1 /*
2 * Copyright (c) 1997, 2013, 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 import java.util.function.Consumer;
29
30 /**
31 * Doubly-linked list implementation of the {@code List} and {@code Deque}
32 * interfaces. Implements all optional list operations, and permits all
33 * elements (including {@code null}).
34 *
35 * <p>All of the operations perform as could be expected for a doubly-linked
36 * list. Operations that index into the list will traverse the list from
37 * the beginning or the end, whichever is closer to the specified index.
38 *
39 * <p><strong>Note that this implementation is not synchronized.</strong>
40 * If multiple threads access a linked list concurrently, and at least
41 * one of the threads modifies the list structurally, it <i>must</i> be
42 * synchronized externally. (A structural modification is any operation
43 * that adds or deletes one or more elements; merely setting the value of
44 * an element is not a structural modification.) This is typically
45 * accomplished by synchronizing on some object that naturally
46 * encapsulates the list.
47 *
48 * If no such object exists, the list should be "wrapped" using the
49 * {@link Collections#synchronizedList Collections.synchronizedList}
50 * method. This is best done at creation time, to prevent accidental
51 * unsynchronized access to the list:<pre>
52 * List list = Collections.synchronizedList(new LinkedList(...));</pre>
53 *
54 * <p>The iterators returned by this class's {@code iterator} and
55 * {@code listIterator} methods are <i>fail-fast</i>: if the list is
56 * structurally modified at any time after the iterator is created, in
57 * any way except through the Iterator's own {@code remove} or
58 * {@code add} methods, the iterator will throw a {@link
59 * ConcurrentModificationException}. Thus, in the face of concurrent
60 * modification, the iterator fails quickly and cleanly, rather than
61 * risking arbitrary, non-deterministic behavior at an undetermined
62 * time in the future.
63 *
64 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
65 * as it is, generally speaking, impossible to make any hard guarantees in the
66 * presence of unsynchronized concurrent modification. Fail-fast iterators
67 * throw {@code ConcurrentModificationException} on a best-effort basis.
68 * Therefore, it would be wrong to write a program that depended on this
69 * exception for its correctness: <i>the fail-fast behavior of iterators
70 * should be used only to detect bugs.</i>
71 *
72 * <p>This class is a member of the
73 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
74 * Java Collections Framework</a>.
75 *
76 * @author Josh Bloch
77 * @see List
78 * @see ArrayList
79 * @since 1.2
80 * @param <E> the type of elements held in this collection
81 */
82
83 public class LinkedList<E>
84 extends AbstractSequentialList<E>
85 implements List<E>, Deque<E>, Cloneable, java.io.Serializable
86 {
87 transient int size = 0;
88
89 /**
90 * Pointer to first node.
91 * Invariant: (first == null && last == null) ||
92 * (first.prev == null && first.item != null)
93 */
94 transient Node<E> first;
95
96 /**
97 * Pointer to last node.
98 * Invariant: (first == null && last == null) ||
99 * (last.next == null && last.item != null)
100 */
101 transient Node<E> last;
102
103 /**
104 * Constructs an empty list.
105 */
106 public LinkedList() {
107 }
108
109 /**
110 * Constructs a list containing the elements of the specified
111 * collection, in the order they are returned by the collection's
112 * iterator.
113 *
114 * @param c the collection whose elements are to be placed into this list
115 * @throws NullPointerException if the specified collection is null
116 */
117 public LinkedList(Collection<? extends E> c) {
118 this();
119 addAll(c);
120 }
121
122 /**
123 * Links e as first element.
124 */
125 private void linkFirst(E e) {
126 final Node<E> f = first;
127 final Node<E> newNode = new Node<>(null, e, f);
128 first = newNode;
129 if (f == null)
130 last = newNode;
131 else
132 f.prev = newNode;
133 size++;
134 modCount++;
135 }
136
137 /**
138 * Links e as last element.
139 */
140 void linkLast(E e) {
141 final Node<E> l = last;
142 final Node<E> newNode = new Node<>(l, e, null);
143 last = newNode;
144 if (l == null)
145 first = newNode;
146 else
147 l.next = newNode;
148 size++;
149 modCount++;
150 }
151
152 /**
153 * Inserts element e before non-null Node succ.
