1 /*
2 * Copyright (c) 1997, 2011, 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 * Doubly-linked list implementation of the {@code List} and {@code Deque}
30 * interfaces. Implements all optional list operations, and permits all
31 * elements (including {@code null}).
32 *
33 * <p>All of the operations perform as could be expected for a doubly-linked
34 * list. Operations that index into the list will traverse the list from
35 * the beginning or the end, whichever is closer to the specified index.
36 *
37 * <p><strong>Note that this implementation is not synchronized.</strong>
38 * If multiple threads access a linked list concurrently, and at least
39 * one of the threads modifies the list structurally, it <i>must</i> be
40 * synchronized externally. (A structural modification is any operation
41 * that adds or deletes one or more elements; merely setting the value of
42 * an element is not a structural modification.) This is typically
43 * accomplished by synchronizing on some object that naturally
44 * encapsulates the list.
45 *
46 * If no such object exists, the list should be "wrapped" using the
47 * {@link Collections#synchronizedList Collections.synchronizedList}
48 * method. This is best done at creation time, to prevent accidental
49 * unsynchronized access to the list:<pre>
50 * List list = Collections.synchronizedList(new LinkedList(...));</pre>
51 *
52 * <p>The iterators returned by this class's {@code iterator} and
53 * {@code listIterator} methods are <i>fail-fast</i>: if the list is
54 * structurally modified at any time after the iterator is created, in
55 * any way except through the Iterator's own {@code remove} or
56 * {@code add} methods, the iterator will throw a {@link
57 * ConcurrentModificationException}. Thus, in the face of concurrent
58 * modification, the iterator fails quickly and cleanly, rather than
59 * risking arbitrary, non-deterministic behavior at an undetermined
60 * time in the future.
61 *
62 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
63 * as it is, generally speaking, impossible to make any hard guarantees in the
64 * presence of unsynchronized concurrent modification. Fail-fast iterators
65 * throw {@code ConcurrentModificationException} on a best-effort basis.
66 * Therefore, it would be wrong to write a program that depended on this
67 * exception for its correctness: <i>the fail-fast behavior of iterators
68 * should be used only to detect bugs.</i>
69 *
70 * <p>This class is a member of the
71 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
72 * Java Collections Framework</a>.
73 *
74 * @author Josh Bloch
75 * @see List
76 * @see ArrayList
77 * @since 1.2
78 * @param <E> the type of elements held in this collection
79 */
80
81 public class LinkedList<E>
82 extends AbstractSequentialList<E>
83 implements List<E>, Deque<E>, Cloneable, java.io.Serializable
84 {
85 transient int size = 0;
86
87 /**
88 * Pointer to first node.
89 * Invariant: (first == null && last == null) ||
90 * (first.prev == null && first.item != null)
91 */
92 transient Node<E> first;
93
94 /**
95 * Pointer to last node.
96 * Invariant: (first == null && last == null) ||
97 * (last.next == null && last.item != null)
98 */
99 transient Node<E> last;
100
101 /**
102 * Constructs an empty list.
103 */
104 public LinkedList() {
105 }
106
107 /**
108 * Constructs a list containing the elements of the specified
109 * collection, in the order they are returned by the collection's
110 * iterator.
111 *
112 * @param c the collection whose elements are to be placed into this list
113 * @throws NullPointerException if the specified collection is null
114 */
115 public LinkedList(Collection<? extends E> c) {
116 this();
117 addAll(c);
118 }
119
120 /**
121 * Links e as first element.
122 */
123 private void linkFirst(E e) {
124 final Node<E> f = first;
125 final Node<E> newNode = new Node<>(null, e, f);
126 first = newNode;
127 if (f == null)
128 last = newNode;
129 else
130 f.prev = newNode;
131 size++;
132 modCount++;
133 }
134
135 /**
136 * Links e as last element.
