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. Implements all
30 * optional list operations, and permits all elements (including
31 * {@code null}). In addition to implementing the {@code List} interface,
32 * the {@code LinkedList} class provides uniformly named methods to
33 * {@code get}, {@code remove} and {@code insert} an element at the
34 * beginning and end of the list. These operations allow linked lists to be
35 * used as a stack, {@linkplain Queue queue}, or {@linkplain Deque
36 * double-ended queue}.
37 *
38 * <p>The class implements the {@code Deque} interface, providing
39 * first-in-first-out queue operations for {@code add},
40 * {@code poll}, along with other stack and deque operations.
41 *
42 * <p>All of the operations perform as could be expected for a doubly-linked
43 * list. Operations that index into the list will traverse the list from
44 * the beginning or the end, whichever is closer to the specified index.
45 *
46 * <p><strong>Note that this implementation is not synchronized.</strong>
47 * If multiple threads access a linked list concurrently, and at least
48 * one of the threads modifies the list structurally, it <i>must</i> be
49 * synchronized externally. (A structural modification is any operation
50 * that adds or deletes one or more elements; merely setting the value of
51 * an element is not a structural modification.) This is typically
52 * accomplished by synchronizing on some object that naturally
53 * encapsulates the list.
54 *
55 * If no such object exists, the list should be "wrapped" using the
56 * {@link Collections#synchronizedList Collections.synchronizedList}
57 * method. This is best done at creation time, to prevent accidental
58 * unsynchronized access to the list:<pre>
59 * List list = Collections.synchronizedList(new LinkedList(...));</pre>
60 *
61 * <p>The iterators returned by this class's {@code iterator} and
62 * {@code listIterator} methods are <i>fail-fast</i>: if the list is
63 * structurally modified at any time after the iterator is created, in
64 * any way except through the Iterator's own {@code remove} or
65 * {@code add} methods, the iterator will throw a {@link
66 * ConcurrentModificationException}. Thus, in the face of concurrent
67 * modification, the iterator fails quickly and cleanly, rather than
68 * risking arbitrary, non-deterministic behavior at an undetermined
69 * time in the future.
70 *
71 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
72 * as it is, generally speaking, impossible to make any hard guarantees in the
73 * presence of unsynchronized concurrent modification. Fail-fast iterators
74 * throw {@code ConcurrentModificationException} on a best-effort basis.
75 * Therefore, it would be wrong to write a program that depended on this
76 * exception for its correctness: <i>the fail-fast behavior of iterators
77 * should be used only to detect bugs.</i>
78 *
79 * <p>This class is a member of the
80 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
81 * Java Collections Framework</a>.
82 *
83 * @author Josh Bloch
84 * @see List
85 * @see ArrayList
86 * @since 1.2
87 * @param <E> the type of elements held in this collection
88 */
89
90 public class LinkedList<E>
91 extends AbstractSequentialList<E>
92 implements List<E>, Deque<E>, Cloneable, java.io.Serializable
93 {
94 transient int size = 0;
95
96 /**
97 * Pointer to first node.
98 * Invariant: (first == null && last == null) ||
99 * (first.prev == null && first.item != null)
100 */
101 transient Node<E> first;
102
103 /**
104 * Pointer to last node.
105 * Invariant: (first == null && last == null) ||
106 * (last.next == null && last.item != null)
107 */
108 transient Node<E> last;
109
110 /**
111 * Constructs an empty list.
112 */
113 public LinkedList() {
114 }
115
116 /**
117 * Constructs a list containing the elements of the specified
118 * collection, in the order they are returned by the collection's
119 * iterator.
120 *
121 * @param c the collection whose elements are to be placed into this list
122 * @throws NullPointerException if the specified collection is null
123 */
124 public LinkedList(Collection<? extends E> c) {
125 this();
126 addAll(c);
127 }
128
129 /**
130 * Links e as first element.