154 */
155 void linkBefore(E e, Node<E> succ) {
156 // assert succ != null;
157 final Node<E> pred = succ.prev;
158 final Node<E> newNode = new Node<>(pred, e, succ);
159 succ.prev = newNode;
160 if (pred == null)
161 first = newNode;
162 else
163 pred.next = newNode;
164 size++;
165 modCount++;
166 }
167
168 /**
169 * Unlinks non-null first node f.
170 */
171 private E unlinkFirst(Node<E> f) {
172 // assert f == first && f != null;
173 final E element = f.item;
174 final Node<E> next = f.next;
175 f.item = null;
176 f.next = null; // help GC
177 first = next;
178 if (next == null)
179 last = null;
180 else
181 next.prev = null;
182 size--;
183 modCount++;
184 return element;
185 }
186
187 /**
188 * Unlinks non-null last node l.
189 */
190 private E unlinkLast(Node<E> l) {
191 // assert l == last && l != null;
192 final E element = l.item;
193 final Node<E> prev = l.prev;
194 l.item = null;
195 l.prev = null; // help GC
196 last = prev;
197 if (prev == null)
198 first = null;
199 else
200 prev.next = null;
201 size--;
202 modCount++;
203 return element;
204 }
205
206 /**
207 * Unlinks non-null node x.
208 */
209 E unlink(Node<E> x) {
210 // assert x != null;
211 final E element = x.item;
212 final Node<E> next = x.next;
213 final Node<E> prev = x.prev;
214
215 if (prev == null) {
216 first = next;
217 } else {
218 prev.next = next;
219 x.prev = null;
220 }
221
222 if (next == null) {
223 last = prev;
224 } else {
225 next.prev = prev;
226 x.next = null;
227 }
228
229 x.item = null;
230 size--;
231 modCount++;
232 return element;
233 }
234
235 /**
236 * Returns the first element in this list.
237 *
238 * @return the first element in this list
239 * @throws NoSuchElementException if this list is empty
240 */
241 public E getFirst() {
242 final Node<E> f = first;
243 if (f == null)
244 throw new NoSuchElementException();
245 return f.item;
246 }
247
248 /**
249 * Returns the last element in this list.
250 *
251 * @return the last element in this list
252 * @throws NoSuchElementException if this list is empty
253 */
254 public E getLast() {
255 final Node<E> l = last;
256 if (l == null)
257 throw new NoSuchElementException();
258 return l.item;
259 }
260
261 /**
262 * Removes and returns the first element from this list.
263 *
264 * @return the first element from this list
265 * @throws NoSuchElementException if this list is empty
266 */
267 public E removeFirst() {
268 final Node<E> f = first;
269 if (f == null)
270 throw new NoSuchElementException();
271 return unlinkFirst(f);
272 }
273
274 /**
275 * Removes and returns the last element from this list.
276 *
277 * @return the last element from this list
278 * @throws NoSuchElementException if this list is empty
279 */
280 public E removeLast() {
281 final Node<E> l = last;
282 if (l == null)
283 throw new NoSuchElementException();
284 return unlinkLast(l);
285 }
286
287 /**
288 * Inserts the specified element at the beginning of this list.
289 *
290 * @param e the element to add
291 */
292 public void addFirst(E e) {
293 linkFirst(e);
294 }
295
296 /**
297 * Appends the specified element to the end of this list.
298 *
299 * <p>This method is equivalent to {@link #add}.
300 *
301 * @param e the element to add
302 */
303 public void addLast(E e) {
304 linkLast(e);
305 }
306
307 /**
308 * Returns {@code true} if this list contains the specified element.
309 * More formally, returns {@code true} if and only if this list contains
310 * at least one element {@code e} such that
311 * <tt>(o==null ? e==null : o.equals(e))</tt>.
312 *
313 * @param o element whose presence in this list is to be tested
314 * @return {@code true} if this list contains the specified element
315 */
316 public boolean contains(Object o) {
317 return indexOf(o) != -1;
318 }
319
320 /**
321 * Returns the number of elements in this list.
322 *
323 * @return the number of elements in this list
324 */
325 public int size() {
326 return size;
327 }
328
329 /**
330 * Appends the specified element to the end of this list.
331 *
332 * <p>This method is equivalent to {@link #addLast}.
333 *
334 * @param e element to be appended to this list
335 * @return {@code true} (as specified by {@link Collection#add})
336 */
337 public boolean add(E e) {
338 linkLast(e);
339 return true;
340 }
341
342 /**
343 * Removes the first occurrence of the specified element from this list,
344 * if it is present. If this list does not contain the element, it is
345 * unchanged. More formally, removes the element with the lowest index
346 * {@code i} such that
347 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
348 * (if such an element exists). Returns {@code true} if this list
349 * contained the specified element (or equivalently, if this list
350 * changed as a result of the call).
