new Itr();
298 }
299
300 /**
301 * {@inheritDoc}
302 *
303 * @implSpec
304 * This implementation returns {@code listIterator(0)}.
305 *
306 * @see #listIterator(int)
307 */
308 public ListIterator<E> listIterator() {
309 return listIterator(0);
310 }
311
312 /**
313 * {@inheritDoc}
314 *
315 * @implSpec
316 * This implementation returns a straightforward implementation of the
317 * {@code ListIterator} interface that extends the implementation of the
318 * {@code Iterator} interface returned by the {@code iterator()} method.
319 * The {@code ListIterator} implementation relies on the backing list's
320 * {@code get(int)}, {@code set(int, E)}, {@code add(int, E)}
321 * and {@code remove(int)} methods.
322 *
323 * <p>Note that the list iterator returned by this implementation will
324 * throw an {@link UnsupportedOperationException} in response to its
325 * {@code remove}, {@code set} and {@code add} methods unless the
326 * list's {@code remove(int)}, {@code set(int, E)}, and
327 * {@code add(int, E)} methods are overridden.
328 *
329 * <p>This implementation can be made to throw runtime exceptions in the
330 * face of concurrent modification, as described in the specification for
331 * the (protected) {@link #modCount} field.
332 *
333 * @throws IndexOutOfBoundsException {@inheritDoc}
334 */
335 public ListIterator<E> listIterator(final int index) {
336 rangeCheckForAdd(index);
337
338 return new ListItr(index);
339 }
340
341 private class Itr implements Iterator<E> {
342 /**
343 * Index of element to be returned by subsequent call to next.
344 */
345 int cursor = 0;
346
347 /**
348 * Index of element returned by most recent call to next or
349 * previous. Reset to -1 if this element is deleted by a call
350 * to remove.
351 */
352 int lastRet = -1;
353
354 /**
355 * The modCount value that the iterator believes that the backing
356 * List should have. If this expectation is violated, the iterator
357 * has detected concurrent modification.
358 */
359 int expectedModCount = modCount;
360
361 public boolean hasNext() {
362 return cursor != size();
363 }
364
365 public E next() {
366 checkForComodification();
367 try {
368 int i = cursor;
369 E next = get(i);
370 lastRet = i;
371 cursor = i + 1;
372 return next;
373 } catch (IndexOutOfBoundsException e) {
374 checkForComodification();
375 throw new NoSuchElementException();
376 }
377 }
378
379 public void remove() {
380 if (lastRet < 0)
381 throw new IllegalStateException();
382 checkForComodification();
383
384 try {
385 AbstractList.this.remove(lastRet);
386 if (lastRet < cursor)
387 cursor--;
388 lastRet = -1;
389 expectedModCount = modCount;
390 } catch (IndexOutOfBoundsException e) {
391 throw new ConcurrentModificationException();
392 }
393 }
394
395 final void checkForComodification() {
396 if (modCount != expectedModCount)
397 throw new ConcurrentModificationException();
398 }
399 }
400
401 private class ListItr extends Itr implements ListIterator<E> {
402 ListItr(int index) {
403 cursor = index;
404 }
405
406 public boolean hasPrevious() {
407 return cursor != 0;
408 }
409
410 public E previous() {
411 checkForComodification();
412 try {
413 int i = cursor - 1;
414 E previous = get(i);
415 lastRet = cursor = i;
416 return previous;
417 } catch (IndexOutOfBoundsException e) {
418 checkForComodification();
419 throw new NoSuchElementException();
420 }
421 }
422
423 public int nextIndex() {
424 return cursor;
425 }
426
427 public int previousIndex() {
428 return cursor-1;
429 }
430
431 public void set(E e) {
432 if (lastRet < 0)
433 throw new IllegalStateException();
434 checkForComodification();
435
436 try {
437 AbstractList.this.set(lastRet, e);
438 expectedModCount = modCount;
439 } catch (IndexOutOfBoundsException ex) {
440 throw new ConcurrentModificationException();
441 }
442 }
443
444 public void add(E e) {
445 checkForComodification();
446
447 try {
448 int i = cursor;
449 AbstractList.this.add(i, e);
450 lastRet = -1;
451 cursor = i + 1;
452 expectedModCount = modCount;
453 } catch (IndexOutOfBoundsException ex) {
454 throw new ConcurrentModificationException();
455 }
456 }
457 }
458
459 /**
460 * {@inheritDoc}
461 *
462 * @implSpec
463 * This implementation returns a list that subclasses
464 * {@code AbstractList}. The subclass stores, in private fields, the
465 * offset of the subList within the backing list, the size of the subList
466 * (which can change over its lifetime), and the expected
467 * {@code modCount} value of the backing list. There are two variants
468 * of the subclass, one of which implements {@code RandomAccess}.
