1 /*
2 * Copyright (c) 1997, 2006, 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 * Linked list implementation of the {@code List} interface.
30 * Implements all optional list operations, and permits all elements
31 * (including {@code null}). In addition to implementing the {@code
32 * List} interface, the {@code LinkedList} class provides uniformly
33 * named methods to {@code get}, {@code remove} and {@code add} an
34 * element at the beginning (<i>operation</i>{@code First}) and end
35 * (<i>operation</i>{@code Last}) of the list. These operations allow
36 * linked lists to be used as a stack, {@linkplain Queue queue}, or
37 * {@linkplain Deque deque} (double-ended queue); by implementing the
38 * {@link Deque} interface, support is included for first-in-first-out
39 * operations for {@code add} and {@code poll}.
40 *
41 * <p>All of the operations perform as could be expected for a doubly-linked
42 * list. Operations that index into the list will traverse the list from
43 * the beginning or the end, whichever is closer to the specified index.
44 *
45 * <p><strong>Note that this implementation is not synchronized.</strong>
46 * If multiple threads access a linked list concurrently, and at least
47 * one of the threads modifies the list structurally, it <i>must</i> be
48 * synchronized externally. (A structural modification is any operation
49 * that adds or deletes one or more elements; merely setting the value of
50 * an element is not a structural modification.) This is typically
51 * accomplished by synchronizing on some object that naturally
52 * encapsulates the list.
53 *
54 * If no such object exists, the list should be "wrapped" using the
55 * {@link Collections#synchronizedList Collections.synchronizedList}
56 * method. This is best done at creation time, to prevent accidental
57 * unsynchronized access to the list:<pre>
58 * List list = Collections.synchronizedList(new LinkedList(...));</pre>
59 *
60 * <p>The iterators returned by this class's {@code iterator} and
61 * {@code listIterator} methods are <i>fail-fast</i>: if the list is
62 * structurally modified at any time after the iterator is created, in
63 * any way except through the Iterator's own {@code remove} or
64 * {@code add} methods, the iterator will throw a {@link
65 * ConcurrentModificationException}. Thus, in the face of concurrent
66 * modification, the iterator fails quickly and cleanly, rather than
67 * risking arbitrary, non-deterministic behavior at an undetermined
68 * time in the future.
69 *
70 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
71 * as it is, generally speaking, impossible to make any hard guarantees in the
72 * presence of unsynchronized concurrent modification. Fail-fast iterators
73 * throw {@code ConcurrentModificationException} on a best-effort basis.
74 * Therefore, it would be wrong to write a program that depended on this
75 * exception for its correctness: <i>the fail-fast behavior of iterators
76 * should be used only to detect bugs.</i>
77 *
78 * <p>This class is a member of the
79 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
80 * Java Collections Framework</a>.
81 *
82 * @author Josh Bloch
83 * @see List
84 * @see ArrayList
85 * @since 1.2
86 * @param <E> the type of elements held in this collection
87 */
88
89 public class LinkedList<E>
90 extends AbstractSequentialList<E>
91 implements List<E>, Deque<E>, Cloneable, java.io.Serializable
92 {
93 transient int size = 0;
94
95 /**
96 * Pointer to first node.
97 * Invariant: (first == null && last == null) ||
98 * (first.prev == null && first.item != null)
99 */
100 transient Node<E> first;
101
102 /**
103 * Pointer to last node.
104 * Invariant: (first == null && last == null) ||
105 * (last.next == null && last.item != null)
106 */
107 transient Node<E> last;
108
109 /**
110 * Constructs an empty list.
111 */
112 public LinkedList() {
113 }
114
115 /**
116 * Constructs a list containing the elements of the specified
117 * collection, in the order they are returned by the collection's
118 * iterator.
119 *
120 * @param c the collection whose elements are to be placed into this list
121 * @throws NullPointerException if the specified collection is null
122 */
123 public LinkedList(Collection<? extends E> c) {
124 this();
125 addAll(c);
126 }
127
128 /**
129 * Links e as first element.