351 *
352 * @param o element to be removed from this list, if present
353 * @return {@code true} if this list contained the specified element
354 */
355 public boolean remove(Object o) {
356 if (o == null) {
357 for (Node<E> x = first; x != null; x = x.next) {
358 if (x.item == null) {
359 unlink(x);
360 return true;
361 }
362 }
363 } else {
364 for (Node<E> x = first; x != null; x = x.next) {
365 if (o.equals(x.item)) {
366 unlink(x);
367 return true;
368 }
369 }
370 }
371 return false;
372 }
373
374 /**
375 * Appends all of the elements in the specified collection to the end of
376 * this list, in the order that they are returned by the specified
377 * collection's iterator. The behavior of this operation is undefined if
378 * the specified collection is modified while the operation is in
379 * progress. (Note that this will occur if the specified collection is
380 * this list, and it's nonempty.)
381 *
382 * @param c collection containing elements to be added to this list
383 * @return {@code true} if this list changed as a result of the call
384 * @throws NullPointerException if the specified collection is null
385 */
386 public boolean addAll(Collection<? extends E> c) {
387 return addAll(size, c);
388 }
389
390 /**
391 * Inserts all of the elements in the specified collection into this
392 * list, starting at the specified position. Shifts the element
393 * currently at that position (if any) and any subsequent elements to
394 * the right (increases their indices). The new elements will appear
395 * in the list in the order that they are returned by the
396 * specified collection's iterator.
397 *
398 * @param index index at which to insert the first element
399 * from the specified collection
400 * @param c collection containing elements to be added to this list
401 * @return {@code true} if this list changed as a result of the call
402 * @throws IndexOutOfBoundsException {@inheritDoc}
403 * @throws NullPointerException if the specified collection is null
404 */
405 public boolean addAll(int index, Collection<? extends E> c) {
406 checkPositionIndex(index);
407
408 Object[] a = c.toArray();
409 int numNew = a.length;
410 if (numNew == 0)
411 return false;
412
413 Node<E> pred, succ;
414 if (index == size) {
415 succ = null;
416 pred = last;
417 } else {
418 succ = node(index);
419 pred = succ.prev;
420 }
421
422 for (Object o : a) {
423 @SuppressWarnings("unchecked") E e = (E) o;
424 Node<E> newNode = new Node<>(pred, e, null);
425 if (pred == null)
426 first = newNode;
427 else
428 pred.next = newNode;
429 pred = newNode;
430 }
431
432 if (succ == null) {
433 last = pred;
434 } else {
435 pred.next = succ;
436 succ.prev = pred;
437 }
438
439 size += numNew;
440 modCount++;
441 return true;
442 }
443
444 /**
445 * Removes all of the elements from this list.
446 * The list will be empty after this call returns.
447 */
448 public void clear() {
449 // Clearing all of the links between nodes is "unnecessary", but:
450 // - helps a generational GC if the discarded nodes inhabit
451 // more than one generation
452 // - is sure to free memory even if there is a reachable Iterator
453 for (Node<E> x = first; x != null; ) {
454 Node<E> next = x.next;
455 x.item = null;
456 x.next = null;
457 x.prev = null;
458 x = next;
459 }
460 first = last = null;
461 size = 0;
462 modCount++;
463 }
464
465
466 // Positional Access Operations
467
468 /**
469 * Returns the element at the specified position in this list.
470 *
471 * @param index index of the element to return
472 * @return the element at the specified position in this list
473 * @throws IndexOutOfBoundsException {@inheritDoc}
474 */
475 public E get(int index) {
476 checkElementIndex(index);
477 return node(index).item;
478 }
479
480 /**
481 * Replaces the element at the specified position in this list with the
482 * specified element.
483 *
484 * @param index index of the element to replace
485 * @param element element to be stored at the specified position
486 * @return the element previously at the specified position
487 * @throws IndexOutOfBoundsException {@inheritDoc}
488 */
489 public E set(int index, E element) {
490 checkElementIndex(index);
491 Node<E> x = node(index);
492 E oldVal = x.item;
493 x.item = element;
494 return oldVal;
495 }
496
497 /**
498 * Inserts the specified element at the specified position in this list.
499 * Shifts the element currently at that position (if any) and any
500 * subsequent elements to the right (adds one to their indices).