137 */
138 void linkLast(E e) {
139 final Node<E> l = last;
140 final Node<E> newNode = new Node<>(l, e, null);
141 last = newNode;
142 if (l == null)
143 first = newNode;
144 else
145 l.next = newNode;
146 size++;
147 modCount++;
148 }
149
150 /**
151 * Inserts element e before non-null Node succ.
152 */
153 void linkBefore(E e, Node<E> succ) {
154 // assert succ != null;
155 final Node<E> pred = succ.prev;
156 final Node<E> newNode = new Node<>(pred, e, succ);
157 succ.prev = newNode;
158 if (pred == null)
159 first = newNode;
160 else
161 pred.next = newNode;
162 size++;
163 modCount++;
164 }
165
166 /**
167 * Unlinks non-null first node f.
168 */
169 private E unlinkFirst(Node<E> f) {
170 // assert f == first && f != null;
171 final E element = f.item;
172 final Node<E> next = f.next;
173 f.item = null;
174 f.next = null; // help GC
175 first = next;
176 if (next == null)
177 last = null;
178 else
179 next.prev = null;
180 size--;
181 modCount++;
182 return element;
183 }
184
185 /**
186 * Unlinks non-null last node l.
187 */
188 private E unlinkLast(Node<E> l) {
189 // assert l == last && l != null;
190 final E element = l.item;
191 final Node<E> prev = l.prev;
192 l.item = null;
193 l.prev = null; // help GC
194 last = prev;
195 if (prev == null)
196 first = null;
197 else
198 prev.next = null;
199 size--;
200 modCount++;
201 return element;
202 }
203
204 /**
205 * Unlinks non-null node x.
206 */
207 E unlink(Node<E> x) {
208 // assert x != null;
209 final E element = x.item;
210 final Node<E> next = x.next;
211 final Node<E> prev = x.prev;
212
213 if (prev == null) {
214 first = next;
215 } else {
216 prev.next = next;
217 x.prev = null;
218 }
219
220 if (next == null) {
221 last = prev;
222 } else {
223 next.prev = prev;
224 x.next = null;
225 }
226
227 x.item = null;
228 size--;
229 modCount++;
230 return element;
231 }
232
233 /**
234 * Returns the first element in this list.
235 *
236 * @return the first element in this list
237 * @throws NoSuchElementException if this list is empty
238 */
239 public E getFirst() {
240 final Node<E> f = first;
241 if (f == null)
242 throw new NoSuchElementException();
243 return f.item;
244 }
245
246 /**
247 * Returns the last element in this list.
248 *
249 * @return the last element in this list
250 * @throws NoSuchElementException if this list is empty
251 */
252 public E getLast() {
253 final Node<E> l = last;
254 if (l == null)
255 throw new NoSuchElementException();
256 return l.item;
257 }
258
259 /**
260 * Removes and returns the first element from this list.
261 *
262 * @return the first element from this list
263 * @throws NoSuchElementException if this list is empty
264 */
265 public E removeFirst() {
266 final Node<E> f = first;
267 if (f == null)
268 throw new NoSuchElementException();
269 return unlinkFirst(f);
270 }
271
272 /**
273 * Removes and returns the last element from this list.
274 *
275 * @return the last element from this list
276 * @throws NoSuchElementException if this list is empty
277 */
278 public E removeLast() {
279 final Node<E> l = last;
280 if (l == null)
281 throw new NoSuchElementException();
282 return unlinkLast(l);
283 }
284
285 /**
286 * Inserts the specified element at the beginning of this list.
287 *
288 * @param e the element to add
289 */
290 public void addFirst(E e) {
291 linkFirst(e);
292 }
293
294 /**
295 * Appends the specified element to the end of this list.
296 *
297 * <p>This method is equivalent to {@link #add}.
298 *
299 * @param e the element to add
300 */
301 public void addLast(E e) {
302 linkLast(e);
303 }
304
305 /**
306 * Returns {@code true} if this list contains the specified element.
307 * More formally, returns {@code true} if and only if this list contains
308 * at least one element {@code e} such that
309 * <tt>(o==null ? e==null : o.equals(e))</tt>.