130 */
131 private void linkFirst(E e) {
132 final Node<E> f = first;
133 final Node<E> newNode = new Node<E>(null, e, f);
134 first = newNode;
135 if (f == null)
136 last = newNode;
137 else
138 f.prev = newNode;
139 size++;
140 modCount++;
141 }
142
143 /**
144 * Links e as last element.
145 */
146 void linkLast(E e) {
147 final Node<E> l = last;
148 final Node<E> newNode = new Node<E>(l, e, null);
149 last = newNode;
150 if (l == null)
151 first = newNode;
152 else
153 l.next = newNode;
154 size++;
155 modCount++;
156 }
157
158 /**
159 * Inserts element e before non-null Node succ.
160 */
161 void linkBefore(E e, Node<E> succ) {
162 // assert succ != null;
163 final Node<E> pred = succ.prev;
164 final Node<E> newNode = new Node<E>(pred, e, succ);
165 succ.prev = newNode;
166 if (pred == null)
167 first = newNode;
168 else
169 pred.next = newNode;
170 size++;
171 modCount++;
172 }
173
174 /**
175 * Unlinks non-null first node f.
176 */
177 private E unlinkFirst(Node<E> f) {
178 // assert f == first && f != null;
179 final E element = f.item;
180 final Node<E> next = f.next;
181 f.item = null;
182 f.next = null; // help GC
183 first = next;
184 if (next == null)
185 last = null;
186 else
187 next.prev = null;
188 size--;
189 modCount++;
190 return element;
191 }
192
193 /**
194 * Unlinks non-null last node l.
195 */
196 private E unlinkLast(Node<E> l) {
197 // assert l == last && l != null;
198 final E element = l.item;
199 final Node<E> prev = l.prev;
200 l.item = null;
201 l.prev = null; // help GC
202 last = prev;
203 if (prev == null)
204 first = null;
205 else
206 prev.next = null;
207 size--;
208 modCount++;
209 return element;
210 }
211
212 /**
213 * Unlinks non-null node x.
214 */
215 E unlink(Node<E> x) {
216 // assert x != null;
217 final E element = x.item;
218 final Node<E> next = x.next;
219 final Node<E> prev = x.prev;
220
221 if (prev == null) {
222 first = next;
223 } else {
224 prev.next = next;
225 x.prev = null;
226 }
227
228 if (next == null) {
229 last = prev;
230 } else {
231 next.prev = prev;
232 x.next = null;
233 }
234
235 x.item = null;
236 size--;
237 modCount++;
238 return element;
239 }
240
241 /**
242 * Returns the first element in this list.
243 *
244 * @return the first element in this list
245 * @throws NoSuchElementException if this list is empty
246 */
247 public E getFirst() {
248 final Node<E> f = first;
249 if (f == null)
250 throw new NoSuchElementException();
251 return f.item;
252 }
253
254 /**
255 * Returns the last element in this list.
256 *
257 * @return the last element in this list
258 * @throws NoSuchElementException if this list is empty
259 */
260 public E getLast() {
261 final Node<E> l = last;
262 if (l == null)
263 throw new NoSuchElementException();
264 return l.item;
265 }
266
267 /**
268 * Removes and returns the first element from this list.
269 *
270 * @return the first element from this list
271 * @throws NoSuchElementException if this list is empty
272 */
273 public E removeFirst() {
274 final Node<E> f = first;
275 if (f == null)
276 throw new NoSuchElementException();
277 return unlinkFirst(f);
278 }
279
280 /**
281 * Removes and returns the last element from this list.
282 *
283 * @return the last element from this list
284 * @throws NoSuchElementException if this list is empty
285 */
286 public E removeLast() {
287 final Node<E> l = last;
288 if (l == null)
289 throw new NoSuchElementException();
290 return unlinkLast(l);
291 }
292
293 /**
294 * Inserts the specified element at the beginning of this list.
295 *
296 * @param e the element to add
297 */
298 public void addFirst(E e) {
299 linkFirst(e);
300 }
301
302 /**
303 * Appends the specified element to the end of this list.