501 *
502 * @param index index at which the specified element is to be inserted
503 * @param element element to be inserted
504 * @throws IndexOutOfBoundsException {@inheritDoc}
505 */
506 public void add(int index, E element) {
507 checkPositionIndex(index);
508
509 if (index == size)
510 linkLast(element);
511 else
512 linkBefore(element, node(index));
513 }
514
515 /**
516 * Removes the element at the specified position in this list. Shifts any
517 * subsequent elements to the left (subtracts one from their indices).
518 * Returns the element that was removed from the list.
519 *
520 * @param index the index of the element to be removed
521 * @return the element previously at the specified position
522 * @throws IndexOutOfBoundsException {@inheritDoc}
523 */
524 public E remove(int index) {
525 checkElementIndex(index);
526 return unlink(node(index));
527 }
528
529 /**
530 * Tells if the argument is the index of an existing element.
531 */
532 private boolean isElementIndex(int index) {
533 return index >= 0 && index < size;
534 }
535
536 /**
537 * Tells if the argument is the index of a valid position for an
538 * iterator or an add operation.
539 */
540 private boolean isPositionIndex(int index) {
541 return index >= 0 && index <= size;
542 }
543
544 /**
545 * Constructs an IndexOutOfBoundsException detail message.
546 * Of the many possible refactorings of the error handling code,
547 * this "outlining" performs best with both server and client VMs.
548 */
549 private String outOfBoundsMsg(int index) {
550 return "Index: "+index+", Size: "+size;
551 }
552
553 private void checkElementIndex(int index) {
554 if (!isElementIndex(index))
555 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
556 }
557
558 private void checkPositionIndex(int index) {
559 if (!isPositionIndex(index))
560 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
561 }
562
563 /**
564 * Returns the (non-null) Node at the specified element index.
565 */
566 Node<E> node(int index) {
567 // assert isElementIndex(index);
568
569 if (index < (size >> 1)) {
570 Node<E> x = first;
571 for (int i = 0; i < index; i++)
572 x = x.next;
573 return x;
574 } else {
575 Node<E> x = last;
576 for (int i = size - 1; i > index; i--)
577 x = x.prev;
578 return x;
579 }
580 }
581
582 // Search Operations
583
584 /**
585 * Returns the index of the first occurrence of the specified element
586 * in this list, or -1 if this list does not contain the element.
587 * More formally, returns the lowest index {@code i} such that
588 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
589 * or -1 if there is no such index.
590 *
591 * @param o element to search for
592 * @return the index of the first occurrence of the specified element in
593 * this list, or -1 if this list does not contain the element
594 */
595 public int indexOf(Object o) {
596 int index = 0;
597 if (o == null) {
598 for (Node<E> x = first; x != null; x = x.next) {
599 if (x.item == null)
600 return index;
601 index++;
602 }
603 } else {
604 for (Node<E> x = first; x != null; x = x.next) {
605 if (o.equals(x.item))
606 return index;
607 index++;
608 }
609 }
610 return -1;
611 }
612
613 /**
614 * Returns the index of the last occurrence of the specified element
615 * in this list, or -1 if this list does not contain the element.
616 * More formally, returns the highest index {@code i} such that
617 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
618 * or -1 if there is no such index.
619 *
620 * @param o element to search for
621 * @return the index of the last occurrence of the specified element in
622 * this list, or -1 if this list does not contain the element
623 */
624 public int lastIndexOf(Object o) {
625 int index = size;
626 if (o == null) {
627 for (Node<E> x = last; x != null; x = x.prev) {
628 index--;
629 if (x.item == null)
630 return index;
631 }
632 } else {
633 for (Node<E> x = last; x != null; x = x.prev) {
634 index--;
635 if (o.equals(x.item))
636 return index;
637 }
638 }
639 return -1;
640 }
641
642 // Queue operations.
643
644 /**
645 * Retrieves, but does not remove, the head (first element) of this list.
646 *
647 * @return the head of this list, or {@code null} if this list is empty
648 * @since 1.5
649 */
650 public E peek() {
651 final Node<E> f = first;
652 return (f == null) ? null : f.item;
653 }
654
655 /**
656 * Retrieves, but does not remove, the head (first element) of this list.
657 *
658 * @return the head of this list
659 * @throws NoSuchElementException if this list is empty
660 * @since 1.5
661 */
662 public E element() {
663 return getFirst();
664 }
665
666 /**
667 * Retrieves and removes the head (first element) of this list.