469 * If this list implements {@code RandomAccess} the returned list will
470 * be an instance of the subclass that implements {@code RandomAccess}.
471 *
472 * <p>The subclass's {@code set(int, E)}, {@code get(int)},
473 * {@code add(int, E)}, {@code remove(int)}, {@code addAll(int,
474 * Collection)} and {@code removeRange(int, int)} methods all
475 * delegate to the corresponding methods on the backing abstract list,
476 * after bounds-checking the index and adjusting for the offset. The
477 * {@code addAll(Collection c)} method merely returns {@code addAll(size,
478 * c)}.
479 *
480 * <p>The {@code listIterator(int)} method returns a "wrapper object"
481 * over a list iterator on the backing list, which is created with the
482 * corresponding method on the backing list. The {@code iterator} method
483 * merely returns {@code listIterator()}, and the {@code size} method
484 * merely returns the subclass's {@code size} field.
485 *
486 * <p>All methods first check to see if the actual {@code modCount} of
487 * the backing list is equal to its expected value, and throw a
488 * {@code ConcurrentModificationException} if it is not.
489 *
490 * @throws IndexOutOfBoundsException if an endpoint index value is out of range
491 * {@code (fromIndex < 0 || toIndex > size)}
492 * @throws IllegalArgumentException if the endpoint indices are out of order
493 * {@code (fromIndex > toIndex)}
494 */
495 public List<E> subList(int fromIndex, int toIndex) {
496 subListRangeCheck(fromIndex, toIndex, size());
497 return (this instanceof RandomAccess ?
498 new RandomAccessSubList(this, 0, fromIndex, toIndex) :
499 new SubList(this, 0, fromIndex, toIndex));
500 }
501
502 static void subListRangeCheck(int fromIndex, int toIndex, int size) {
503 if (fromIndex < 0)
504 throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
505 if (toIndex > size)
506 throw new IndexOutOfBoundsException("toIndex = " + toIndex);
507 if (fromIndex > toIndex)
508 throw new IllegalArgumentException("fromIndex(" + fromIndex +
509 ") > toIndex(" + toIndex + ")");
510 }
511
512 private class SubList extends AbstractList<E> {
513 final AbstractList<E> parent;
514 final int offset;
515 int size;
516
517 SubList(AbstractList<E> parent,
518 int offset, int fromIndex, int toIndex) {
519 this.parent = parent;
520 this.offset = offset + fromIndex;
521 this.size = toIndex - fromIndex;
522 this.modCount = AbstractList.this.modCount;
523 }
524
525 public E set(int index, E element) {
526 rangeCheck(index);
527 checkForComodification();
528 return AbstractList.this.set(index + offset, element);
529 }
530
531 public E get(int index) {
532 rangeCheck(index);
533 checkForComodification();
534 return AbstractList.this.get(index + offset);
535 }
536
537 public int size() {
538 checkForComodification();
539 return size;
540 }
541
542 public void add(int index, E element) {
543 rangeCheckForAdd(index);
544 checkForComodification();
545 AbstractList.this.add(index + offset, element);
546 updateSizeAndModCount(1, AbstractList.this.modCount);
547 }
548
549 public E remove(int index) {
550 rangeCheck(index);
551 checkForComodification();
552 E result = AbstractList.this.remove(index + offset);
553 updateSizeAndModCount(-1, AbstractList.this.modCount);
554 return result;
555 }
556
557 protected void removeRange(int fromIndex, int toIndex) {
558 checkForComodification();
559 AbstractList.this.removeRange(offset + fromIndex,
560 offset + toIndex);
561 updateSizeAndModCount(fromIndex - toIndex,
562 AbstractList.this.modCount);
563 }
564
565 public boolean addAll(Collection<? extends E> c) {
566 return addAll(size, c);
567 }
568
569 public boolean addAll(int index, Collection<? extends E> c) {
570 rangeCheckForAdd(index);
571 int cSize = c.size();