304 *
305 * <p>This method is equivalent to {@link #add}.
306 *
307 * @param e the element to add
308 */
309 public void addLast(E e) {
310 linkLast(e);
311 }
312
313 /**
314 * Returns {@code true} if this list contains the specified element.
315 * More formally, returns {@code true} if and only if this list contains
316 * at least one element {@code e} such that
317 * <tt>(o==null ? e==null : o.equals(e))</tt>.
318 *
319 * @param o element whose presence in this list is to be tested
320 * @return {@code true} if this list contains the specified element
321 */
322 public boolean contains(Object o) {
323 return indexOf(o) != -1;
324 }
325
326 /**
327 * Returns the number of elements in this list.
328 *
329 * @return the number of elements in this list
330 */
331 public int size() {
332 return size;
333 }
334
335 /**
336 * Appends the specified element to the end of this list.
337 *
338 * <p>This method is equivalent to {@link #addLast}.
339 *
340 * @param e element to be appended to this list
341 * @return {@code true} (as specified by {@link Collection#add})
342 */
343 public boolean add(E e) {
344 linkLast(e);
345 return true;
346 }
347
348 /**
349 * Removes the first occurrence of the specified element from this list,
350 * if it is present. If this list does not contain the element, it is
351 * unchanged. More formally, removes the element with the lowest index
352 * {@code i} such that
353 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
354 * (if such an element exists). Returns {@code true} if this list
355 * contained the specified element (or equivalently, if this list
356 * changed as a result of the call).
357 *
358 * @param o element to be removed from this list, if present
359 * @return {@code true} if this list contained the specified element
360 */
361 public boolean remove(Object o) {
362 if (o == null) {
363 for (Node<E> x = first; x != null; x = x.next) {
364 if (x.item == null) {
365 unlink(x);
366 return true;
367 }
368 }
369 } else {
370 for (Node<E> x = first; x != null; x = x.next) {
371 if (o.equals(x.item)) {
372 unlink(x);
373 return true;
374 }
375 }
376 }
377 return false;
378 }
379
380 /**
381 * Appends all of the elements in the specified collection to the end of
382 * this list, in the order that they are returned by the specified
383 * collection's iterator. The behavior of this operation is undefined if
384 * the specified collection is modified while the operation is in
385 * progress. (Note that this will occur if the specified collection is
386 * this list, and it's nonempty.)
387 *
388 * @param c collection containing elements to be added to this list
389 * @return {@code true} if this list changed as a result of the call
390 * @throws NullPointerException if the specified collection is null
391 */
392 public boolean addAll(Collection<? extends E> c) {
393 return addAll(size, c);
394 }
395
396 /**
397 * Inserts all of the elements in the specified collection into this
398 * list, starting at the specified position. Shifts the element
399 * currently at that position (if any) and any subsequent elements to
400 * the right (increases their indices). The new elements will appear
401 * in the list in the order that they are returned by the
402 * specified collection's iterator.
403 *
404 * @param index index at which to insert the first element
405 * from the specified collection
406 * @param c collection containing elements to be added to this list
407 * @return {@code true} if this list changed as a result of the call
408 * @throws IndexOutOfBoundsException {@inheritDoc}
409 * @throws NullPointerException if the specified collection is null
410 */
411 public boolean addAll(int index, Collection<? extends E> c) {
412 checkPositionIndex(index);
413
414 Object[] a = c.toArray();
415 int numNew = a.length;
416 if (numNew == 0)
417 return false;
418
419 Node<E> pred, succ;
420 if (index == size) {
421 succ = null;
422 pred = last;
423 } else {
424 succ = node(index);
425 pred = succ.prev;
426 }
427
428 for (Object o : a) {
429 @SuppressWarnings("unchecked") E e = (E) o;
430 Node<E> newNode = new Node<E>(pred, e, null);
431 if (pred == null)
432 first = newNode;
433 else
434 pred.next = newNode;
435 pred = newNode;
436 }
437
438 if (succ == null) {
439 last = pred;
440 } else {
441 pred.next = succ;
442 succ.prev = pred;
443 }
444
445 size += numNew;
446 modCount++;
447 return true;
448 }
449
450 /**
451 * Removes all of the elements from this list.