310 *
311 * @param o element whose presence in this list is to be tested
312 * @return {@code true} if this list contains the specified element
313 */
314 public boolean contains(Object o) {
315 return indexOf(o) != -1;
316 }
317
318 /**
319 * Returns the number of elements in this list.
320 *
321 * @return the number of elements in this list
322 */
323 public int size() {
324 return size;
325 }
326
327 /**
328 * Appends the specified element to the end of this list.
329 *
330 * <p>This method is equivalent to {@link #addLast}.
331 *
332 * @param e element to be appended to this list
333 * @return {@code true} (as specified by {@link Collection#add})
334 */
335 public boolean add(E e) {
336 linkLast(e);
337 return true;
338 }
339
340 /**
341 * Removes the first occurrence of the specified element from this list,
342 * if it is present. If this list does not contain the element, it is
343 * unchanged. More formally, removes the element with the lowest index
344 * {@code i} such that
345 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
346 * (if such an element exists). Returns {@code true} if this list
347 * contained the specified element (or equivalently, if this list
348 * changed as a result of the call).
349 *
350 * @param o element to be removed from this list, if present
351 * @return {@code true} if this list contained the specified element
352 */
353 public boolean remove(Object o) {
354 if (o == null) {
355 for (Node<E> x = first; x != null; x = x.next) {
356 if (x.item == null) {
357 unlink(x);
358 return true;
359 }
360 }
361 } else {
362 for (Node<E> x = first; x != null; x = x.next) {
363 if (o.equals(x.item)) {
364 unlink(x);
365 return true;
366 }
367 }
368 }
369 return false;
370 }
371
372 /**
373 * Appends all of the elements in the specified collection to the end of
374 * this list, in the order that they are returned by the specified
375 * collection's iterator. The behavior of this operation is undefined if
376 * the specified collection is modified while the operation is in
377 * progress. (Note that this will occur if the specified collection is
378 * this list, and it's nonempty.)
379 *
380 * @param c collection containing elements to be added to this list
381 * @return {@code true} if this list changed as a result of the call
382 * @throws NullPointerException if the specified collection is null
383 */
384 public boolean addAll(Collection<? extends E> c) {
385 return addAll(size, c);
386 }
387
388 /**
389 * Inserts all of the elements in the specified collection into this
390 * list, starting at the specified position. Shifts the element
391 * currently at that position (if any) and any subsequent elements to
392 * the right (increases their indices). The new elements will appear
393 * in the list in the order that they are returned by the
394 * specified collection's iterator.
395 *
396 * @param index index at which to insert the first element
397 * from the specified collection
398 * @param c collection containing elements to be added to this list
399 * @return {@code true} if this list changed as a result of the call
400 * @throws IndexOutOfBoundsException {@inheritDoc}
401 * @throws NullPointerException if the specified collection is null
402 */
403 public boolean addAll(int index, Collection<? extends E> c) {
404 checkPositionIndex(index);
405
406 Object[] a = c.toArray();
407 int numNew = a.length;
408 if (numNew == 0)
409 return false;
410
411 Node<E> pred, succ;
412 if (index == size) {
413 succ = null;
414 pred = last;
415 } else {
416 succ = node(index);
417 pred = succ.prev;
418 }
419
420 for (Object o : a) {
421 @SuppressWarnings("unchecked") E e = (E) o;
422 Node<E> newNode = new Node<>(pred, e, null);
423 if (pred == null)
424 first = newNode;
425 else
426 pred.next = newNode;
427 pred = newNode;
428 }
429
430 if (succ == null) {
431 last = pred;
432 } else {
433 pred.next = succ;
434 succ.prev = pred;
435 }
436
437 size += numNew;
438 modCount++;
439 return true;
440 }
441
442 /**
443 * Removes all of the elements from this list.
444 * The list will be empty after this call returns.
