1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Josh Bloch of Google Inc. and released to the public domain,
32 * as explained at http://creativecommons.org/publicdomain/zero/1.0/.
33 */
34
35 package java.util;
36
37 import java.io.Serializable;
38 import java.util.function.Consumer;
39 import sun.misc.SharedSecrets;
40
41 /**
42 * Resizable-array implementation of the {@link Deque} interface. Array
43 * deques have no capacity restrictions; they grow as necessary to support
44 * usage. They are not thread-safe; in the absence of external
45 * synchronization, they do not support concurrent access by multiple threads.
46 * Null elements are prohibited. This class is likely to be faster than
47 * {@link Stack} when used as a stack, and faster than {@link LinkedList}
48 * when used as a queue.
49 *
50 * <p>Most {@code ArrayDeque} operations run in amortized constant time.
51 * Exceptions include {@link #remove(Object) remove}, {@link
52 * #removeFirstOccurrence removeFirstOccurrence}, {@link #removeLastOccurrence
53 * removeLastOccurrence}, {@link #contains contains}, {@link #iterator
54 * iterator.remove()}, and the bulk operations, all of which run in linear
55 * time.
56 *
57 * <p>The iterators returned by this class's {@code iterator} method are
58 * <i>fail-fast</i>: If the deque is modified at any time after the iterator
59 * is created, in any way except through the iterator's own {@code remove}
60 * method, the iterator will generally throw a {@link
61 * ConcurrentModificationException}. Thus, in the face of concurrent
62 * modification, the iterator fails quickly and cleanly, rather than risking
63 * arbitrary, non-deterministic behavior at an undetermined time in the
64 * future.
65 *
66 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
67 * as it is, generally speaking, impossible to make any hard guarantees in the
68 * presence of unsynchronized concurrent modification. Fail-fast iterators
69 * throw {@code ConcurrentModificationException} on a best-effort basis.
70 * Therefore, it would be wrong to write a program that depended on this
71 * exception for its correctness: <i>the fail-fast behavior of iterators
72 * should be used only to detect bugs.</i>
73 *
74 * <p>This class and its iterator implement all of the
75 * <em>optional</em> methods of the {@link Collection} and {@link
76 * Iterator} interfaces.
77 *
78 * <p>This class is a member of the
79 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
80 * Java Collections Framework</a>.
81 *
82 * @author Josh Bloch and Doug Lea
83 * @since 1.6
84 * @param <E> the type of elements held in this collection
85 */
86 public class ArrayDeque<E> extends AbstractCollection<E>
87 implements Deque<E>, Cloneable, Serializable
88 {
89 /**
90 * The array in which the elements of the deque are stored.
91 * The capacity of the deque is the length of this array, which is
92 * always a power of two. The array is never allowed to become
93 * full, except transiently within an addX method where it is
94 * resized (see doubleCapacity) immediately upon becoming full,
95 * thus avoiding head and tail wrapping around to equal each
96 * other. We also guarantee that all array cells not holding
97 * deque elements are always null.
98 */
99 transient Object[] elements; // non-private to simplify nested class access
100
101 /**
102 * The index of the element at the head of the deque (which is the
103 * element that would be removed by remove() or pop()); or an
104 * arbitrary number equal to tail if the deque is empty.
105 */
106 transient int head;
107
108 /**
109 * The index at which the next element would be added to the tail
110 * of the deque (via addLast(E), add(E), or push(E)).
111 */
112 transient int tail;
113
114 /**
115 * The minimum capacity that we'll use for a newly created deque.
116 * Must be a power of 2.
117 */
118 private static final int MIN_INITIAL_CAPACITY = 8;
119
120 // ****** Array allocation and resizing utilities ******
121
122 private static int calculateSize(int numElements) {
123 int initialCapacity = MIN_INITIAL_CAPACITY;
124 // Find the best power of two to hold elements.
125 // Tests "<=" because arrays aren't kept full.
126 if (numElements >= initialCapacity) {
127 initialCapacity = numElements;
128 initialCapacity |= (initialCapacity >>> 1);
129 initialCapacity |= (initialCapacity >>> 2);
130 initialCapacity |= (initialCapacity >>> 4);
131 initialCapacity |= (initialCapacity >>> 8);
132 initialCapacity |= (initialCapacity >>> 16);
133 initialCapacity++;
134
135 if (initialCapacity < 0) // Too many elements, must back off
136 initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
137 }
138 return initialCapacity;
139 }
140
141 /**
142 * Allocates empty array to hold the given number of elements.