452 * The list will be empty after this call returns.
453 */
454 public void clear() {
455 // Clearing all of the links between nodes is "unnecessary", but:
456 // - helps a generational GC if the discarded nodes inhabit
457 // more than one generation
458 // - is sure to free memory even if there is a reachable Iterator
459 for (Node<E> x = first; x != null; ) {
460 Node<E> next = x.next;
461 x.item = null;
462 x.next = null;
463 x.prev = null;
464 x = next;
465 }
466 first = last = null;
467 size = 0;
468 modCount++;
469 }
470
471
472 // Positional Access Operations
473
474 /**
475 * Returns the element at the specified position in this list.
476 *
477 * @param index index of the element to return
478 * @return the element at the specified position in this list
479 * @throws IndexOutOfBoundsException {@inheritDoc}
480 */
481 public E get(int index) {
482 checkElementIndex(index);
483 return node(index).item;
484 }
485
486 /**
487 * Replaces the element at the specified position in this list with the
488 * specified element.
489 *
490 * @param index index of the element to replace
491 * @param element element to be stored at the specified position
492 * @return the element previously at the specified position
493 * @throws IndexOutOfBoundsException {@inheritDoc}
494 */
495 public E set(int index, E element) {
496 checkElementIndex(index);
497 Node<E> x = node(index);
498 E oldVal = x.item;
499 x.item = element;
500 return oldVal;
501 }
502
503 /**
504 * Inserts the specified element at the specified position in this list.
505 * Shifts the element currently at that position (if any) and any
506 * subsequent elements to the right (adds one to their indices).
507 *
508 * @param index index at which the specified element is to be inserted
509 * @param element element to be inserted
510 * @throws IndexOutOfBoundsException {@inheritDoc}
511 */
512 public void add(int index, E element) {
513 checkPositionIndex(index);
514
515 if (index == size)
516 linkLast(element);
517 else
518 linkBefore(element, node(index));
519 }
520
521 /**
522 * Removes the element at the specified position in this list. Shifts any
523 * subsequent elements to the left (subtracts one from their indices).
524 * Returns the element that was removed from the list.
525 *
526 * @param index the index of the element to be removed
527 * @return the element previously at the specified position
528 * @throws IndexOutOfBoundsException {@inheritDoc}
529 */
530 public E remove(int index) {
531 checkElementIndex(index);
532 return unlink(node(index));
533 }
534
535 /**
536 * Tells if the argument is the index of an existing element.
537 */
538 private boolean isElementIndex(int index) {
539 return index >= 0 && index < size;
540 }
541
542 /**
543 * Tells if the argument is the index of a valid position for an
544 * iterator or an add operation.
545 */
546 private boolean isPositionIndex(int index) {
547 return index >= 0 && index <= size;
548 }
549
550 /**
551 * Constructs an IndexOutOfBoundsException detail message.
552 * Of the many possible refactorings of the error handling code,
553 * this "outlining" performs best with both server and client VMs.
554 */
555 private String outOfBoundsMsg(int index) {
556 return "Index: "+index+", Size: "+size;
557 }
558
559 private void checkElementIndex(int index) {
560 if (!isElementIndex(index))
561 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
562 }
563
564 private void checkPositionIndex(int index) {
565 if (!isPositionIndex(index))
566 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
567 }
568
569 /**
570 * Returns the (non-null) Node at the specified element index.
571 */
572 Node<E> node(int index) {
573 // assert isElementIndex(index);
574
575 if (index < (size >> 1)) {
576 Node<E> x = first;
577 for (int i = 0; i < index; i++)
578 x = x.next;
579 return x;
580 } else {
581 Node<E> x = last;
582 for (int i = size - 1; i > index; i--)
583 x = x.prev;
584 return x;
585 }
586 }
587
588 // Search Operations
589
590 /**
591 * Returns the index of the first occurrence of the specified element
592 * in this list, or -1 if this list does not contain the element.