445 */
446 public void clear() {
447 // Clearing all of the links between nodes is "unnecessary", but:
448 // - helps a generational GC if the discarded nodes inhabit
449 // more than one generation
450 // - is sure to free memory even if there is a reachable Iterator
451 for (Node<E> x = first; x != null; ) {
452 Node<E> next = x.next;
453 x.item = null;
454 x.next = null;
455 x.prev = null;
456 x = next;
457 }
458 first = last = null;
459 size = 0;
460 modCount++;
461 }
462
463
464 // Positional Access Operations
465
466 /**
467 * Returns the element at the specified position in this list.
468 *
469 * @param index index of the element to return
470 * @return the element at the specified position in this list
471 * @throws IndexOutOfBoundsException {@inheritDoc}
472 */
473 public E get(int index) {
474 checkElementIndex(index);
475 return node(index).item;
476 }
477
478 /**
479 * Replaces the element at the specified position in this list with the
480 * specified element.
481 *
482 * @param index index of the element to replace
483 * @param element element to be stored at the specified position
484 * @return the element previously at the specified position
485 * @throws IndexOutOfBoundsException {@inheritDoc}
486 */
487 public E set(int index, E element) {
488 checkElementIndex(index);
489 Node<E> x = node(index);
490 E oldVal = x.item;
491 x.item = element;
492 return oldVal;
493 }
494
495 /**
496 * Inserts the specified element at the specified position in this list.
497 * Shifts the element currently at that position (if any) and any
498 * subsequent elements to the right (adds one to their indices).
499 *
500 * @param index index at which the specified element is to be inserted
501 * @param element element to be inserted
502 * @throws IndexOutOfBoundsException {@inheritDoc}
503 */
504 public void add(int index, E element) {
505 checkPositionIndex(index);
506
507 if (index == size)
508 linkLast(element);
509 else
510 linkBefore(element, node(index));
511 }
512
513 /**
514 * Removes the element at the specified position in this list. Shifts any
515 * subsequent elements to the left (subtracts one from their indices).
516 * Returns the element that was removed from the list.
517 *
518 * @param index the index of the element to be removed
519 * @return the element previously at the specified position
520 * @throws IndexOutOfBoundsException {@inheritDoc}
521 */
522 public E remove(int index) {
523 checkElementIndex(index);
524 return unlink(node(index));
525 }
526
527 /**
528 * Tells if the argument is the index of an existing element.
529 */
530 private boolean isElementIndex(int index) {
531 return index >= 0 && index < size;
532 }
533
534 /**
535 * Tells if the argument is the index of a valid position for an
536 * iterator or an add operation.
537 */
538 private boolean isPositionIndex(int index) {
539 return index >= 0 && index <= size;
540 }
541
542 /**
543 * Constructs an IndexOutOfBoundsException detail message.
544 * Of the many possible refactorings of the error handling code,
545 * this "outlining" performs best with both server and client VMs.
546 */
547 private String outOfBoundsMsg(int index) {
548 return "Index: "+index+", Size: "+size;
549 }
550
551 private void checkElementIndex(int index) {
552 if (!isElementIndex(index))
553 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
554 }
555
556 private void checkPositionIndex(int index) {
557 if (!isPositionIndex(index))
558 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
559 }
560
561 /**
562 * Returns the (non-null) Node at the specified element index.
563 */
564 Node<E> node(int index) {
565 // assert isElementIndex(index);
566
567 if (index < (size >> 1)) {
568 Node<E> x = first;
569 for (int i = 0; i < index; i++)
570 x = x.next;
571 return x;
572 } else {
573 Node<E> x = last;
574 for (int i = size - 1; i > index; i--)
575 x = x.prev;
576 return x;
577 }
578 }
579
580 // Search Operations
581
582 /**
583 * Returns the index of the first occurrence of the specified element
584 * in this list, or -1 if this list does not contain the element.
585 * More formally, returns the lowest index {@code i} such that
586 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
587 * or -1 if there is no such index.