131 */
132 private void linkFirst(E e) {
133 final Node<E> f = first;
134 final Node<E> newNode = new Node<E>(null, e, f);
135 first = newNode;
136 if (f == null)
137 last = newNode;
138 else
139 f.prev = newNode;
140 size++;
141 modCount++;
142 }
143
144 /**
145 * Links e as last element.
146 */
147 void linkLast(E e) {
148 final Node<E> l = last;
149 final Node<E> newNode = new Node<E>(l, e, null);
150 last = newNode;
151 if (l == null)
152 first = newNode;
153 else
154 l.next = newNode;
155 size++;
156 modCount++;
157 }
158
159 /**
160 * Inserts element e before non-null Node succ.
161 */
162 void linkBefore(E e, Node<E> succ) {
163 // assert succ != null;
164 final Node<E> pred = succ.prev;
165 final Node<E> newNode = new Node<E>(pred, e, succ);
166 succ.prev = newNode;
167 if (pred == null)
168 first = newNode;
169 else
170 pred.next = newNode;
171 size++;
172 modCount++;
173 }
174
175 /**
176 * Unlinks non-null first node f.
177 */
178 private E unlinkFirst(Node<E> f) {
179 // assert f == first && f != null;
180 final E element = f.item;
181 final Node<E> next = f.next;
182 f.item = null;
183 f.next = null; // help GC
184 first = next;
185 if (next == null)
186 last = null;
187 else
188 next.prev = null;
189 size--;
190 modCount++;
191 return element;
192 }
193
194 /**
195 * Unlinks non-null last node l.
196 */
197 private E unlinkLast(Node<E> l) {
198 // assert l == last && l != null;
199 final E element = l.item;
200 final Node<E> prev = l.prev;
201 l.item = null;
202 l.prev = null; // help GC
203 last = prev;
204 if (prev == null)
205 first = null;
206 else
207 prev.next = null;
208 size--;
209 modCount++;
210 return element;
211 }
212
213 /**
214 * Unlinks non-null node x.
215 */
216 E unlink(Node<E> x) {
217 // assert x != null;
218 final E element = x.item;
219 final Node<E> next = x.next;
220 final Node<E> prev = x.prev;
221
222 if (prev == null) {
223 first = next;
224 } else {
225 prev.next = next;
226 x.prev = null;
227 }
228
229 if (next == null) {
230 last = prev;
231 } else {
232 next.prev = prev;
233 x.next = null;
234 }
235
236 x.item = null;
237 size--;
238 modCount++;
239 return element;
240 }
241
242 /**
243 * Returns the first element in this list.
244 *
245 * @return the first element in this list
246 * @throws NoSuchElementException if this list is empty
247 */
248 public E getFirst() {
249 final Node<E> f = first;
250 if (f == null)
251 throw new NoSuchElementException();
252 return f.item;
253 }
254
255 /**
256 * Returns the last element in this list.
257 *
258 * @return the last element in this list
259 * @throws NoSuchElementException if this list is empty
260 */
261 public E getLast() {
262 final Node<E> l = last;
263 if (l == null)
264 throw new NoSuchElementException();
265 return l.item;
266 }
267
268 /**
269 * Removes and returns the first element from this list.
270 *
271 * @return the first element from this list
272 * @throws NoSuchElementException if this list is empty
273 */
274 public E removeFirst() {
275 final Node<E> f = first;
276 if (f == null)
277 throw new NoSuchElementException();
278 return unlinkFirst(f);
279 }
280
281 /**
282 * Removes and returns the last element from this list.
283 *
284 * @return the last element from this list
285 * @throws NoSuchElementException if this list is empty
286 */
287 public E removeLast() {
288 final Node<E> l = last;
289 if (l == null)
290 throw new NoSuchElementException();
291 return unlinkLast(l);
292 }
293
294 /**
295 * Inserts the specified element at the beginning of this list.
296 *
297 * @param e the element to add
298 */
299 public void addFirst(E e) {
300 linkFirst(e);
301 }
302
303 /**
304 * Appends the specified element to the end of this list.
305 *
306 * <p>This method is equivalent to {@link #add}.