593 * More formally, returns the lowest index {@code i} such that
594 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
595 * or -1 if there is no such index.
596 *
597 * @param o element to search for
598 * @return the index of the first occurrence of the specified element in
599 * this list, or -1 if this list does not contain the element
600 */
601 public int indexOf(Object o) {
602 int index = 0;
603 if (o == null) {
604 for (Node<E> x = first; x != null; x = x.next) {
605 if (x.item == null)
606 return index;
607 index++;
608 }
609 } else {
610 for (Node<E> x = first; x != null; x = x.next) {
611 if (o.equals(x.item))
612 return index;
613 index++;
614 }
615 }
616 return -1;
617 }
618
619 /**
620 * Returns the index of the last occurrence of the specified element
621 * in this list, or -1 if this list does not contain the element.
622 * More formally, returns the highest index {@code i} such that
623 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
624 * or -1 if there is no such index.
625 *
626 * @param o element to search for
627 * @return the index of the last occurrence of the specified element in
628 * this list, or -1 if this list does not contain the element
629 */
630 public int lastIndexOf(Object o) {
631 int index = size;
632 if (o == null) {
633 for (Node<E> x = last; x != null; x = x.prev) {
634 index--;
635 if (x.item == null)
636 return index;
637 }
638 } else {
639 for (Node<E> x = last; x != null; x = x.prev) {
640 index--;
641 if (o.equals(x.item))
642 return index;
643 }
644 }
645 return -1;
646 }
647
648 // Queue operations.
649
650 /**
651 * Retrieves, but does not remove, the head (first element) of this list.
652 *
653 * @return the head of this list, or {@code null} if this list is empty
654 * @since 1.5
655 */
656 public E peek() {
657 final Node<E> f = first;
658 return (f == null) ? null : f.item;
659 }
660
661 /**
662 * Retrieves, but does not remove, the head (first element) of this list.
663 *
664 * @return the head of this list
665 * @throws NoSuchElementException if this list is empty
666 * @since 1.5
667 */
668 public E element() {
669 return getFirst();
670 }
671
672 /**
673 * Retrieves and removes the head (first element) of this list.
674 *
675 * @return the head of this list, or {@code null} if this list is empty
676 * @since 1.5
677 */
678 public E poll() {
679 final Node<E> f = first;
680 return (f == null) ? null : unlinkFirst(f);
681 }
682
683 /**
684 * Retrieves and removes the head (first element) of this list.
685 *
686 * @return the head of this list
687 * @throws NoSuchElementException if this list is empty
688 * @since 1.5
689 */
690 public E remove() {
691 return removeFirst();
692 }
693
694 /**
695 * Adds the specified element as the tail (last element) of this list.
696 *
697 * @param e the element to add
698 * @return {@code true} (as specified by {@link Queue#offer})
699 * @since 1.5
700 */
701 public boolean offer(E e) {
702 return add(e);
703 }
704
705 // Deque operations
706 /**
707 * Inserts the specified element at the front of this list.
708 *
709 * @param e the element to insert
710 * @return {@code true} (as specified by {@link Deque#offerFirst})
711 * @since 1.6
712 */
713 public boolean offerFirst(E e) {
714 addFirst(e);
715 return true;
716 }
717
718 /**
719 * Inserts the specified element at the end of this list.
720 *
721 * @param e the element to insert
722 * @return {@code true} (as specified by {@link Deque#offerLast})
723 * @since 1.6
724 */
725 public boolean offerLast(E e) {
726 addLast(e);
727 return true;
728 }
729
730 /**
731 * Retrieves, but does not remove, the first element of this list,
732 * or returns {@code null} if this list is empty.
733 *
734 * @return the first element of this list, or {@code null}
735 * if this list is empty
736 * @since 1.6
737 */
738 public E peekFirst() {
739 final Node<E> f = first;
740 return (f == null) ? null : f.item;
741 }
742
743 /**
744 * Retrieves, but does not remove, the last element of this list,
745 * or returns {@code null} if this list is empty.
