1 /*
2 * Copyright (c) 1997, 2018, 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}/java.base/java/util/package-summary.html#CollectionsFramework">
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 */
92 transient Node<E> first;
93
94 /**
95 * Pointer to last node.
96 */
97 transient Node<E> last;
98
99 /*
100 void dataStructureInvariants() {
101 assert (size == 0)
102 ? (first == null && last == null)
103 : (first.prev == null && last.next == null);
104 }
105 */
106
107 /**
108 * Constructs an empty list.
109 */
110 public LinkedList() {
111 }
112
113 /**
114 * Constructs a list containing the elements of the specified
115 * collection, in the order they are returned by the collection's
116 * iterator.
117 *
118 * @param c the collection whose elements are to be placed into this list
119 * @throws NullPointerException if the specified collection is null
120 */
121 public LinkedList(Collection<? extends E> c) {
122 this();
123 addAll(c);
124 }
125
126 /**
127 * Links e as first element.
128 */
129 private void linkFirst(E e) {
130 final Node<E> f = first;
131 final Node<E> newNode = new Node<>(null, e, f);
132 first = newNode;
133 if (f == null)
134 last = newNode;
135 else
136 f.prev = newNode;
137 size++;
138 modCount++;
139 }
140
141 /**
142 * Links e as last element.
143 */
144 void linkLast(E e) {
145 final Node<E> l = last;
146 final Node<E> newNode = new Node<>(l, e, null);
147 last = newNode;
148 if (l == null)
149 first = newNode;
150 else
151 l.next = newNode;
152 size++;
153 modCount++;
154 }
155
156 /**
157 * Inserts element e before non-null Node succ.
158 */
159 void linkBefore(E e, Node<E> succ) {
160 // assert succ != null;
161 final Node<E> pred = succ.prev;
162 final Node<E> newNode = new Node<>(pred, e, succ);
163 succ.prev = newNode;
164 if (pred == null)
165 first = newNode;
166 else
167 pred.next = newNode;
168 size++;
169 modCount++;
170 }
171
172 /**
173 * Unlinks non-null first node f.
174 */
175 private E unlinkFirst(Node<E> f) {
176 // assert f == first && f != null;
177 final E element = f.item;
178 final Node<E> next = f.next;
179 f.item = null;
180 f.next = null; // help GC
181 first = next;
182 if (next == null)
183 last = null;
184 else
185 next.prev = null;
186 size--;
187 modCount++;
188 return element;
189 }
190
191 /**
192 * Unlinks non-null last node l.
193 */
194 private E unlinkLast(Node<E> l) {
195 // assert l == last && l != null;
196 final E element = l.item;
197 final Node<E> prev = l.prev;
198 l.item = null;
199 l.prev = null; // help GC
200 last = prev;
201 if (prev == null)
202 first = null;
203 else
204 prev.next = null;
205 size--;
206 modCount++;
207 return element;
208 }
209
210 /**
211 * Unlinks non-null node x.
212 */
213 E unlink(Node<E> x) {
214 // assert x != null;
215 final E element = x.item;
216 final Node<E> next = x.next;
217 final Node<E> prev = x.prev;
218
219 if (prev == null) {
220 first = next;
221 } else {
222 prev.next = next;
223 x.prev = null;
224 }
225
226 if (next == null) {
227 last = prev;
228 } else {
229 next.prev = prev;
230 x.next = null;
231 }
232
233 x.item = null;
234 size--;
235 modCount++;
236 return element;
237 }
238
239 /**
240 * Returns the first element in this list.
241 *
242 * @return the first element in this list
243 * @throws NoSuchElementException if this list is empty
244 */
245 public E getFirst() {
246 final Node<E> f = first;
247 if (f == null)
248 throw new NoSuchElementException();
249 return f.item;
250 }
251
252 /**
253 * Returns the last element in this list.
254 *
255 * @return the last element in this list
256 * @throws NoSuchElementException if this list is empty
257 */
258 public E getLast() {
259 final Node<E> l = last;
260 if (l == null)
261 throw new NoSuchElementException();
262 return l.item;
263 }
264
265 /**
266 * Removes and returns the first element from this list.