307 *
308 * @param e the element to add
309 */
310 public void addLast(E e) {
311 linkLast(e);
312 }
313
314 /**
315 * Returns {@code true} if this list contains the specified element.
316 * More formally, returns {@code true} if and only if this list contains
317 * at least one element {@code e} such that
318 * <tt>(o==null ? e==null : o.equals(e))</tt>.
319 *
320 * @param o element whose presence in this list is to be tested
321 * @return {@code true} if this list contains the specified element
322 */
323 public boolean contains(Object o) {
324 return indexOf(o) != -1;
325 }
326
327 /**
328 * Returns the number of elements in this list.
329 *
330 * @return the number of elements in this list
331 */
332 public int size() {
333 return size;
334 }
335
336 /**
337 * Appends the specified element to the end of this list.
338 *
339 * <p>This method is equivalent to {@link #addLast}.
340 *
341 * @param e element to be appended to this list
342 * @return {@code true} (as specified by {@link Collection#add})
343 */
344 public boolean add(E e) {
345 linkLast(e);
346 return true;
347 }
348
349 /**
350 * Removes the first occurrence of the specified element from this list,
351 * if it is present. If this list does not contain the element, it is
352 * unchanged. More formally, removes the element with the lowest index
353 * {@code i} such that
354 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
355 * (if such an element exists). Returns {@code true} if this list
356 * contained the specified element (or equivalently, if this list
357 * changed as a result of the call).
358 *
359 * @param o element to be removed from this list, if present
360 * @return {@code true} if this list contained the specified element
361 */
362 public boolean remove(Object o) {
363 if (o == null) {
364 for (Node<E> x = first; x != null; x = x.next) {
365 if (x.item == null) {
366 unlink(x);
367 return true;
368 }
369 }
370 } else {
371 for (Node<E> x = first; x != null; x = x.next) {
372 if (o.equals(x.item)) {
373 unlink(x);
374 return true;
375 }
376 }
377 }
378 return false;
379 }
380
381 /**
382 * Appends all of the elements in the specified collection to the end of
383 * this list, in the order that they are returned by the specified
384 * collection's iterator. The behavior of this operation is undefined if
385 * the specified collection is modified while the operation is in
386 * progress. (Note that this will occur if the specified collection is
387 * this list, and it's nonempty.)
388 *
389 * @param c collection containing elements to be added to this list
390 * @return {@code true} if this list changed as a result of the call
391 * @throws NullPointerException if the specified collection is null
392 */
393 public boolean addAll(Collection<? extends E> c) {
394 return addAll(size, c);
395 }
396
397 /**
398 * Inserts all of the elements in the specified collection into this
399 * list, starting at the specified position. Shifts the element
400 * currently at that position (if any) and any subsequent elements to
401 * the right (increases their indices). The new elements will appear
402 * in the list in the order that they are returned by the
403 * specified collection's iterator.
404 *
405 * @param index index at which to insert the first element
406 * from the specified collection
407 * @param c collection containing elements to be added to this list
408 * @return {@code true} if this list changed as a result of the call
409 * @throws IndexOutOfBoundsException {@inheritDoc}
410 * @throws NullPointerException if the specified collection is null
411 */
412 public boolean addAll(int index, Collection<? extends E> c) {
413 checkPositionIndex(index);
414
415 Object[] a = c.toArray();
416 int numNew = a.length;
417 if (numNew == 0)
418 return false;
419
420 Node<E> pred, succ;
421 if (index == size) {
422 succ = null;
423 pred = last;
424 } else {
425 succ = node(index);
426 pred = succ.prev;
427 }
428
429 for (Object o : a) {
430 @SuppressWarnings("unchecked") E e = (E) o;
431 Node<E> newNode = new Node<E>(pred, e, null);
432 if (pred == null)
433 first = newNode;
434 else
435 pred.next = newNode;
436 pred = newNode;
437 }
438
439 if (succ == null) {
440 last = pred;
441 } else {
442 pred.next = succ;
443 succ.prev = pred;
444 }
445
446 size += numNew;
447 modCount++;
448 return true;
449 }
450
451 /**
452 * Removes all of the elements from this list.