572 if (cSize == 0)
573 return false;
574 checkForComodification();
575 AbstractList.this.addAll(offset + index, c);
576 updateSizeAndModCount(cSize,
577 AbstractList.this.modCount);
578 return true;
579 }
580
581 public Iterator<E> iterator() {
582 return listIterator();
583 }
584
585 public ListIterator<E> listIterator(final int index) {
586 checkForComodification();
587 rangeCheckForAdd(index);
588
589 return new ListIterator<E>() {
590 private final ListIterator<E> i =
591 AbstractList.this.listIterator(offset + index);
592
593 public boolean hasNext() {
594 return nextIndex() < size;
595 }
596
597 public E next() {
598 if (hasNext())
599 return i.next();
600 else
601 throw new NoSuchElementException();
602 }
603
604 public boolean hasPrevious() {
605 return previousIndex() >= 0;
606 }
607
608 public E previous() {
609 if (hasPrevious())
610 return i.previous();
611 else
612 throw new NoSuchElementException();
613 }
614
615 public int nextIndex() {
616 return i.nextIndex() - offset;
617 }
618
619 public int previousIndex() {
620 return i.previousIndex() - offset;
621 }
622
623 public void remove() {
624 i.remove();
625 updateSizeAndModCount(-1,
626 AbstractList.this.modCount);
627 }
628
629 public void set(E e) {
630 i.set(e);
631 }
632
633 public void add(E e) {
634 i.add(e);
635 updateSizeAndModCount(1,
636 AbstractList.this.modCount);
637 }
638 };
639 }
640
641 public List<E> subList(int fromIndex, int toIndex) {
642 subListRangeCheck(fromIndex, toIndex, size);
643 return AbstractList.this.new SubList(this, offset, fromIndex,
644 toIndex);
645 }
646
647 private void rangeCheck(int index) {
648 if (index < 0 || index >= size)
649 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
650 }
651
652 private void rangeCheckForAdd(int index) {
653 if (index < 0 || index > size)
654 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
655 }
656
657 private String outOfBoundsMsg(int index) {
658 return "Index: " + index + ", Size: " + size;
659 }
660
661 private void checkForComodification() {
662 if (AbstractList.this.modCount != this.modCount)
663 throw new ConcurrentModificationException();
664 }
665
666 private void updateSizeAndModCount(int sizeChange, int modCount) {
667 AbstractList<E> alist = this;
668 do {
669 SubList slist = (SubList)alist;
670 slist.size += sizeChange;
671 slist.modCount = modCount;
672 alist = slist.parent;
673 } while (alist instanceof AbstractList.SubList);
674 }
675 }
676
677 private class RandomAccessSubList extends SubList implements RandomAccess {
678 RandomAccessSubList(AbstractList<E> list, int offset,
679 int fromIndex, int toIndex) {
680 super(list, offset, fromIndex, toIndex);
681 }
682
683 public List<E> subList(int fromIndex, int toIndex) {
684 subListRangeCheck(fromIndex, toIndex, size);
685 return AbstractList.this.new RandomAccessSubList(this, offset,
686 fromIndex, toIndex);
687 }
688 }
689
690 // Comparison and hashing
691
692 /**
693 * Compares the specified object with this list for equality. Returns
694 * {@code true} if and only if the specified object is also a list, both
695 * lists have the same size, and all corresponding pairs of elements in
696 * the two lists are <i>equal</i>. (Two elements {@code e1} and
697 * {@code e2} are <i>equal</i> if {@code (e1==null ? e2==null :
698 * e1.equals(e2))}.) In other words, two lists are defined to be
699 * equal if they contain the same elements in the same order.
700 *
701 * @implSpec
702 * This implementation first checks if the specified object is this
703 * list. If so, it returns {@code true}; if not, it checks if the
704 * specified object is a list. If not, it returns {@code false}; if so,
705 * it iterates over both lists, comparing corresponding pairs of elements.