588 *
589 * @param o element to search for
590 * @return the index of the first occurrence of the specified element in
591 * this list, or -1 if this list does not contain the element
592 */
593 public int indexOf(Object o) {
594 int index = 0;
595 if (o == null) {
596 for (Node<E> x = first; x != null; x = x.next) {
597 if (x.item == null)
598 return index;
599 index++;
600 }
601 } else {
602 for (Node<E> x = first; x != null; x = x.next) {
603 if (o.equals(x.item))
604 return index;
605 index++;
606 }
607 }
608 return -1;
609 }
610
611 /**
612 * Returns the index of the last occurrence of the specified element
613 * in this list, or -1 if this list does not contain the element.
614 * More formally, returns the highest index {@code i} such that
615 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
616 * or -1 if there is no such index.
617 *
618 * @param o element to search for
619 * @return the index of the last occurrence of the specified element in
620 * this list, or -1 if this list does not contain the element
621 */
622 public int lastIndexOf(Object o) {
623 int index = size;
624 if (o == null) {
625 for (Node<E> x = last; x != null; x = x.prev) {
626 index--;
627 if (x.item == null)
628 return index;
629 }
630 } else {
631 for (Node<E> x = last; x != null; x = x.prev) {
632 index--;
633 if (o.equals(x.item))
634 return index;
635 }
636 }
637 return -1;
638 }
639
640 // Queue operations.
641
642 /**
643 * Retrieves, but does not remove, the head (first element) of this list.
644 *
645 * @return the head of this list, or {@code null} if this list is empty
646 * @since 1.5
647 */
648 public E peek() {
649 final Node<E> f = first;
650 return (f == null) ? null : f.item;
651 }
652
653 /**
654 * Retrieves, but does not remove, the head (first element) of this list.
655 *
656 * @return the head of this list
657 * @throws NoSuchElementException if this list is empty
658 * @since 1.5
659 */
660 public E element() {
661 return getFirst();
662 }
663
664 /**
665 * Retrieves and removes the head (first element) of this list.
666 *
667 * @return the head of this list, or {@code null} if this list is empty
668 * @since 1.5
669 */
670 public E poll() {
671 final Node<E> f = first;
672 return (f == null) ? null : unlinkFirst(f);
673 }
674
675 /**
676 * Retrieves and removes the head (first element) of this list.
677 *
678 * @return the head of this list
679 * @throws NoSuchElementException if this list is empty
680 * @since 1.5
681 */
682 public E remove() {
683 return removeFirst();
684 }
685
686 /**
687 * Adds the specified element as the tail (last element) of this list.
688 *
689 * @param e the element to add
690 * @return {@code true} (as specified by {@link Queue#offer})
691 * @since 1.5
692 */
693 public boolean offer(E e) {
694 return add(e);
695 }
696
697 // Deque operations
698 /**
699 * Inserts the specified element at the front of this list.
700 *
701 * @param e the element to insert
702 * @return {@code true} (as specified by {@link Deque#offerFirst})
703 * @since 1.6
704 */
705 public boolean offerFirst(E e) {
706 addFirst(e);
707 return true;
708 }
709
710 /**
711 * Inserts the specified element at the end of this list.
712 *
713 * @param e the element to insert
714 * @return {@code true} (as specified by {@link Deque#offerLast})
715 * @since 1.6
716 */
717 public boolean offerLast(E e) {
718 addLast(e);
719 return true;
720 }
721
722 /**
723 * Retrieves, but does not remove, the first element of this list,
724 * or returns {@code null} if this list is empty.
725 *
726 * @return the first element of this list, or {@code null}
727 * if this list is empty
728 * @since 1.6
729 */
730 public E peekFirst() {
731 final Node<E> f = first;
732 return (f == null) ? null : f.item;
733 }
734
735 /**
736 * Retrieves, but does not remove, the last element of this list,
737 * or returns {@code null} if this list is empty.
738 *
739 * @return the last element of this list, or {@code null}
740 * if this list is empty
741 * @since 1.6
742 */
743 public E peekLast() {
744 final Node<E> l = last;
745 return (l == null) ? null : l.item;
746 }
747
748 /**
749 * Retrieves and removes the first element of this list,
750 * or returns {@code null} if this list is empty.