453 * The list will be empty after this call returns.
454 */
455 public void clear() {
456 // Clearing all of the links between nodes is "unnecessary", but:
457 // - helps a generational GC if the discarded nodes inhabit
458 // more than one generation
459 // - is sure to free memory even if there is a reachable Iterator
460 for (Node<E> x = first; x != null; ) {
461 Node<E> next = x.next;
462 x.item = null;
463 x.next = null;
464 x.prev = null;
465 x = next;
466 }
467 first = last = null;
468 size = 0;
469 modCount++;
470 }
471
472
473 // Positional Access Operations
474
475 /**
476 * Returns the element at the specified position in this list.
477 *
478 * @param index index of the element to return
479 * @return the element at the specified position in this list
480 * @throws IndexOutOfBoundsException {@inheritDoc}
481 */
482 public E get(int index) {
483 checkElementIndex(index);
484 return node(index).item;
485 }
486
487 /**
488 * Replaces the element at the specified position in this list with the
489 * specified element.
490 *
491 * @param index index of the element to replace
492 * @param element element to be stored at the specified position
493 * @return the element previously at the specified position
494 * @throws IndexOutOfBoundsException {@inheritDoc}
495 */
496 public E set(int index, E element) {
497 checkElementIndex(index);
498 Node<E> x = node(index);
499 E oldVal = x.item;
500 x.item = element;
501 return oldVal;
502 }
503
504 /**
505 * Inserts the specified element at the specified position in this list.
506 * Shifts the element currently at that position (if any) and any
507 * subsequent elements to the right (adds one to their indices).
508 *
509 * @param index index at which the specified element is to be inserted
510 * @param element element to be inserted
511 * @throws IndexOutOfBoundsException {@inheritDoc}
512 */
513 public void add(int index, E element) {
514 checkPositionIndex(index);
515
516 if (index == size)
517 linkLast(element);
518 else
519 linkBefore(element, node(index));
520 }
521
522 /**
523 * Removes the element at the specified position in this list. Shifts any
524 * subsequent elements to the left (subtracts one from their indices).
525 * Returns the element that was removed from the list.
526 *
527 * @param index the index of the element to be removed
528 * @return the element previously at the specified position
529 * @throws IndexOutOfBoundsException {@inheritDoc}
530 */
531 public E remove(int index) {
532 checkElementIndex(index);
533 return unlink(node(index));
534 }
535
536 /**
537 * Tells if the argument is the index of an existing element.
538 */
539 private boolean isElementIndex(int index) {
540 return index >= 0 && index < size;
541 }
542
543 /**
544 * Tells if the argument is the index of a valid position for an
545 * iterator or an add operation.
546 */
547 private boolean isPositionIndex(int index) {
548 return index >= 0 && index <= size;
549 }
550
551 /**
552 * Constructs an IndexOutOfBoundsException detail message.
553 * Of the many possible refactorings of the error handling code,
554 * this "outlining" performs best with both server and client VMs.
555 */
556 private String outOfBoundsMsg(int index) {
557 return "Index: "+index+", Size: "+size;
558 }
559
560 private void checkElementIndex(int index) {
561 if (!isElementIndex(index))
562 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
563 }
564
565 private void checkPositionIndex(int index) {
566 if (!isPositionIndex(index))
567 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
568 }
569
570 /**
571 * Returns the (non-null) Node at the specified element index.
572 */
573 Node<E> node(int index) {
574 // assert isElementIndex(index);
575
576 if (index < (size >> 1)) {
577 Node<E> x = first;
578 for (int i = 0; i < index; i++)
579 x = x.next;
580 return x;
581 } else {
582 Node<E> x = last;
583 for (int i = size - 1; i > index; i--)
584 x = x.prev;
585 return x;
586 }
587 }
588
589 // Search Operations
590
591 /**
592 * Returns the index of the first occurrence of the specified element
593 * in this list, or -1 if this list does not contain the element.