267 *
268 * @return the first element from this list
269 * @throws NoSuchElementException if this list is empty
270 */
271 public E removeFirst() {
272 final Node<E> f = first;
273 if (f == null)
274 throw new NoSuchElementException();
275 return unlinkFirst(f);
276 }
277
278 /**
279 * Removes and returns the last element from this list.
280 *
281 * @return the last element from this list
282 * @throws NoSuchElementException if this list is empty
283 */
284 public E removeLast() {
285 final Node<E> l = last;
286 if (l == null)
287 throw new NoSuchElementException();
288 return unlinkLast(l);
289 }
290
291 /**
292 * Inserts the specified element at the beginning of this list.
293 *
294 * @param e the element to add
295 */
296 public void addFirst(E e) {
297 linkFirst(e);
298 }
299
300 /**
301 * Appends the specified element to the end of this list.
302 *
303 * <p>This method is equivalent to {@link #add}.
304 *
305 * @param e the element to add
306 */
307 public void addLast(E e) {
308 linkLast(e);
309 }
310
311 /**
312 * Returns {@code true} if this list contains the specified element.
313 * More formally, returns {@code true} if and only if this list contains
314 * at least one element {@code e} such that
315 * {@code Objects.equals(o, e)}.
316 *
317 * @param o element whose presence in this list is to be tested
318 * @return {@code true} if this list contains the specified element
319 */
320 public boolean contains(Object o) {
321 return indexOf(o) >= 0;
322 }
323
324 /**
325 * Returns the number of elements in this list.
326 *
327 * @return the number of elements in this list
328 */
329 public int size() {
330 return size;
331 }
332
333 /**
334 * Appends the specified element to the end of this list.
335 *
336 * <p>This method is equivalent to {@link #addLast}.
337 *
338 * @param e element to be appended to this list
339 * @return {@code true} (as specified by {@link Collection#add})
340 */
341 public boolean add(E e) {
342 linkLast(e);
343 return true;
344 }
345
346 /**
347 * Removes the first occurrence of the specified element from this list,
348 * if it is present. If this list does not contain the element, it is
349 * unchanged. More formally, removes the element with the lowest index
350 * {@code i} such that
351 * {@code Objects.equals(o, get(i))}
352 * (if such an element exists). Returns {@code true} if this list
353 * contained the specified element (or equivalently, if this list
354 * changed as a result of the call).
355 *
356 * @param o element to be removed from this list, if present
357 * @return {@code true} if this list contained the specified element
358 */
359 public boolean remove(Object o) {
360 if (o == null) {
361 for (Node<E> x = first; x != null; x = x.next) {
362 if (x.item == null) {
363 unlink(x);
364 return true;
365 }
366 }
367 } else {
368 for (Node<E> x = first; x != null; x = x.next) {
369 if (o.equals(x.item)) {
370 unlink(x);
371 return true;
372 }
373 }
374 }
375 return false;
376 }
377
378 /**
379 * Appends all of the elements in the specified collection to the end of
380 * this list, in the order that they are returned by the specified
381 * collection's iterator. The behavior of this operation is undefined if
382 * the specified collection is modified while the operation is in
383 * progress. (Note that this will occur if the specified collection is
384 * this list, and it's nonempty.)
385 *
386 * @param c collection containing elements to be added to this list
387 * @return {@code true} if this list changed as a result of the call
388 * @throws NullPointerException if the specified collection is null
389 */
390 public boolean addAll(Collection<? extends E> c) {
391 return addAll(size, c);
392 }
393
394 /**
395 * Inserts all of the elements in the specified collection into this
396 * list, starting at the specified position. Shifts the element
397 * currently at that position (if any) and any subsequent elements to
398 * the right (increases their indices). The new elements will appear
399 * in the list in the order that they are returned by the
400 * specified collection's iterator.