746 *
747 * @return the last element of this list, or {@code null}
748 * if this list is empty
749 * @since 1.6
750 */
751 public E peekLast() {
752 final Node<E> l = last;
753 return (l == null) ? null : l.item;
754 }
755
756 /**
757 * Retrieves and removes the first element of this list,
758 * or returns {@code null} if this list is empty.
759 *
760 * @return the first element of this list, or {@code null} if
761 * this list is empty
762 * @since 1.6
763 */
764 public E pollFirst() {
765 final Node<E> f = first;
766 return (f == null) ? null : unlinkFirst(f);
767 }
768
769 /**
770 * Retrieves and removes the last element of this list,
771 * or returns {@code null} if this list is empty.
772 *
773 * @return the last element of this list, or {@code null} if
774 * this list is empty
775 * @since 1.6
776 */
777 public E pollLast() {
778 final Node<E> l = last;
779 return (l == null) ? null : unlinkLast(l);
780 }
781
782 /**
783 * Pushes an element onto the stack represented by this list. In other
784 * words, inserts the element at the front of this list.
785 *
786 * <p>This method is equivalent to {@link #addFirst}.
787 *
788 * @param e the element to push
789 * @since 1.6
790 */
791 public void push(E e) {
792 addFirst(e);
793 }
794
795 /**
796 * Pops an element from the stack represented by this list. In other
797 * words, removes and returns the first element of this list.
798 *
799 * <p>This method is equivalent to {@link #removeFirst()}.
800 *
801 * @return the element at the front of this list (which is the top
802 * of the stack represented by this list)
803 * @throws NoSuchElementException if this list is empty
804 * @since 1.6
805 */
806 public E pop() {
807 return removeFirst();
808 }
809
810 /**
811 * Removes the first occurrence of the specified element in this
812 * list (when traversing the list from head to tail). If the list
813 * does not contain the element, it is unchanged.
814 *
815 * @param o element to be removed from this list, if present
816 * @return {@code true} if the list contained the specified element
817 * @since 1.6
818 */
819 public boolean removeFirstOccurrence(Object o) {
820 return remove(o);
821 }
822
823 /**
824 * Removes the last occurrence of the specified element in this
825 * list (when traversing the list from head to tail). If the list
826 * does not contain the element, it is unchanged.
827 *
828 * @param o element to be removed from this list, if present
829 * @return {@code true} if the list contained the specified element
830 * @since 1.6
831 */
832 public boolean removeLastOccurrence(Object o) {
833 if (o == null) {
834 for (Node<E> x = last; x != null; x = x.prev) {
835 if (x.item == null) {
836 unlink(x);
837 return true;
838 }
839 }
840 } else {
841 for (Node<E> x = last; x != null; x = x.prev) {
842 if (o.equals(x.item)) {
843 unlink(x);
844 return true;
845 }
846 }
847 }
848 return false;
849 }
850
851 /**
852 * Returns a list-iterator of the elements in this list (in proper
853 * sequence), starting at the specified position in the list.
854 * Obeys the general contract of {@code List.listIterator(int)}.<p>
855 *
856 * The list-iterator is <i>fail-fast</i>: if the list is structurally
857 * modified at any time after the Iterator is created, in any way except
858 * through the list-iterator's own {@code remove} or {@code add}
859 * methods, the list-iterator will throw a
860 * {@code ConcurrentModificationException}. Thus, in the face of
861 * concurrent modification, the iterator fails quickly and cleanly, rather
862 * than risking arbitrary, non-deterministic behavior at an undetermined
863 * time in the future.