594 * More formally, returns the lowest index {@code i} such that
595 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
596 * or -1 if there is no such index.
597 *
598 * @param o element to search for
599 * @return the index of the first occurrence of the specified element in
600 * this list, or -1 if this list does not contain the element
601 */
602 public int indexOf(Object o) {
603 int index = 0;
604 if (o == null) {
605 for (Node<E> x = first; x != null; x = x.next) {
606 if (x.item == null)
607 return index;
608 index++;
609 }
610 } else {
611 for (Node<E> x = first; x != null; x = x.next) {
612 if (o.equals(x.item))
613 return index;
614 index++;
615 }
616 }
617 return -1;
618 }
619
620 /**
621 * Returns the index of the last occurrence of the specified element
622 * in this list, or -1 if this list does not contain the element.
623 * More formally, returns the highest index {@code i} such that
624 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
625 * or -1 if there is no such index.
626 *
627 * @param o element to search for
628 * @return the index of the last occurrence of the specified element in
629 * this list, or -1 if this list does not contain the element
630 */
631 public int lastIndexOf(Object o) {
632 int index = size;
633 if (o == null) {
634 for (Node<E> x = last; x != null; x = x.prev) {
635 index--;
636 if (x.item == null)
637 return index;
638 }
639 } else {
640 for (Node<E> x = last; x != null; x = x.prev) {
641 index--;
642 if (o.equals(x.item))
643 return index;
644 }
645 }
646 return -1;
647 }
648
649 // Queue operations.
650
651 /**
652 * Retrieves, but does not remove, the head (first element) of this list.
653 *
654 * @return the head of this list, or {@code null} if this list is empty
655 * @since 1.5
656 */
657 public E peek() {
658 final Node<E> f = first;
659 return (f == null) ? null : f.item;
660 }
661
662 /**
663 * Retrieves, but does not remove, the head (first element) of this list.
664 *
665 * @return the head of this list
666 * @throws NoSuchElementException if this list is empty
667 * @since 1.5
668 */
669 public E element() {
670 return getFirst();
671 }
672
673 /**
674 * Retrieves and removes the head (first element) of this list.
675 *
676 * @return the head of this list, or {@code null} if this list is empty
677 * @since 1.5
678 */
679 public E poll() {
680 final Node<E> f = first;
681 return (f == null) ? null : unlinkFirst(f);
682 }
683
684 /**
685 * Retrieves and removes the head (first element) of this list.
686 *
687 * @return the head of this list
688 * @throws NoSuchElementException if this list is empty
689 * @since 1.5
690 */
691 public E remove() {
692 return removeFirst();
693 }
694
695 /**
696 * Adds the specified element as the tail (last element) of this list.
697 *
698 * @param e the element to add
699 * @return {@code true} (as specified by {@link Queue#offer})
700 * @since 1.5
701 */
702 public boolean offer(E e) {
703 return add(e);
704 }
705
706 // Deque operations
707 /**
708 * Inserts the specified element at the front of this list.
709 *
710 * @param e the element to insert
711 * @return {@code true} (as specified by {@link Deque#offerFirst})
712 * @since 1.6
713 */
714 public boolean offerFirst(E e) {
715 addFirst(e);
716 return true;
717 }
718
719 /**
720 * Inserts the specified element at the end of this list.
721 *
722 * @param e the element to insert
723 * @return {@code true} (as specified by {@link Deque#offerLast})
724 * @since 1.6
725 */
726 public boolean offerLast(E e) {
727 addLast(e);
728 return true;
729 }
730
731 /**
732 * Retrieves, but does not remove, the first element of this list,
733 * or returns {@code null} if this list is empty.
734 *
735 * @return the first element of this list, or {@code null}
736 * if this list is empty
737 * @since 1.6
738 */
739 public E peekFirst() {
740 final Node<E> f = first;
741 return (f == null) ? null : f.item;
742 }
743
744 /**
745 * Retrieves, but does not remove, the last element of this list,
746 * or returns {@code null} if this list is empty.