401 *
402 * @param index index at which to insert the first element
403 * from the specified collection
404 * @param c collection containing elements to be added to this list
405 * @return {@code true} if this list changed as a result of the call
406 * @throws IndexOutOfBoundsException {@inheritDoc}
407 * @throws NullPointerException if the specified collection is null
408 */
409 public boolean addAll(int index, Collection<? extends E> c) {
410 checkPositionIndex(index);
411
412 Object[] a = c.toArray();
413 int numNew = a.length;
414 if (numNew == 0)
415 return false;
416
417 Node<E> pred, succ;
418 if (index == size) {
419 succ = null;
420 pred = last;
421 } else {
422 succ = node(index);
423 pred = succ.prev;
424 }
425
426 for (Object o : a) {
427 @SuppressWarnings("unchecked") E e = (E) o;
428 Node<E> newNode = new Node<>(pred, e, null);
429 if (pred == null)
430 first = newNode;
431 else
432 pred.next = newNode;
433 pred = newNode;
434 }
435
436 if (succ == null) {
437 last = pred;
438 } else {
439 pred.next = succ;
440 succ.prev = pred;
441 }
442
443 size += numNew;
444 modCount++;
445 return true;
446 }
447
448 /**
449 * Removes all of the elements from this list.
450 * The list will be empty after this call returns.
451 */
452 public void clear() {
453 // Clearing all of the links between nodes is "unnecessary", but:
454 // - helps a generational GC if the discarded nodes inhabit
455 // more than one generation
456 // - is sure to free memory even if there is a reachable Iterator
457 for (Node<E> x = first; x != null; ) {
458 Node<E> next = x.next;
459 x.item = null;
460 x.next = null;
461 x.prev = null;
462 x = next;
463 }
464 first = last = null;
465 size = 0;
466 modCount++;
467 }
468
469
470 // Positional Access Operations
471
472 /**
473 * Returns the element at the specified position in this list.
474 *
475 * @param index index of the element to return
476 * @return the element at the specified position in this list
477 * @throws IndexOutOfBoundsException {@inheritDoc}
478 */
479 public E get(int index) {
480 checkElementIndex(index);
481 return node(index).item;
482 }
483
484 /**
485 * Replaces the element at the specified position in this list with the
486 * specified element.
487 *
488 * @param index index of the element to replace
489 * @param element element to be stored at the specified position
490 * @return the element previously at the specified position
491 * @throws IndexOutOfBoundsException {@inheritDoc}
492 */
493 public E set(int index, E element) {
494 checkElementIndex(index);
495 Node<E> x = node(index);
496 E oldVal = x.item;
497 x.item = element;
498 return oldVal;
499 }
500
501 /**
502 * Inserts the specified element at the specified position in this list.
503 * Shifts the element currently at that position (if any) and any
504 * subsequent elements to the right (adds one to their indices).
505 *
506 * @param index index at which the specified element is to be inserted
507 * @param element element to be inserted
508 * @throws IndexOutOfBoundsException {@inheritDoc}
509 */
510 public void add(int index, E element) {
511 checkPositionIndex(index);
512
513 if (index == size)
514 linkLast(element);
515 else
516 linkBefore(element, node(index));
517 }
518
519 /**
520 * Removes the element at the specified position in this list. Shifts any
521 * subsequent elements to the left (subtracts one from their indices).
522 * Returns the element that was removed from the list.
523 *
524 * @param index the index of the element to be removed
525 * @return the element previously at the specified position
526 * @throws IndexOutOfBoundsException {@inheritDoc}
527 */
528 public E remove(int index) {
529 checkElementIndex(index);
530 return unlink(node(index));
531 }
532
533 /**
534 * Tells if the argument is the index of an existing element.
535 */
536 private boolean isElementIndex(int index) {
537 return index >= 0 && index < size;
538 }
539
540 /**
541 * Tells if the argument is the index of a valid position for an
542 * iterator or an add operation.
543 */
544 private boolean isPositionIndex(int index) {
545 return index >= 0 && index <= size;
546 }
547
548 /**
549 * Constructs an IndexOutOfBoundsException detail message.
550 * Of the many possible refactorings of the error handling code,
551 * this "outlining" performs best with both server and client VMs.
552 */
553 private String outOfBoundsMsg(int index) {
554 return "Index: "+index+", Size: "+size;
555 }
556
557 private void checkElementIndex(int index) {
558 if (!isElementIndex(index))
559 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
560 }
561
562 private void checkPositionIndex(int index) {
563 if (!isPositionIndex(index))
564 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
565 }
566
567 /**
568 * Returns the (non-null) Node at the specified element index.