668 *
669 * @return the head of this list, or {@code null} if this list is empty
670 * @since 1.5
671 */
672 public E poll() {
673 final Node<E> f = first;
674 return (f == null) ? null : unlinkFirst(f);
675 }
676
677 /**
678 * Retrieves and removes the head (first element) of this list.
679 *
680 * @return the head of this list
681 * @throws NoSuchElementException if this list is empty
682 * @since 1.5
683 */
684 public E remove() {
685 return removeFirst();
686 }
687
688 /**
689 * Adds the specified element as the tail (last element) of this list.
690 *
691 * @param e the element to add
692 * @return {@code true} (as specified by {@link Queue#offer})
693 * @since 1.5
694 */
695 public boolean offer(E e) {
696 return add(e);
697 }
698
699 // Deque operations
700 /**
701 * Inserts the specified element at the front of this list.
702 *
703 * @param e the element to insert
704 * @return {@code true} (as specified by {@link Deque#offerFirst})
705 * @since 1.6
706 */
707 public boolean offerFirst(E e) {
708 addFirst(e);
709 return true;
710 }
711
712 /**
713 * Inserts the specified element at the end of this list.
714 *
715 * @param e the element to insert
716 * @return {@code true} (as specified by {@link Deque#offerLast})
717 * @since 1.6
718 */
719 public boolean offerLast(E e) {
720 addLast(e);
721 return true;
722 }
723
724 /**
725 * Retrieves, but does not remove, the first element of this list,
726 * or returns {@code null} if this list is empty.
727 *
728 * @return the first element of this list, or {@code null}
729 * if this list is empty
730 * @since 1.6
731 */
732 public E peekFirst() {
733 final Node<E> f = first;
734 return (f == null) ? null : f.item;
735 }
736
737 /**
738 * Retrieves, but does not remove, the last element of this list,
739 * or returns {@code null} if this list is empty.
740 *
741 * @return the last element of this list, or {@code null}
742 * if this list is empty
743 * @since 1.6
744 */
745 public E peekLast() {
746 final Node<E> l = last;
747 return (l == null) ? null : l.item;
748 }
749
750 /**
751 * Retrieves and removes the first element of this list,
752 * or returns {@code null} if this list is empty.
753 *
754 * @return the first element of this list, or {@code null} if
755 * this list is empty
756 * @since 1.6
757 */
758 public E pollFirst() {
759 final Node<E> f = first;
760 return (f == null) ? null : unlinkFirst(f);
761 }
762
763 /**
764 * Retrieves and removes the last element of this list,
765 * or returns {@code null} if this list is empty.
766 *
767 * @return the last element of this list, or {@code null} if
768 * this list is empty
769 * @since 1.6
770 */
771 public E pollLast() {
772 final Node<E> l = last;
773 return (l == null) ? null : unlinkLast(l);
774 }
775
776 /**
777 * Pushes an element onto the stack represented by this list. In other
778 * words, inserts the element at the front of this list.
779 *
780 * <p>This method is equivalent to {@link #addFirst}.
781 *
782 * @param e the element to push
783 * @since 1.6
784 */
785 public void push(E e) {
786 addFirst(e);
787 }
788
789 /**
790 * Pops an element from the stack represented by this list. In other
791 * words, removes and returns the first element of this list.
792 *
793 * <p>This method is equivalent to {@link #removeFirst()}.
794 *
795 * @return the element at the front of this list (which is the top
796 * of the stack represented by this list)
797 * @throws NoSuchElementException if this list is empty
798 * @since 1.6
799 */
800 public E pop() {
801 return removeFirst();
802 }
803
804 /**
805 * Removes the first occurrence of the specified element in this
806 * list (when traversing the list from head to tail). If the list
807 * does not contain the element, it is unchanged.
808 *
809 * @param o element to be removed from this list, if present
810 * @return {@code true} if the list contained the specified element
811 * @since 1.6
812 */
813 public boolean removeFirstOccurrence(Object o) {
814 return remove(o);
815 }
816
817 /**
818 * Removes the last occurrence of the specified element in this
819 * list (when traversing the list from head to tail). If the list
820 * does not contain the element, it is unchanged.