747 *
748 * @return the last element of this list, or {@code null}
749 * if this list is empty
750 * @since 1.6
751 */
752 public E peekLast() {
753 final Node<E> l = last;
754 return (l == null) ? null : l.item;
755 }
756
757 /**
758 * Retrieves and removes the first element of this list,
759 * or returns {@code null} if this list is empty.
760 *
761 * @return the first element of this list, or {@code null} if
762 * this list is empty
763 * @since 1.6
764 */
765 public E pollFirst() {
766 final Node<E> f = first;
767 return (f == null) ? null : unlinkFirst(f);
768 }
769
770 /**
771 * Retrieves and removes the last element of this list,
772 * or returns {@code null} if this list is empty.
773 *
774 * @return the last element of this list, or {@code null} if
775 * this list is empty
776 * @since 1.6
777 */
778 public E pollLast() {
779 final Node<E> l = last;
780 return (l == null) ? null : unlinkLast(l);
781 }
782
783 /**
784 * Pushes an element onto the stack represented by this list. In other
785 * words, inserts the element at the front of this list.
786 *
787 * <p>This method is equivalent to {@link #addFirst}.
788 *
789 * @param e the element to push
790 * @since 1.6
791 */
792 public void push(E e) {
793 addFirst(e);
794 }
795
796 /**
797 * Pops an element from the stack represented by this list. In other
798 * words, removes and returns the first element of this list.
799 *
800 * <p>This method is equivalent to {@link #removeFirst()}.
801 *
802 * @return the element at the front of this list (which is the top
803 * of the stack represented by this list)
804 * @throws NoSuchElementException if this list is empty
805 * @since 1.6
806 */
807 public E pop() {
808 return removeFirst();
809 }
810
811 /**
812 * Removes the first occurrence of the specified element in this
813 * list (when traversing the list from head to tail). If the list
814 * does not contain the element, it is unchanged.
815 *
816 * @param o element to be removed from this list, if present
817 * @return {@code true} if the list contained the specified element
818 * @since 1.6
819 */
820 public boolean removeFirstOccurrence(Object o) {
821 return remove(o);
822 }
823
824 /**
825 * Removes the last occurrence of the specified element in this
826 * list (when traversing the list from head to tail). If the list
827 * does not contain the element, it is unchanged.
828 *
829 * @param o element to be removed from this list, if present
830 * @return {@code true} if the list contained the specified element
831 * @since 1.6
832 */
833 public boolean removeLastOccurrence(Object o) {
834 if (o == null) {
835 for (Node<E> x = last; x != null; x = x.prev) {
836 if (x.item == null) {
837 unlink(x);
838 return true;
839 }
840 }
841 } else {
842 for (Node<E> x = last; x != null; x = x.prev) {
843 if (o.equals(x.item)) {
844 unlink(x);
845 return true;
846 }
847 }
848 }
849 return false;
850 }
851
852 /**
853 * Returns a list-iterator of the elements in this list (in proper
854 * sequence), starting at the specified position in the list.
855 * Obeys the general contract of {@code List.listIterator(int)}.<p>
856 *
857 * The list-iterator is <i>fail-fast</i>: if the list is structurally
858 * modified at any time after the Iterator is created, in any way except
859 * through the list-iterator's own {@code remove} or {@code add}
860 * methods, the list-iterator will throw a
861 * {@code ConcurrentModificationException}. Thus, in the face of
862 * concurrent modification, the iterator fails quickly and cleanly, rather
863 * than risking arbitrary, non-deterministic behavior at an undetermined
864 * time in the future.