569 */
570 Node<E> node(int index) {
571 // assert isElementIndex(index);
572
573 if (index < (size >> 1)) {
574 Node<E> x = first;
575 for (int i = 0; i < index; i++)
576 x = x.next;
577 return x;
578 } else {
579 Node<E> x = last;
580 for (int i = size - 1; i > index; i--)
581 x = x.prev;
582 return x;
583 }
584 }
585
586 // Search Operations
587
588 /**
589 * Returns the index of the first occurrence of the specified element
590 * in this list, or -1 if this list does not contain the element.
591 * More formally, returns the lowest index {@code i} such that
592 * {@code Objects.equals(o, get(i))},
593 * or -1 if there is no such index.
594 *
595 * @param o element to search for
596 * @return the index of the first occurrence of the specified element in
597 * this list, or -1 if this list does not contain the element
598 */
599 public int indexOf(Object o) {
600 int index = 0;
601 if (o == null) {
602 for (Node<E> x = first; x != null; x = x.next) {
603 if (x.item == null)
604 return index;
605 index++;
606 }
607 } else {
608 for (Node<E> x = first; x != null; x = x.next) {
609 if (o.equals(x.item))
610 return index;
611 index++;
612 }
613 }
614 return -1;
615 }
616
617 /**
618 * Returns the index of the last occurrence of the specified element
619 * in this list, or -1 if this list does not contain the element.
620 * More formally, returns the highest index {@code i} such that
621 * {@code Objects.equals(o, get(i))},
622 * or -1 if there is no such index.
623 *
624 * @param o element to search for
625 * @return the index of the last occurrence of the specified element in
626 * this list, or -1 if this list does not contain the element
627 */
628 public int lastIndexOf(Object o) {
629 int index = size;
630 if (o == null) {
631 for (Node<E> x = last; x != null; x = x.prev) {
632 index--;
633 if (x.item == null)
634 return index;
635 }
636 } else {
637 for (Node<E> x = last; x != null; x = x.prev) {
638 index--;
639 if (o.equals(x.item))
640 return index;
641 }
642 }
643 return -1;
644 }
645
646 // Queue operations.
647
648 /**
649 * Retrieves, but does not remove, the head (first element) of this list.
650 *
651 * @return the head of this list, or {@code null} if this list is empty
652 * @since 1.5
653 */
654 public E peek() {
655 final Node<E> f = first;
656 return (f == null) ? null : f.item;
657 }
658
659 /**
660 * Retrieves, but does not remove, the head (first element) of this list.
661 *
662 * @return the head of this list
663 * @throws NoSuchElementException if this list is empty
664 * @since 1.5
665 */
666 public E element() {
667 return getFirst();
668 }
669
670 /**
671 * Retrieves and removes the head (first element) of this list.
672 *
673 * @return the head of this list, or {@code null} if this list is empty
674 * @since 1.5
675 */
676 public E poll() {
677 final Node<E> f = first;
678 return (f == null) ? null : unlinkFirst(f);
679 }
680
681 /**
682 * Retrieves and removes the head (first element) of this list.
683 *
684 * @return the head of this list
685 * @throws NoSuchElementException if this list is empty
686 * @since 1.5
687 */
688 public E remove() {
689 return removeFirst();
690 }
691
692 /**
693 * Adds the specified element as the tail (last element) of this list.
694 *
695 * @param e the element to add
696 * @return {@code true} (as specified by {@link Queue#offer})
697 * @since 1.5
698 */
699 public boolean offer(E e) {
700 return add(e);
701 }
702
703 // Deque operations
704 /**
705 * Inserts the specified element at the front of this list.
706 *
707 * @param e the element to insert
708 * @return {@code true} (as specified by {@link Deque#offerFirst})
709 * @since 1.6
710 */
711 public boolean offerFirst(E e) {
712 addFirst(e);
713 return true;
714 }
715
716 /**
717 * Inserts the specified element at the end of this list.