751 *
752 * @return the first element of this list, or {@code null} if
753 * this list is empty
754 * @since 1.6
755 */
756 public E pollFirst() {
757 final Node<E> f = first;
758 return (f == null) ? null : unlinkFirst(f);
759 }
760
761 /**
762 * Retrieves and removes the last element of this list,
763 * or returns {@code null} if this list is empty.
764 *
765 * @return the last element of this list, or {@code null} if
766 * this list is empty
767 * @since 1.6
768 */
769 public E pollLast() {
770 final Node<E> l = last;
771 return (l == null) ? null : unlinkLast(l);
772 }
773
774 /**
775 * Pushes an element onto the stack represented by this list. In other
776 * words, inserts the element at the front of this list.
777 *
778 * <p>This method is equivalent to {@link #addFirst}.
779 *
780 * @param e the element to push
781 * @since 1.6
782 */
783 public void push(E e) {
784 addFirst(e);
785 }
786
787 /**
788 * Pops an element from the stack represented by this list. In other
789 * words, removes and returns the first element of this list.
790 *
791 * <p>This method is equivalent to {@link #removeFirst()}.
792 *
793 * @return the element at the front of this list (which is the top
794 * of the stack represented by this list)
795 * @throws NoSuchElementException if this list is empty
796 * @since 1.6
797 */
798 public E pop() {
799 return removeFirst();
800 }
801
802 /**
803 * Removes the first occurrence of the specified element in this
804 * list (when traversing the list from head to tail). If the list
805 * does not contain the element, it is unchanged.
806 *
807 * @param o element to be removed from this list, if present
808 * @return {@code true} if the list contained the specified element
809 * @since 1.6
810 */
811 public boolean removeFirstOccurrence(Object o) {
812 return remove(o);
813 }
814
815 /**
816 * Removes the last occurrence of the specified element in this
817 * list (when traversing the list from head to tail). If the list
818 * does not contain the element, it is unchanged.
819 *
820 * @param o element to be removed from this list, if present
821 * @return {@code true} if the list contained the specified element
822 * @since 1.6
823 */
824 public boolean removeLastOccurrence(Object o) {
825 if (o == null) {
826 for (Node<E> x = last; x != null; x = x.prev) {
827 if (x.item == null) {
828 unlink(x);
829 return true;
830 }
831 }
832 } else {
833 for (Node<E> x = last; x != null; x = x.prev) {
834 if (o.equals(x.item)) {
835 unlink(x);
836 return true;
837 }
838 }
839 }
840 return false;
841 }
842
843 /**
844 * Returns a list-iterator of the elements in this list (in proper
845 * sequence), starting at the specified position in the list.
846 * Obeys the general contract of {@code List.listIterator(int)}.<p>
847 *
848 * The list-iterator is <i>fail-fast</i>: if the list is structurally
849 * modified at any time after the Iterator is created, in any way except
850 * through the list-iterator's own {@code remove} or {@code add}
851 * methods, the list-iterator will throw a
852 * {@code ConcurrentModificationException}. Thus, in the face of
853 * concurrent modification, the iterator fails quickly and cleanly, rather
854 * than risking arbitrary, non-deterministic behavior at an undetermined
855 * time in the future.