865 *
866 * @param index index of the first element to be returned from the
867 * list-iterator (by a call to {@code next})
868 * @return a ListIterator of the elements in this list (in proper
869 * sequence), starting at the specified position in the list
870 * @throws IndexOutOfBoundsException {@inheritDoc}
871 * @see List#listIterator(int)
872 */
873 public ListIterator<E> listIterator(int index) {
874 checkPositionIndex(index);
875 return new ListItr(index);
876 }
877
878 private class ListItr implements ListIterator<E> {
879 private Node<E> lastReturned = null;
880 private Node<E> next;
881 private int nextIndex;
882 private int expectedModCount = modCount;
883
884 ListItr(int index) {
885 // assert isPositionIndex(index);
886 next = (index == size) ? null : node(index);
887 nextIndex = index;
888 }
889
890 public boolean hasNext() {
891 return nextIndex < size;
892 }
893
894 public E next() {
895 checkForComodification();
896 if (!hasNext())
897 throw new NoSuchElementException();
898
899 lastReturned = next;
900 next = next.next;
901 nextIndex++;
902 return lastReturned.item;
903 }
904
905 public boolean hasPrevious() {
906 return nextIndex > 0;
907 }
908
909 public E previous() {
910 checkForComodification();
911 if (!hasPrevious())
912 throw new NoSuchElementException();
913
914 lastReturned = next = (next == null) ? last : next.prev;
915 nextIndex--;
916 return lastReturned.item;
917 }
918
919 public int nextIndex() {
920 return nextIndex;
921 }
922
923 public int previousIndex() {
924 return nextIndex - 1;
925 }
926
927 public void remove() {
928 checkForComodification();
929 if (lastReturned == null)
930 throw new IllegalStateException();
931
932 Node<E> lastNext = lastReturned.next;
933 unlink(lastReturned);
934 if (next == lastReturned)
935 next = lastNext;
936 else
937 nextIndex--;
938 lastReturned = null;
939 expectedModCount++;
940 }
941
942 public void set(E e) {
943 if (lastReturned == null)
944 throw new IllegalStateException();
945 checkForComodification();
946 lastReturned.item = e;
947 }
948
949 public void add(E e) {
950 checkForComodification();
951 lastReturned = null;
952 if (next == null)
953 linkLast(e);
954 else
955 linkBefore(e, next);
956 nextIndex++;
957 expectedModCount++;
958 }
959
960 final void checkForComodification() {
961 if (modCount != expectedModCount)
962 throw new ConcurrentModificationException();
963 }
964 }
965
966 private static class Node<E> {
967 E item;
968 Node<E> next;
969 Node<E> prev;
970
971 Node(Node<E> prev, E element, Node<E> next) {
972 this.item = element;
973 this.next = next;
974 this.prev = prev;
975 }
976 }
977
978 /**
979 * @since 1.6
980 */
981 public Iterator<E> descendingIterator() {
982 return new DescendingIterator();
983 }
984
985 /**
986 * Adapter to provide descending iterators via ListItr.previous
987 */
988 private class DescendingIterator implements Iterator<E> {
989 private final ListItr itr = new ListItr(size());
990 public boolean hasNext() {
991 return itr.hasPrevious();
992 }
993 public E next() {
994 return itr.previous();
995 }
996 public void remove() {
997 itr.remove();
998 }
999 }
1000
1001 @SuppressWarnings("unchecked")
1002 private LinkedList<E> superClone() {
1003 try {
1004 return (LinkedList<E>) super.clone();
1005 } catch (CloneNotSupportedException e) {
1006 throw new InternalError();
1007 }
1008 }
1009
1010 /**
1011 * Returns a shallow copy of this {@code LinkedList}. (The elements
1012 * themselves are not cloned.)
1013 *
1014 * @return a shallow copy of this {@code LinkedList} instance
1015 */
1016 public Object clone() {
1017 LinkedList<E> clone = superClone();
1018
1019 // Put clone into "virgin" state
1020 clone.first = clone.last = null;
1021 clone.size = 0;
1022 clone.modCount = 0;
1023
1024 // Initialize clone with our elements
1025 for (Node<E> x = first; x != null; x = x.next)
1026 clone.add(x.item);
1027
1028 return clone;
1029 }
1030
1031 /**
1032 * Returns an array containing all of the elements in this list
1033 * in proper sequence (from first to last element).
1034 *
1035 * <p>The returned array will be "safe" in that no references to it are
1036 * maintained by this list. (In other words, this method must allocate
1037 * a new array). The caller is thus free to modify the returned array.