718 *
719 * @param e the element to insert
720 * @return {@code true} (as specified by {@link Deque#offerLast})
721 * @since 1.6
722 */
723 public boolean offerLast(E e) {
724 addLast(e);
725 return true;
726 }
727
728 /**
729 * Retrieves, but does not remove, the first element of this list,
730 * or returns {@code null} if this list is empty.
731 *
732 * @return the first element of this list, or {@code null}
733 * if this list is empty
734 * @since 1.6
735 */
736 public E peekFirst() {
737 final Node<E> f = first;
738 return (f == null) ? null : f.item;
739 }
740
741 /**
742 * Retrieves, but does not remove, the last element of this list,
743 * or returns {@code null} if this list is empty.
744 *
745 * @return the last element of this list, or {@code null}
746 * if this list is empty
747 * @since 1.6
748 */
749 public E peekLast() {
750 final Node<E> l = last;
751 return (l == null) ? null : l.item;
752 }
753
754 /**
755 * Retrieves and removes the first element of this list,
756 * or returns {@code null} if this list is empty.
757 *
758 * @return the first element of this list, or {@code null} if
759 * this list is empty
760 * @since 1.6
761 */
762 public E pollFirst() {
763 final Node<E> f = first;
764 return (f == null) ? null : unlinkFirst(f);
765 }
766
767 /**
768 * Retrieves and removes the last element of this list,
769 * or returns {@code null} if this list is empty.
770 *
771 * @return the last element of this list, or {@code null} if
772 * this list is empty
773 * @since 1.6
774 */
775 public E pollLast() {
776 final Node<E> l = last;
777 return (l == null) ? null : unlinkLast(l);
778 }
779
780 /**
781 * Pushes an element onto the stack represented by this list. In other
782 * words, inserts the element at the front of this list.
783 *
784 * <p>This method is equivalent to {@link #addFirst}.
785 *
786 * @param e the element to push
787 * @since 1.6
788 */
789 public void push(E e) {
790 addFirst(e);
791 }
792
793 /**
794 * Pops an element from the stack represented by this list. In other
795 * words, removes and returns the first element of this list.
796 *
797 * <p>This method is equivalent to {@link #removeFirst()}.
798 *
799 * @return the element at the front of this list (which is the top
800 * of the stack represented by this list)
801 * @throws NoSuchElementException if this list is empty
802 * @since 1.6
803 */
804 public E pop() {
805 return removeFirst();
806 }
807
808 /**
809 * Removes the first occurrence of the specified element in this
810 * list (when traversing the list from head to tail). If the list
811 * does not contain the element, it is unchanged.
812 *
813 * @param o element to be removed from this list, if present
814 * @return {@code true} if the list contained the specified element
815 * @since 1.6
816 */
817 public boolean removeFirstOccurrence(Object o) {
818 return remove(o);
819 }
820
821 /**
822 * Removes the last occurrence of the specified element in this
823 * list (when traversing the list from head to tail). If the list
824 * does not contain the element, it is unchanged.
825 *
826 * @param o element to be removed from this list, if present
827 * @return {@code true} if the list contained the specified element
828 * @since 1.6
829 */
830 public boolean removeLastOccurrence(Object o) {
831 if (o == null) {
832 for (Node<E> x = last; x != null; x = x.prev) {
833 if (x.item == null) {
834 unlink(x);
835 return true;
836 }
837 }
838 } else {
839 for (Node<E> x = last; x != null; x = x.prev) {
840 if (o.equals(x.item)) {
841 unlink(x);
842 return true;
843 }
844 }
845 }
846 return false;
847 }
848
849 /**
850 * Returns a list-iterator of the elements in this list (in proper
851 * sequence), starting at the specified position in the list.
852 * Obeys the general contract of {@code List.listIterator(int)}.<p>
853 *
854 * The list-iterator is <i>fail-fast</i>: if the list is structurally
855 * modified at any time after the Iterator is created, in any way except
856 * through the list-iterator's own {@code remove} or {@code add}
857 * methods, the list-iterator will throw a
858 * {@code ConcurrentModificationException}. Thus, in the face of
859 * concurrent modification, the iterator fails quickly and cleanly, rather
860 * than risking arbitrary, non-deterministic behavior at an undetermined
861 * time in the future.