856 *
857 * @param index index of the first element to be returned from the
858 * list-iterator (by a call to {@code next})
859 * @return a ListIterator of the elements in this list (in proper
860 * sequence), starting at the specified position in the list
861 * @throws IndexOutOfBoundsException {@inheritDoc}
862 * @see List#listIterator(int)
863 */
864 public ListIterator<E> listIterator(int index) {
865 checkPositionIndex(index);
866 return new ListItr(index);
867 }
868
869 private class ListItr implements ListIterator<E> {
870 private Node<E> lastReturned = null;
871 private Node<E> next;
872 private int nextIndex;
873 private int expectedModCount = modCount;
874
875 ListItr(int index) {
876 // assert isPositionIndex(index);
877 next = (index == size) ? null : node(index);
878 nextIndex = index;
879 }
880
881 public boolean hasNext() {
882 return nextIndex < size;
883 }
884
885 public E next() {
886 checkForComodification();
887 if (!hasNext())
888 throw new NoSuchElementException();
889
890 lastReturned = next;
891 next = next.next;
892 nextIndex++;
893 return lastReturned.item;
894 }
895
896 public boolean hasPrevious() {
897 return nextIndex > 0;
898 }
899
900 public E previous() {
901 checkForComodification();
902 if (!hasPrevious())
903 throw new NoSuchElementException();
904
905 lastReturned = next = (next == null) ? last : next.prev;
906 nextIndex--;
907 return lastReturned.item;
908 }
909
910 public int nextIndex() {
911 return nextIndex;
912 }
913
914 public int previousIndex() {
915 return nextIndex - 1;
916 }
917
918 public void remove() {
919 checkForComodification();
920 if (lastReturned == null)
921 throw new IllegalStateException();
922
923 Node<E> lastNext = lastReturned.next;
924 unlink(lastReturned);
925 if (next == lastReturned)
926 next = lastNext;
927 else
928 nextIndex--;
929 lastReturned = null;
930 expectedModCount++;
931 }
932
933 public void set(E e) {
934 if (lastReturned == null)
935 throw new IllegalStateException();
936 checkForComodification();
937 lastReturned.item = e;
938 }
939
940 public void add(E e) {
941 checkForComodification();
942 lastReturned = null;
943 if (next == null)
944 linkLast(e);
945 else
946 linkBefore(e, next);
947 nextIndex++;
948 expectedModCount++;
949 }
950
951 final void checkForComodification() {
952 if (modCount != expectedModCount)
953 throw new ConcurrentModificationException();
954 }
955 }
956
957 private static class Node<E> {
958 E item;
959 Node<E> next;
960 Node<E> prev;
961
962 Node(Node<E> prev, E element, Node<E> next) {
963 this.item = element;
964 this.next = next;
965 this.prev = prev;
966 }
967 }
968
969 /**
970 * @since 1.6
971 */
972 public Iterator<E> descendingIterator() {
973 return new DescendingIterator();
974 }
975
976 /**
977 * Adapter to provide descending iterators via ListItr.previous
978 */
979 private class DescendingIterator implements Iterator<E> {
980 private final ListItr itr = new ListItr(size());
981 public boolean hasNext() {
982 return itr.hasPrevious();
983 }
984 public E next() {
985 return itr.previous();
986 }
987 public void remove() {
988 itr.remove();
989 }
990 }
991
992 @SuppressWarnings("unchecked")
993 private LinkedList<E> superClone() {
994 try {
995 return (LinkedList<E>) super.clone();
996 } catch (CloneNotSupportedException e) {
997 throw new InternalError(e);
998 }
999 }
1000
1001 /**
1002 * Returns a shallow copy of this {@code LinkedList}. (The elements
1003 * themselves are not cloned.)
1004 *
1005 * @return a shallow copy of this {@code LinkedList} instance
1006 */
1007 public Object clone() {
1008 LinkedList<E> clone = superClone();
1009
1010 // Put clone into "virgin" state
1011 clone.first = clone.last = null;
1012 clone.size = 0;
1013 clone.modCount = 0;
1014
1015 // Initialize clone with our elements
1016 for (Node<E> x = first; x != null; x = x.next)
1017 clone.add(x.item);
1018
1019 return clone;
1020 }
1021
1022 /**
1023 * Returns an array containing all of the elements in this list
1024 * in proper sequence (from first to last element).
1025 *
1026 * <p>The returned array will be "safe" in that no references to it are
1027 * maintained by this list. (In other words, this method must allocate
1028 * a new array). The caller is thus free to modify the returned array.