143 *
144 * @param numElements the number of elements to hold
145 */
146 private void allocateElements(int numElements) {
147 elements = new Object[calculateSize(numElements)];
148 }
149
150 /**
151 * Doubles the capacity of this deque. Call only when full, i.e.,
152 * when head and tail have wrapped around to become equal.
153 */
154 private void doubleCapacity() {
155 assert head == tail;
156 int p = head;
157 int n = elements.length;
158 int r = n - p; // number of elements to the right of p
159 int newCapacity = n << 1;
160 if (newCapacity < 0)
161 throw new IllegalStateException("Sorry, deque too big");
162 Object[] a = new Object[newCapacity];
163 System.arraycopy(elements, p, a, 0, r);
164 System.arraycopy(elements, 0, a, r, p);
165 elements = a;
166 head = 0;
167 tail = n;
168 }
169
170 /**
171 * Copies the elements from our element array into the specified array,
172 * in order (from first to last element in the deque). It is assumed
173 * that the array is large enough to hold all elements in the deque.
174 *
175 * @return its argument
176 */
177 private <T> T[] copyElements(T[] a) {
178 if (head < tail) {
179 System.arraycopy(elements, head, a, 0, size());
180 } else if (head > tail) {
181 int headPortionLen = elements.length - head;
182 System.arraycopy(elements, head, a, 0, headPortionLen);
183 System.arraycopy(elements, 0, a, headPortionLen, tail);
184 }
185 return a;
186 }
187
188 /**
189 * Constructs an empty array deque with an initial capacity
190 * sufficient to hold 16 elements.
191 */
192 public ArrayDeque() {
193 elements = new Object[16];
194 }
195
196 /**
197 * Constructs an empty array deque with an initial capacity
198 * sufficient to hold the specified number of elements.
199 *
200 * @param numElements lower bound on initial capacity of the deque
201 */
202 public ArrayDeque(int numElements) {
203 allocateElements(numElements);
204 }
205
206 /**
207 * Constructs a deque containing the elements of the specified
208 * collection, in the order they are returned by the collection's
209 * iterator. (The first element returned by the collection's
210 * iterator becomes the first element, or <i>front</i> of the
211 * deque.)
212 *
213 * @param c the collection whose elements are to be placed into the deque
214 * @throws NullPointerException if the specified collection is null
215 */
216 public ArrayDeque(Collection<? extends E> c) {
217 allocateElements(c.size());
218 addAll(c);
219 }
220
221 // The main insertion and extraction methods are addFirst,
222 // addLast, pollFirst, pollLast. The other methods are defined in
223 // terms of these.
224
225 /**
226 * Inserts the specified element at the front of this deque.
227 *
228 * @param e the element to add
229 * @throws NullPointerException if the specified element is null
230 */
231 public void addFirst(E e) {
232 if (e == null)
233 throw new NullPointerException();
234 elements[head = (head - 1) & (elements.length - 1)] = e;
235 if (head == tail)
236 doubleCapacity();
237 }
238
239 /**
240 * Inserts the specified element at the end of this deque.
241 *
242 * <p>This method is equivalent to {@link #add}.
243 *
244 * @param e the element to add
245 * @throws NullPointerException if the specified element is null
246 */
247 public void addLast(E e) {
248 if (e == null)
249 throw new NullPointerException();
250 elements[tail] = e;
251 if ( (tail = (tail + 1) & (elements.length - 1)) == head)
252 doubleCapacity();
253 }
254
255 /**
256 * Inserts the specified element at the front of this deque.
257 *
258 * @param e the element to add
259 * @return {@code true} (as specified by {@link Deque#offerFirst})
260 * @throws NullPointerException if the specified element is null
261 */
262 public boolean offerFirst(E e) {
263 addFirst(e);
264 return true;
265 }
266
267 /**
268 * Inserts the specified element at the end of this deque.