862 *
863 * @param index index of the first element to be returned from the
864 * list-iterator (by a call to {@code next})
865 * @return a ListIterator of the elements in this list (in proper
866 * sequence), starting at the specified position in the list
867 * @throws IndexOutOfBoundsException {@inheritDoc}
868 * @see List#listIterator(int)
869 */
870 public ListIterator<E> listIterator(int index) {
871 checkPositionIndex(index);
872 return new ListItr(index);
873 }
874
875 private class ListItr implements ListIterator<E> {
876 private Node<E> lastReturned;
877 private Node<E> next;
878 private int nextIndex;
879 private int expectedModCount = modCount;
880
881 ListItr(int index) {
882 // assert isPositionIndex(index);
883 next = (index == size) ? null : node(index);
884 nextIndex = index;
885 }
886
887 public boolean hasNext() {
888 return nextIndex < size;
889 }
890
891 public E next() {
892 checkForComodification();
893 if (!hasNext())
894 throw new NoSuchElementException();
895
896 lastReturned = next;
897 next = next.next;
898 nextIndex++;
899 return lastReturned.item;
900 }
901
902 public boolean hasPrevious() {
903 return nextIndex > 0;
904 }
905
906 public E previous() {
907 checkForComodification();
908 if (!hasPrevious())
909 throw new NoSuchElementException();
910
911 lastReturned = next = (next == null) ? last : next.prev;
912 nextIndex--;
913 return lastReturned.item;
914 }
915
916 public int nextIndex() {
917 return nextIndex;
918 }
919
920 public int previousIndex() {
921 return nextIndex - 1;
922 }
923
924 public void remove() {
925 checkForComodification();
926 if (lastReturned == null)
927 throw new IllegalStateException();
928
929 Node<E> lastNext = lastReturned.next;
930 unlink(lastReturned);
931 if (next == lastReturned)
932 next = lastNext;
933 else
934 nextIndex--;
935 lastReturned = null;
936 expectedModCount++;
937 }
938
939 public void set(E e) {
940 if (lastReturned == null)
941 throw new IllegalStateException();
942 checkForComodification();
943 lastReturned.item = e;
944 }
945
946 public void add(E e) {
947 checkForComodification();
948 lastReturned = null;
949 if (next == null)
950 linkLast(e);
951 else
952 linkBefore(e, next);
953 nextIndex++;
954 expectedModCount++;
955 }
956
957 public void forEachRemaining(Consumer<? super E> action) {
958 Objects.requireNonNull(action);
959 while (modCount == expectedModCount && nextIndex < size) {
960 action.accept(next.item);
961 lastReturned = next;
962 next = next.next;
963 nextIndex++;
964 }
965 checkForComodification();
966 }
967
968 final void checkForComodification() {
969 if (modCount != expectedModCount)
970 throw new ConcurrentModificationException();
971 }
972 }
973
974 private static class Node<E> {
975 E item;
976 Node<E> next;
977 Node<E> prev;
978
979 Node(Node<E> prev, E element, Node<E> next) {
980 this.item = element;
981 this.next = next;
982 this.prev = prev;
983 }
984 }
985
986 /**
987 * @since 1.6
988 */
989 public Iterator<E> descendingIterator() {
990 return new DescendingIterator();
991 }
992
993 /**
994 * Adapter to provide descending iterators via ListItr.previous
995 */
996 private class DescendingIterator implements Iterator<E> {
997 private final ListItr itr = new ListItr(size());
998 public boolean hasNext() {
999 return itr.hasPrevious();
1000 }
1001 public E next() {
1002 return itr.previous();
1003 }
1004 public void remove() {
1005 itr.remove();
1006 }
1007 }
1008
1009 @SuppressWarnings("unchecked")
1010 private LinkedList<E> superClone() {
1011 try {
1012 return (LinkedList<E>) super.clone();
1013 } catch (CloneNotSupportedException e) {
1014 throw new InternalError(e);
1015 }
1016 }
1017
1018 /**
1019 * Returns a shallow copy of this {@code LinkedList}. (The elements
1020 * themselves are not cloned.)