864 *
865 * @param index index of the first element to be returned from the
866 * list-iterator (by a call to {@code next})
867 * @return a ListIterator of the elements in this list (in proper
868 * sequence), starting at the specified position in the list
869 * @throws IndexOutOfBoundsException {@inheritDoc}
870 * @see List#listIterator(int)
871 */
872 public ListIterator<E> listIterator(int index) {
873 checkPositionIndex(index);
874 return new ListItr(index);
875 }
876
877 private class ListItr implements ListIterator<E> {
878 private Node<E> lastReturned = null;
879 private Node<E> next;
880 private int nextIndex;
881 private int expectedModCount = modCount;
882
883 ListItr(int index) {
884 // assert isPositionIndex(index);
885 next = (index == size) ? null : node(index);
886 nextIndex = index;
887 }
888
889 public boolean hasNext() {
890 return nextIndex < size;
891 }
892
893 public E next() {
894 checkForComodification();
895 if (!hasNext())
896 throw new NoSuchElementException();
897
898 lastReturned = next;
899 next = next.next;
900 nextIndex++;
901 return lastReturned.item;
902 }
903
904 public boolean hasPrevious() {
905 return nextIndex > 0;
906 }
907
908 public E previous() {
909 checkForComodification();
910 if (!hasPrevious())
911 throw new NoSuchElementException();
912
913 lastReturned = next = (next == null) ? last : next.prev;
914 nextIndex--;
915 return lastReturned.item;
916 }
917
918 public int nextIndex() {
919 return nextIndex;
920 }
921
922 public int previousIndex() {
923 return nextIndex - 1;
924 }
925
926 public void remove() {
927 checkForComodification();
928 if (lastReturned == null)
929 throw new IllegalStateException();
930
931 Node<E> lastNext = lastReturned.next;
932 unlink(lastReturned);
933 if (next == lastReturned)
934 next = lastNext;
935 else
936 nextIndex--;
937 lastReturned = null;
938 expectedModCount++;
939 }
940
941 public void set(E e) {
942 if (lastReturned == null)
943 throw new IllegalStateException();
944 checkForComodification();
945 lastReturned.item = e;
946 }
947
948 public void add(E e) {
949 checkForComodification();
950 lastReturned = null;
951 if (next == null)
952 linkLast(e);
953 else
954 linkBefore(e, next);
955 nextIndex++;
956 expectedModCount++;
957 }
958
959 final void checkForComodification() {
960 if (modCount != expectedModCount)
961 throw new ConcurrentModificationException();
962 }
963 }
964
965 private static class Node<E> {
966 E item;
967 Node<E> next;
968 Node<E> prev;
969
970 Node(Node<E> prev, E element, Node<E> next) {
971 this.item = element;
972 this.next = next;
973 this.prev = prev;
974 }
975 }
976
977 /**
978 * @since 1.6
979 */
980 public Iterator<E> descendingIterator() {
981 return new DescendingIterator();
982 }
983
984 /**
985 * Adapter to provide descending iterators via ListItr.previous
986 */
987 private class DescendingIterator implements Iterator<E> {
988 private final ListItr itr = new ListItr(size());
989 public boolean hasNext() {
990 return itr.hasPrevious();
991 }
992 public E next() {
993 return itr.previous();
994 }
995 public void remove() {
996 itr.remove();
997 }
998 }
999
1000 @SuppressWarnings("unchecked")
1001 private LinkedList<E> superClone() {
1002 try {
1003 return (LinkedList<E>) super.clone();
1004 } catch (CloneNotSupportedException e) {
1005 throw new InternalError();
1006 }
1007 }
1008
1009 /**
1010 * Returns a shallow copy of this {@code LinkedList}. (The elements
1011 * themselves are not cloned.)
1012 *
1013 * @return a shallow copy of this {@code LinkedList} instance
1014 */
1015 public Object clone() {
1016 LinkedList<E> clone = superClone();
1017
1018 // Put clone into "virgin" state
1019 clone.first = clone.last = null;
1020 clone.size = 0;
1021 clone.modCount = 0;
1022
1023 // Initialize clone with our elements
1024 for (Node<E> x = first; x != null; x = x.next)
1025 clone.add(x.item);
1026
1027 return clone;
1028 }
1029
1030 /**
1031 * Returns an array containing all of the elements in this list
1032 * in proper sequence (from first to last element).
1033 *
1034 * <p>The returned array will be "safe" in that no references to it are
1035 * maintained by this list. (In other words, this method must allocate
1036 * a new array). The caller is thus free to modify the returned array.