1038 *
1039 * <p>This method acts as bridge between array-based and collection-based
1040 * APIs.
1041 *
1042 * @return an array containing all of the elements in this list
1043 * in proper sequence
1044 */
1045 public Object[] toArray() {
1046 Object[] result = new Object[size];
1047 int i = 0;
1048 for (Node<E> x = first; x != null; x = x.next)
1049 result[i++] = x.item;
1050 return result;
1051 }
1052
1053 /**
1054 * Returns an array containing all of the elements in this list in
1055 * proper sequence (from first to last element); the runtime type of
1056 * the returned array is that of the specified array. If the list fits
1057 * in the specified array, it is returned therein. Otherwise, a new
1058 * array is allocated with the runtime type of the specified array and
1059 * the size of this list.
1060 *
1061 * <p>If the list fits in the specified array with room to spare (i.e.,
1062 * the array has more elements than the list), the element in the array
1063 * immediately following the end of the list is set to {@code null}.
1064 * (This is useful in determining the length of the list <i>only</i> if
1065 * the caller knows that the list does not contain any null elements.)
1066 *
1067 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1068 * array-based and collection-based APIs. Further, this method allows
1069 * precise control over the runtime type of the output array, and may,
1070 * under certain circumstances, be used to save allocation costs.
1071 *
1072 * <p>Suppose {@code x} is a list known to contain only strings.
1073 * The following code can be used to dump the list into a newly
1074 * allocated array of {@code String}:
1075 *
1076 * <pre>
1077 * String[] y = x.toArray(new String[0]);</pre>
1078 *
1079 * Note that {@code toArray(new Object[0])} is identical in function to
1080 * {@code toArray()}.
1081 *
1082 * @param a the array into which the elements of the list are to
1083 * be stored, if it is big enough; otherwise, a new array of the
1084 * same runtime type is allocated for this purpose.
1085 * @return an array containing the elements of the list
1086 * @throws ArrayStoreException if the runtime type of the specified array
1087 * is not a supertype of the runtime type of every element in
1088 * this list
1089 * @throws NullPointerException if the specified array is null
1090 */
1091 @SuppressWarnings("unchecked")
1092 public <T> T[] toArray(T[] a) {
1093 if (a.length < size)
1094 a = (T[])java.lang.reflect.Array.newInstance(
1095 a.getClass().getComponentType(), size);
1096 int i = 0;
1097 Object[] result = a;
1098 for (Node<E> x = first; x != null; x = x.next)
1099 result[i++] = x.item;
1100
1101 if (a.length > size)
1102 a[size] = null;
1103
1104 return a;
1105 }
1106
1107 private static final long serialVersionUID = 876323262645176354L;
1108
1109 /**
1110 * Saves the state of this {@code LinkedList} instance to a stream
1111 * (that is, serializes it).
1112 *
1113 * @serialData The size of the list (the number of elements it
1114 * contains) is emitted (int), followed by all of its
1115 * elements (each an Object) in the proper order.
1116 */
1117 private void writeObject(java.io.ObjectOutputStream s)
1118 throws java.io.IOException {
1119 // Write out any hidden serialization magic
1120 s.defaultWriteObject();
1121
1122 // Write out size
1123 s.writeInt(size);
1124
1125 // Write out all elements in the proper order.
1126 for (Node<E> x = first; x != null; x = x.next)
1127 s.writeObject(x.item);
1128 }
1129
1130 /**
1131 * Reconstitutes this {@code LinkedList} instance from a stream
1132 * (that is, deserializes it).
1133 */
1134 @SuppressWarnings("unchecked")
1135 private void readObject(java.io.ObjectInputStream s)
1136 throws java.io.IOException, ClassNotFoundException {
1137 // Read in any hidden serialization magic
1138 s.defaultReadObject();
1139
1140 // Read in size
1141 int size = s.readInt();
1142
1143 // Read in all elements in the proper order.
1144 for (int i = 0; i < size; i++)
1145 linkLast((E)s.readObject());
1146 }
1147 }