1021 *
1022 * @return a shallow copy of this {@code LinkedList} instance
1023 */
1024 public Object clone() {
1025 LinkedList<E> clone = superClone();
1026
1027 // Put clone into "virgin" state
1028 clone.first = clone.last = null;
1029 clone.size = 0;
1030 clone.modCount = 0;
1031
1032 // Initialize clone with our elements
1033 for (Node<E> x = first; x != null; x = x.next)
1034 clone.add(x.item);
1035
1036 return clone;
1037 }
1038
1039 /**
1040 * Returns an array containing all of the elements in this list
1041 * in proper sequence (from first to last element).
1042 *
1043 * <p>The returned array will be "safe" in that no references to it are
1044 * maintained by this list. (In other words, this method must allocate
1045 * a new array). The caller is thus free to modify the returned array.
1046 *
1047 * <p>This method acts as bridge between array-based and collection-based
1048 * APIs.
1049 *
1050 * @return an array containing all of the elements in this list
1051 * in proper sequence
1052 */
1053 public Object[] toArray() {
1054 Object[] result = new Object[size];
1055 int i = 0;
1056 for (Node<E> x = first; x != null; x = x.next)
1057 result[i++] = x.item;
1058 return result;
1059 }
1060
1061 /**
1062 * Returns an array containing all of the elements in this list in
1063 * proper sequence (from first to last element); the runtime type of
1064 * the returned array is that of the specified array. If the list fits
1065 * in the specified array, it is returned therein. Otherwise, a new
1066 * array is allocated with the runtime type of the specified array and
1067 * the size of this list.
1068 *
1069 * <p>If the list fits in the specified array with room to spare (i.e.,
1070 * the array has more elements than the list), the element in the array
1071 * immediately following the end of the list is set to {@code null}.
1072 * (This is useful in determining the length of the list <i>only</i> if
1073 * the caller knows that the list does not contain any null elements.)
1074 *
1075 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1076 * array-based and collection-based APIs. Further, this method allows
1077 * precise control over the runtime type of the output array, and may,
1078 * under certain circumstances, be used to save allocation costs.
1079 *
1080 * <p>Suppose {@code x} is a list known to contain only strings.
1081 * The following code can be used to dump the list into a newly
1082 * allocated array of {@code String}:
1083 *
1084 * <pre>
1085 * String[] y = x.toArray(new String[0]);</pre>
1086 *
1087 * Note that {@code toArray(new Object[0])} is identical in function to
1088 * {@code toArray()}.
1089 *
1090 * @param a the array into which the elements of the list are to
1091 * be stored, if it is big enough; otherwise, a new array of the
1092 * same runtime type is allocated for this purpose.
1093 * @return an array containing the elements of the list
1094 * @throws ArrayStoreException if the runtime type of the specified array
1095 * is not a supertype of the runtime type of every element in
1096 * this list
1097 * @throws NullPointerException if the specified array is null
1098 */
1099 @SuppressWarnings("unchecked")
1100 public <T> T[] toArray(T[] a) {
1101 if (a.length < size)
1102 a = (T[])java.lang.reflect.Array.newInstance(
1103 a.getClass().getComponentType(), size);
1104 int i = 0;
1105 Object[] result = a;
1106 for (Node<E> x = first; x != null; x = x.next)
1107 result[i++] = x.item;
1108
1109 if (a.length > size)
1110 a[size] = null;
1111
1112 return a;
1113 }
1114
1115 @java.io.Serial
1116 private static final long serialVersionUID = 876323262645176354L;
1117
1118 /**
1119 * Saves the state of this {@code LinkedList} instance to a stream
1120 * (that is, serializes it).
1121 *
1122 * @serialData The size of the list (the number of elements it
1123 * contains) is emitted (int), followed by all of its
1124 * elements (each an Object) in the proper order.
1125 */
1126 @java.io.Serial
1127 private void writeObject(java.io.ObjectOutputStream s)
1128 throws java.io.IOException {
1129 // Write out any hidden serialization magic
1130 s.defaultWriteObject();
1131
1132 // Write out size
1133 s.writeInt(size);
1134
1135 // Write out all elements in the proper order.