821 *
822 * @param o element to be removed from this list, if present
823 * @return {@code true} if the list contained the specified element
824 * @since 1.6
825 */
826 public boolean removeLastOccurrence(Object o) {
827 if (o == null) {
828 for (Node<E> x = last; x != null; x = x.prev) {
829 if (x.item == null) {
830 unlink(x);
831 return true;
832 }
833 }
834 } else {
835 for (Node<E> x = last; x != null; x = x.prev) {
836 if (o.equals(x.item)) {
837 unlink(x);
838 return true;
839 }
840 }
841 }
842 return false;
843 }
844
845 /**
846 * Returns a list-iterator of the elements in this list (in proper
847 * sequence), starting at the specified position in the list.
848 * Obeys the general contract of {@code List.listIterator(int)}.<p>
849 *
850 * The list-iterator is <i>fail-fast</i>: if the list is structurally
851 * modified at any time after the Iterator is created, in any way except
852 * through the list-iterator's own {@code remove} or {@code add}
853 * methods, the list-iterator will throw a
854 * {@code ConcurrentModificationException}. Thus, in the face of
855 * concurrent modification, the iterator fails quickly and cleanly, rather
856 * than risking arbitrary, non-deterministic behavior at an undetermined
857 * time in the future.
858 *
859 * @param index index of the first element to be returned from the
860 * list-iterator (by a call to {@code next})
861 * @return a ListIterator of the elements in this list (in proper
862 * sequence), starting at the specified position in the list
863 * @throws IndexOutOfBoundsException {@inheritDoc}
864 * @see List#listIterator(int)
865 */
866 public ListIterator<E> listIterator(int index) {
867 checkPositionIndex(index);
868 return new ListItr(index);
869 }
870
871 private class ListItr implements ListIterator<E> {
872 private Node<E> lastReturned;
873 private Node<E> next;
874 private int nextIndex;
875 private int expectedModCount = modCount;
876
877 ListItr(int index) {
878 // assert isPositionIndex(index);
879 next = (index == size) ? null : node(index);
880 nextIndex = index;
881 }
882
883 public boolean hasNext() {
884 return nextIndex < size;
885 }
886
887 public E next() {
888 checkForComodification();
889 if (!hasNext())
890 throw new NoSuchElementException();
891
892 lastReturned = next;
893 next = next.next;
894 nextIndex++;
895 return lastReturned.item;
896 }
897
898 public boolean hasPrevious() {
899 return nextIndex > 0;
900 }
901
902 public E previous() {
903 checkForComodification();
904 if (!hasPrevious())
905 throw new NoSuchElementException();
906
907 lastReturned = next = (next == null) ? last : next.prev;
908 nextIndex--;
909 return lastReturned.item;
910 }
911
912 public int nextIndex() {
913 return nextIndex;
914 }
915
916 public int previousIndex() {
917 return nextIndex - 1;
918 }
919
920 public void remove() {
921 checkForComodification();
922 if (lastReturned == null)
923 throw new IllegalStateException();
924
925 Node<E> lastNext = lastReturned.next;
926 unlink(lastReturned);
927 if (next == lastReturned)
928 next = lastNext;
929 else
930 nextIndex--;
931 lastReturned = null;
932 expectedModCount++;
933 }
934
935 public void set(E e) {
936 if (lastReturned == null)
937 throw new IllegalStateException();
938 checkForComodification();
939 lastReturned.item = e;
940 }
941
942 public void add(E e) {
943 checkForComodification();
944 lastReturned = null;
945 if (next == null)
946 linkLast(e);
947 else
948 linkBefore(e, next);
949 nextIndex++;
950 expectedModCount++;
951 }
952
953 public void forEachRemaining(Consumer<? super E> action) {
954 Objects.requireNonNull(action);
955 while (modCount == expectedModCount && nextIndex < size) {
956 action.accept(next.item);
957 lastReturned = next;
958 next = next.next;
959 nextIndex++;
960 }
961 checkForComodification();
962 }
963
964 final void checkForComodification() {
965 if (modCount != expectedModCount)
966 throw new ConcurrentModificationException();
967 }
968 }
969
970 private static class Node<E> {
971 E item;
972 Node<E> next;
973 Node<E> prev;
974
975 Node(Node<E> prev, E element, Node<E> next) {
976 this.item = element;
977 this.next = next;
978 this.prev = prev;
979 }
980 }
981
982 /**
983 * @since 1.6
984 */
985 public Iterator<E> descendingIterator() {
986 return new DescendingIterator();
987 }
988
989 /**
990 * Adapter to provide descending iterators via ListItr.previous
991 */
992 private class DescendingIterator implements Iterator<E> {
993 private final ListItr itr = new ListItr(size());
994 public boolean hasNext() {
995 return itr.hasPrevious();
996 }
997 public E next() {
998 return itr.previous();
999 }
1000 public void remove() {
1001 itr.remove();
1002 }
1003 }
1004
1005 @SuppressWarnings("unchecked")
1006 private LinkedList<E> superClone() {
1007 try {
1008 return (LinkedList<E>) super.clone();
1009 } catch (CloneNotSupportedException e) {
1010 throw new InternalError(e);
1011 }
1012 }
1013
1014 /**
1015 * Returns a shallow copy of this {@code LinkedList}. (The elements
1016 * themselves are not cloned.)