1037 *
1038 * <p>This method acts as bridge between array-based and collection-based
1039 * APIs.
1040 *
1041 * @return an array containing all of the elements in this list
1042 * in proper sequence
1043 */
1044 public Object[] toArray() {
1045 Object[] result = new Object[size];
1046 int i = 0;
1047 for (Node<E> x = first; x != null; x = x.next)
1048 result[i++] = x.item;
1049 return result;
1050 }
1051
1052 /**
1053 * Returns an array containing all of the elements in this list in
1054 * proper sequence (from first to last element); the runtime type of
1055 * the returned array is that of the specified array. If the list fits
1056 * in the specified array, it is returned therein. Otherwise, a new
1057 * array is allocated with the runtime type of the specified array and
1058 * the size of this list.
1059 *
1060 * <p>If the list fits in the specified array with room to spare (i.e.,
1061 * the array has more elements than the list), the element in the array
1062 * immediately following the end of the list is set to {@code null}.
1063 * (This is useful in determining the length of the list <i>only</i> if
1064 * the caller knows that the list does not contain any null elements.)
1065 *
1066 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1067 * array-based and collection-based APIs. Further, this method allows
1068 * precise control over the runtime type of the output array, and may,
1069 * under certain circumstances, be used to save allocation costs.
1070 *
1071 * <p>Suppose {@code x} is a list known to contain only strings.
1072 * The following code can be used to dump the list into a newly
1073 * allocated array of {@code String}:
1074 *
1075 * <pre>
1076 * String[] y = x.toArray(new String[0]);</pre>
1077 *
1078 * Note that {@code toArray(new Object[0])} is identical in function to
1079 * {@code toArray()}.
1080 *
1081 * @param a the array into which the elements of the list are to
1082 * be stored, if it is big enough; otherwise, a new array of the
1083 * same runtime type is allocated for this purpose.
1084 * @return an array containing the elements of the list
1085 * @throws ArrayStoreException if the runtime type of the specified array
1086 * is not a supertype of the runtime type of every element in
1087 * this list
1088 * @throws NullPointerException if the specified array is null
1089 */
1090 @SuppressWarnings("unchecked")
1091 public <T> T[] toArray(T[] a) {
1092 if (a.length < size)
1093 a = (T[])java.lang.reflect.Array.newInstance(
1094 a.getClass().getComponentType(), size);
1095 int i = 0;
1096 Object[] result = a;
1097 for (Node<E> x = first; x != null; x = x.next)
1098 result[i++] = x.item;
1099
1100 if (a.length > size)
1101 a[size] = null;
1102
1103 return a;
1104 }
1105
1106 private static final long serialVersionUID = 876323262645176354L;
1107
1108 /**
1109 * Saves the state of this {@code LinkedList} instance to a stream
1110 * (that is, serializes it).
1111 *
1112 * @serialData The size of the list (the number of elements it
1113 * contains) is emitted (int), followed by all of its
1114 * elements (each an Object) in the proper order.
1115 */
1116 private void writeObject(java.io.ObjectOutputStream s)
1117 throws java.io.IOException {
1118 // Write out any hidden serialization magic
1119 s.defaultWriteObject();
1120
1121 // Write out size
1122 s.writeInt(size);
1123
1124 // Write out all elements in the proper order.
1125 for (Node<E> x = first; x != null; x = x.next)
1126 s.writeObject(x.item);
1127 }
1128
1129 /**
1130 * Reconstitutes this {@code LinkedList} instance from a stream
1131 * (that is, deserializes it).
1132 */
1133 @SuppressWarnings("unchecked")
1134 private void readObject(java.io.ObjectInputStream s)
1135 throws java.io.IOException, ClassNotFoundException {
1136 // Read in any hidden serialization magic
1137 s.defaultReadObject();
1138
1139 // Read in size
1140 int size = s.readInt();
1141
1142 // Read in all elements in the proper order.
1143 for (int i = 0; i < size; i++)
1144 linkLast((E)s.readObject());
1145 }
1146 }