1136 for (Node<E> x = first; x != null; x = x.next)
1137 s.writeObject(x.item);
1138 }
1139
1140 /**
1141 * Reconstitutes this {@code LinkedList} instance from a stream
1142 * (that is, deserializes it).
1143 */
1144 @SuppressWarnings("unchecked")
1145 @java.io.Serial
1146 private void readObject(java.io.ObjectInputStream s)
1147 throws java.io.IOException, ClassNotFoundException {
1148 // Read in any hidden serialization magic
1149 s.defaultReadObject();
1150
1151 // Read in size
1152 int size = s.readInt();
1153
1154 // Read in all elements in the proper order.
1155 for (int i = 0; i < size; i++)
1156 linkLast((E)s.readObject());
1157 }
1158
1159 /**
1160 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
1161 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
1162 * list.
1163 *
1164 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
1165 * {@link Spliterator#ORDERED}. Overriding implementations should document
1166 * the reporting of additional characteristic values.
1167 *
1168 * @implNote
1169 * The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
1170 * and implements {@code trySplit} to permit limited parallelism..
1171 *
1172 * @return a {@code Spliterator} over the elements in this list
1173 * @since 1.8
1174 */
1175 @Override
1176 public Spliterator<E> spliterator() {
1177 return new LLSpliterator<>(this, -1, 0);
1178 }
1179
1180 /** A customized variant of Spliterators.IteratorSpliterator */
1181 static final class LLSpliterator<E> implements Spliterator<E> {
1182 static final int BATCH_UNIT = 1 << 10; // batch array size increment
1183 static final int MAX_BATCH = 1 << 25; // max batch array size;
1184 final LinkedList<E> list; // null OK unless traversed
1185 Node<E> current; // current node; null until initialized
1186 int est; // size estimate; -1 until first needed
1187 int expectedModCount; // initialized when est set
1188 int batch; // batch size for splits
1189
1190 LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
1191 this.list = list;
1192 this.est = est;
1193 this.expectedModCount = expectedModCount;
1194 }
1195
1196 final int getEst() {
1197 int s; // force initialization
1198 final LinkedList<E> lst;
1199 if ((s = est) < 0) {
1200 if ((lst = list) == null)
1201 s = est = 0;
1202 else {
1203 expectedModCount = lst.modCount;
1204 current = lst.first;
1205 s = est = lst.size;
1206 }
1207 }
1208 return s;
1209 }
1210
1211 public long estimateSize() { return (long) getEst(); }
1212
1213 public Spliterator<E> trySplit() {
1214 Node<E> p;
1215 int s = getEst();
1216 if (s > 1 && (p = current) != null) {
1217 int n = batch + BATCH_UNIT;
1218 if (n > s)
1219 n = s;
1220 if (n > MAX_BATCH)
1221 n = MAX_BATCH;
1222 Object[] a = new Object[n];
1223 int j = 0;
1224 do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
1225 current = p;
1226 batch = j;
1227 est = s - j;
1228 return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
1229 }
1230 return null;
1231 }
1232
1233 public void forEachRemaining(Consumer<? super E> action) {
1234 Node<E> p; int n;
1235 if (action == null) throw new NullPointerException();
1236 if ((n = getEst()) > 0 && (p = current) != null) {
1237 current = null;
1238 est = 0;
1239 do {
1240 E e = p.item;
1241 p = p.next;
1242 action.accept(e);
1243 } while (p != null && --n > 0);
1244 }
1245 if (list.modCount != expectedModCount)
1246 throw new ConcurrentModificationException();
1247 }
1248
1249 public boolean tryAdvance(Consumer<? super E> action) {
1250 Node<E> p;
1251 if (action == null) throw new NullPointerException();
1252 if (getEst() > 0 && (p = current) != null) {
1253 --est;
1254 E e = p.item;
1255 current = p.next;
1256 action.accept(e);
1257 if (list.modCount != expectedModCount)
1258 throw new ConcurrentModificationException();
1259 return true;
1260 }
1261 return false;
1262 }
1263
1264 public int characteristics() {
1265 return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
1266 }
1267 }
1268
1269 }