1017 *
1018 * @return a shallow copy of this {@code LinkedList} instance
1019 */
1020 public Object clone() {
1021 LinkedList<E> clone = superClone();
1022
1023 // Put clone into "virgin" state
1024 clone.first = clone.last = null;
1025 clone.size = 0;
1026 clone.modCount = 0;
1027
1028 // Initialize clone with our elements
1029 for (Node<E> x = first; x != null; x = x.next)
1030 clone.add(x.item);
1031
1032 return clone;
1033 }
1034
1035 /**
1036 * Returns an array containing all of the elements in this list
1037 * in proper sequence (from first to last element).
1038 *
1039 * <p>The returned array will be "safe" in that no references to it are
1040 * maintained by this list. (In other words, this method must allocate
1041 * a new array). The caller is thus free to modify the returned array.
1042 *
1043 * <p>This method acts as bridge between array-based and collection-based
1044 * APIs.
1045 *
1046 * @return an array containing all of the elements in this list
1047 * in proper sequence
1048 */
1049 public Object[] toArray() {
1050 Object[] result = new Object[size];
1051 int i = 0;
1052 for (Node<E> x = first; x != null; x = x.next)
1053 result[i++] = x.item;
1054 return result;
1055 }
1056
1057 /**
1058 * Returns an array containing all of the elements in this list in
1059 * proper sequence (from first to last element); the runtime type of
1060 * the returned array is that of the specified array. If the list fits
1061 * in the specified array, it is returned therein. Otherwise, a new
1062 * array is allocated with the runtime type of the specified array and
1063 * the size of this list.
1064 *
1065 * <p>If the list fits in the specified array with room to spare (i.e.,
1066 * the array has more elements than the list), the element in the array
1067 * immediately following the end of the list is set to {@code null}.
1068 * (This is useful in determining the length of the list <i>only</i> if
1069 * the caller knows that the list does not contain any null elements.)
1070 *
1071 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1072 * array-based and collection-based APIs. Further, this method allows
1073 * precise control over the runtime type of the output array, and may,
1074 * under certain circumstances, be used to save allocation costs.
1075 *
1076 * <p>Suppose {@code x} is a list known to contain only strings.
1077 * The following code can be used to dump the list into a newly
1078 * allocated array of {@code String}:
1079 *
1080 * <pre>
1081 * String[] y = x.toArray(new String[0]);</pre>
1082 *
1083 * Note that {@code toArray(new Object[0])} is identical in function to
1084 * {@code toArray()}.
1085 *
1086 * @param a the array into which the elements of the list are to
1087 * be stored, if it is big enough; otherwise, a new array of the
1088 * same runtime type is allocated for this purpose.
1089 * @return an array containing the elements of the list
1090 * @throws ArrayStoreException if the runtime type of the specified array
1091 * is not a supertype of the runtime type of every element in
1092 * this list
1093 * @throws NullPointerException if the specified array is null
1094 */
1095 @SuppressWarnings("unchecked")
1096 public <T> T[] toArray(T[] a) {
1097 if (a.length < size)
1098 a = (T[])java.lang.reflect.Array.newInstance(
1099 a.getClass().getComponentType(), size);
1100 int i = 0;
1101 Object[] result = a;
1102 for (Node<E> x = first; x != null; x = x.next)
1103 result[i++] = x.item;
1104
1105 if (a.length > size)
1106 a[size] = null;
1107
1108 return a;
1109 }
1110
1111 private static final long serialVersionUID = 876323262645176354L;
1112
1113 /**
1114 * Saves the state of this {@code LinkedList} instance to a stream
1115 * (that is, serializes it).
1116 *
1117 * @serialData The size of the list (the number of elements it
1118 * contains) is emitted (int), followed by all of its
1119 * elements (each an Object) in the proper order.