1029 *
1030 * <p>This method acts as bridge between array-based and collection-based
1031 * APIs.
1032 *
1033 * @return an array containing all of the elements in this list
1034 * in proper sequence
1035 */
1036 public Object[] toArray() {
1037 Object[] result = new Object[size];
1038 int i = 0;
1039 for (Node<E> x = first; x != null; x = x.next)
1040 result[i++] = x.item;
1041 return result;
1042 }
1043
1044 /**
1045 * Returns an array containing all of the elements in this list in
1046 * proper sequence (from first to last element); the runtime type of
1047 * the returned array is that of the specified array. If the list fits
1048 * in the specified array, it is returned therein. Otherwise, a new
1049 * array is allocated with the runtime type of the specified array and
1050 * the size of this list.
1051 *
1052 * <p>If the list fits in the specified array with room to spare (i.e.,
1053 * the array has more elements than the list), the element in the array
1054 * immediately following the end of the list is set to {@code null}.
1055 * (This is useful in determining the length of the list <i>only</i> if
1056 * the caller knows that the list does not contain any null elements.)
1057 *
1058 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1059 * array-based and collection-based APIs. Further, this method allows
1060 * precise control over the runtime type of the output array, and may,
1061 * under certain circumstances, be used to save allocation costs.
1062 *
1063 * <p>Suppose {@code x} is a list known to contain only strings.
1064 * The following code can be used to dump the list into a newly
1065 * allocated array of {@code String}:
1066 *
1067 * <pre>
1068 * String[] y = x.toArray(new String[0]);</pre>
1069 *
1070 * Note that {@code toArray(new Object[0])} is identical in function to
1071 * {@code toArray()}.
1072 *
1073 * @param a the array into which the elements of the list are to
1074 * be stored, if it is big enough; otherwise, a new array of the
1075 * same runtime type is allocated for this purpose.
1076 * @return an array containing the elements of the list
1077 * @throws ArrayStoreException if the runtime type of the specified array
1078 * is not a supertype of the runtime type of every element in
1079 * this list
1080 * @throws NullPointerException if the specified array is null
1081 */
1082 @SuppressWarnings("unchecked")
1083 public <T> T[] toArray(T[] a) {
1084 if (a.length < size)
1085 a = (T[])java.lang.reflect.Array.newInstance(
1086 a.getClass().getComponentType(), size);
1087 int i = 0;
1088 Object[] result = a;
1089 for (Node<E> x = first; x != null; x = x.next)
1090 result[i++] = x.item;
1091
1092 if (a.length > size)
1093 a[size] = null;
1094
1095 return a;
1096 }
1097
1098 private static final long serialVersionUID = 876323262645176354L;
1099
1100 /**
1101 * Saves the state of this {@code LinkedList} instance to a stream
1102 * (that is, serializes it).
1103 *
1104 * @serialData The size of the list (the number of elements it
1105 * contains) is emitted (int), followed by all of its
1106 * elements (each an Object) in the proper order.
1107 */
1108 private void writeObject(java.io.ObjectOutputStream s)
1109 throws java.io.IOException {
1110 // Write out any hidden serialization magic
1111 s.defaultWriteObject();
1112
1113 // Write out size
1114 s.writeInt(size);
1115
1116 // Write out all elements in the proper order.
1117 for (Node<E> x = first; x != null; x = x.next)
1118 s.writeObject(x.item);
1119 }
1120
1121 /**
1122 * Reconstitutes this {@code LinkedList} instance from a stream
1123 * (that is, deserializes it).
1124 */
1125 @SuppressWarnings("unchecked")
1126 private void readObject(java.io.ObjectInputStream s)
1127 throws java.io.IOException, ClassNotFoundException {
1128 // Read in any hidden serialization magic
1129 s.defaultReadObject();
1130
1131 // Read in size
1132 int size = s.readInt();
1133
1134 // Read in all elements in the proper order.
1135 for (int i = 0; i < size; i++)
1136 linkLast((E)s.readObject());
1137 }
1138 }