706 * If any comparison returns {@code false}, this method returns
707 * {@code false}. If either iterator runs out of elements before the
708 * other it returns {@code false} (as the lists are of unequal length);
709 * otherwise it returns {@code true} when the iterations complete.
710 *
711 * @param o the object to be compared for equality with this list
712 * @return {@code true} if the specified object is equal to this list
713 */
714 public boolean equals(Object o) {
715 if (o == this)
716 return true;
717 if (!(o instanceof List))
718 return false;
719
720 ListIterator<E> e1 = listIterator();
721 ListIterator<?> e2 = ((List<?>) o).listIterator();
722 while (e1.hasNext() && e2.hasNext()) {
723 E o1 = e1.next();
724 Object o2 = e2.next();
725 if (!(o1==null ? o2==null : o1.equals(o2)))
726 return false;
727 }
728 return !(e1.hasNext() || e2.hasNext());
729 }
730
731 /**
732 * Returns the hash code value for this list.
733 *
734 * @implSpec
735 * This implementation uses exactly the code that is used to define the
736 * list hash function in the documentation for the {@link List#hashCode}
737 * method.
738 *
739 * @return the hash code value for this list
740 */
741 public int hashCode() {
742 int hashCode = 1;
743 for (E e : this)
744 hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
745 return hashCode;
746 }
747
748 /**
749 * Removes from this list all of the elements whose index is between
750 * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
751 * Shifts any succeeding elements to the left (reduces their index).
752 * This call shortens the list by {@code (toIndex - fromIndex)} elements.
753 * (If {@code toIndex==fromIndex}, this operation has no effect.)
754 *
755 * <p>This method is called by the {@code clear} operation on this list
756 * and its subLists. Overriding this method to take advantage of
757 * the internals of the list implementation can <i>substantially</i>
758 * improve the performance of the {@code clear} operation on this list
759 * and its subLists.
760 *
761 * @implSpec
762 * This implementation gets a list iterator positioned before
763 * {@code fromIndex}, and repeatedly calls {@code ListIterator.next}
764 * followed by {@code ListIterator.remove} until the entire range has
765 * been removed. <b>Note: if {@code ListIterator.remove} requires linear
766 * time, this implementation requires quadratic time.</b>
767 *
768 * @param fromIndex index of first element to be removed
769 * @param toIndex index after last element to be removed
770 */
771 protected void removeRange(int fromIndex, int toIndex) {
772 ListIterator<E> it = listIterator(fromIndex);
773 for (int i=0, n=toIndex-fromIndex; i<n; i++) {
774 it.next();
775 it.remove();
776 }
777 }
778
779 /**
780 * The number of times this list has been <i>structurally modified</i>.
781 * Structural modifications are those that change the size of the
782 * list, or otherwise perturb it in such a fashion that iterations in
783 * progress may yield incorrect results.
784 *
785 * <p>This field is used by the iterator and list iterator implementation
786 * returned by the {@code iterator} and {@code listIterator} methods.
787 * If the value of this field changes unexpectedly, the iterator (or list
788 * iterator) will throw a {@code ConcurrentModificationException} in
789 * response to the {@code next}, {@code remove}, {@code previous},
790 * {@code set} or {@code add} operations. This provides
791 * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
792 * the face of concurrent modification during iteration.
793 *
794 * <p><b>Use of this field by subclasses is optional.</b> If a subclass
795 * wishes to provide fail-fast iterators (and list iterators), then it
796 * merely has to increment this field in its {@code add(int, E)} and
797 * {@code remove(int)} methods (and any other methods that it overrides
798 * that result in structural modifications to the list). A single call to
799 * {@code add(int, E)} or {@code remove(int)} must add no more than
800 * one to this field, or the iterators (and list iterators) will throw
801 * bogus {@code ConcurrentModificationExceptions}. If an implementation
802 * does not wish to provide fail-fast iterators, this field may be
803 * ignored.
804 */
805 protected transient int modCount = 0;
806
807 private void rangeCheckForAdd(int index) {
808 if (index < 0 || index > size())
809 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
810 }
811
812 private String outOfBoundsMsg(int index) {
813 return "Index: " + index + ", Size: " + size();
814 }
815 }