1120 */
1121 private void writeObject(java.io.ObjectOutputStream s)
1122 throws java.io.IOException {
1123 // Write out any hidden serialization magic
1124 s.defaultWriteObject();
1125
1126 // Write out size
1127 s.writeInt(size);
1128
1129 // Write out all elements in the proper order.
1130 for (Node<E> x = first; x != null; x = x.next)
1131 s.writeObject(x.item);
1132 }
1133
1134 /**
1135 * Reconstitutes this {@code LinkedList} instance from a stream
1136 * (that is, deserializes it).
1137 */
1138 @SuppressWarnings("unchecked")
1139 private void readObject(java.io.ObjectInputStream s)
1140 throws java.io.IOException, ClassNotFoundException {
1141 // Read in any hidden serialization magic
1142 s.defaultReadObject();
1143
1144 // Read in size
1145 int size = s.readInt();
1146
1147 // Read in all elements in the proper order.
1148 for (int i = 0; i < size; i++)
1149 linkLast((E)s.readObject());
1150 }
1151
1152 /**
1153 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
1154 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
1155 * list.
1156 *
1157 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
1158 * {@link Spliterator#ORDERED}. Overriding implementations should document
1159 * the reporting of additional characteristic values.
1160 *
1161 * @implNote
1162 * The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
1163 * and implements {@code trySplit} to permit limited parallelism..
1164 *
1165 * @return a {@code Spliterator} over the elements in this list
1166 * @since 1.8
1167 */
1168 @Override
1169 public Spliterator<E> spliterator() {
1170 return new LLSpliterator<>(this, -1, 0);
1171 }
1172
1173 /** A customized variant of Spliterators.IteratorSpliterator */
1174 static final class LLSpliterator<E> implements Spliterator<E> {
1175 static final int BATCH_UNIT = 1 << 10; // batch array size increment
1176 static final int MAX_BATCH = 1 << 25; // max batch array size;
1177 final LinkedList<E> list; // null OK unless traversed
1178 Node<E> current; // current node; null until initialized
1179 int est; // size estimate; -1 until first needed
1180 int expectedModCount; // initialized when est set
1181 int batch; // batch size for splits
1182
1183 LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
1184 this.list = list;
1185 this.est = est;
1186 this.expectedModCount = expectedModCount;
1187 }
1188
1189 final int getEst() {
1190 int s; // force initialization
1191 final LinkedList<E> lst;
1192 if ((s = est) < 0) {
1193 if ((lst = list) == null)
1194 s = est = 0;
1195 else {
1196 expectedModCount = lst.modCount;
1197 current = lst.first;
1198 s = est = lst.size;
1199 }
1200 }
1201 return s;
1202 }
1203
1204 public long estimateSize() { return (long) getEst(); }
1205
1206 public Spliterator<E> trySplit() {
1207 Node<E> p;
1208 int s = getEst();
1209 if (s > 1 && (p = current) != null) {
1210 int n = batch + BATCH_UNIT;
1211 if (n > s)
1212 n = s;
1213 if (n > MAX_BATCH)
1214 n = MAX_BATCH;
1215 Object[] a = new Object[n];
1216 int j = 0;
1217 do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
1218 current = p;
1219 batch = j;
1220 est = s - j;
1221 return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
1222 }
1223 return null;
1224 }
1225
1226 public void forEachRemaining(Consumer<? super E> action) {
1227 Node<E> p; int n;
1228 if (action == null) throw new NullPointerException();
1229 if ((n = getEst()) > 0 && (p = current) != null) {
1230 current = null;
1231 est = 0;
1232 do {
1233 E e = p.item;
1234 p = p.next;
1235 action.accept(e);
1236 } while (p != null && --n > 0);
1237 }
1238 if (list.modCount != expectedModCount)
1239 throw new ConcurrentModificationException();
1240 }
1241
1242 public boolean tryAdvance(Consumer<? super E> action) {
1243 Node<E> p;
1244 if (action == null) throw new NullPointerException();
1245 if (getEst() > 0 && (p = current) != null) {
1246 --est;
1247 E e = p.item;
1248 current = p.next;
1249 action.accept(e);
1250 if (list.modCount != expectedModCount)
1251 throw new ConcurrentModificationException();
1252 return true;
1253 }
1254 return false;
1255 }
1256
1257 public int characteristics() {
1258 return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
1259 }
1260 }
1261
1262 }