269 *
270 * @param e the element to add
271 * @return {@code true} (as specified by {@link Deque#offerLast})
272 * @throws NullPointerException if the specified element is null
273 */
274 public boolean offerLast(E e) {
275 addLast(e);
276 return true;
277 }
278
279 /**
280 * @throws NoSuchElementException {@inheritDoc}
281 */
282 public E removeFirst() {
283 E x = pollFirst();
284 if (x == null)
285 throw new NoSuchElementException();
286 return x;
287 }
288
289 /**
290 * @throws NoSuchElementException {@inheritDoc}
291 */
292 public E removeLast() {
293 E x = pollLast();
294 if (x == null)
295 throw new NoSuchElementException();
296 return x;
297 }
298
299 public E pollFirst() {
300 int h = head;
301 @SuppressWarnings("unchecked")
302 E result = (E) elements[h];
303 // Element is null if deque empty
304 if (result == null)
305 return null;
306 elements[h] = null; // Must null out slot
307 head = (h + 1) & (elements.length - 1);
308 return result;
309 }
310
311 public E pollLast() {
312 int t = (tail - 1) & (elements.length - 1);
313 @SuppressWarnings("unchecked")
314 E result = (E) elements[t];
315 if (result == null)
316 return null;
317 elements[t] = null;
318 tail = t;
319 return result;
320 }
321
322 /**
323 * @throws NoSuchElementException {@inheritDoc}
324 */
325 public E getFirst() {
326 @SuppressWarnings("unchecked")
327 E result = (E) elements[head];
328 if (result == null)
329 throw new NoSuchElementException();
330 return result;
331 }
332
333 /**
334 * @throws NoSuchElementException {@inheritDoc}
335 */
336 public E getLast() {
337 @SuppressWarnings("unchecked")
338 E result = (E) elements[(tail - 1) & (elements.length - 1)];
339 if (result == null)
340 throw new NoSuchElementException();
341 return result;
342 }
343
344 @SuppressWarnings("unchecked")
345 public E peekFirst() {
346 // elements[head] is null if deque empty
347 return (E) elements[head];
348 }
349
350 @SuppressWarnings("unchecked")
351 public E peekLast() {
352 return (E) elements[(tail - 1) & (elements.length - 1)];
353 }
354
355 /**
356 * Removes the first occurrence of the specified element in this
357 * deque (when traversing the deque from head to tail).
358 * If the deque does not contain the element, it is unchanged.
359 * More formally, removes the first element {@code e} such that
360 * {@code o.equals(e)} (if such an element exists).
361 * Returns {@code true} if this deque contained the specified element
362 * (or equivalently, if this deque changed as a result of the call).
363 *
364 * @param o element to be removed from this deque, if present
365 * @return {@code true} if the deque contained the specified element
366 */
367 public boolean removeFirstOccurrence(Object o) {
368 if (o == null)
369 return false;
370 int mask = elements.length - 1;
371 int i = head;
372 Object x;
373 while ( (x = elements[i]) != null) {
374 if (o.equals(x)) {
375 delete(i);
376 return true;
377 }
378 i = (i + 1) & mask;
379 }
380 return false;
381 }
382
383 /**
384 * Removes the last occurrence of the specified element in this
385 * deque (when traversing the deque from head to tail).
386 * If the deque does not contain the element, it is unchanged.
387 * More formally, removes the last element {@code e} such that
388 * {@code o.equals(e)} (if such an element exists).
389 * Returns {@code true} if this deque contained the specified element
390 * (or equivalently, if this deque changed as a result of the call).
391 *
392 * @param o element to be removed from this deque, if present
393 * @return {@code true} if the deque contained the specified element
394 */
395 public boolean removeLastOccurrence(Object o) {
396 if (o == null)
397 return false;
398 int mask = elements.length - 1;
399 int i = (tail - 1) & mask;
400 Object x;
401 while ( (x = elements[i]) != null) {
402 if (o.equals(x)) {
403 delete(i);
404 return true;
405 }
406 i = (i - 1) & mask;
407 }
408 return false;
409 }
410
411 // *** Queue methods ***
412
413 /**
414 * Inserts the specified element at the end of this deque.
415 *
416 * <p>This method is equivalent to {@link #addLast}.
417 *
418 * @param e the element to add
419 * @return {@code true} (as specified by {@link Collection#add})
420 * @throws NullPointerException if the specified element is null
421 */
422 public boolean add(E e) {
423 addLast(e);
424 return true;
425 }
426
427 /**
428 * Inserts the specified element at the end of this deque.
429 *
430 * <p>This method is equivalent to {@link #offerLast}.
431 *
432 * @param e the element to add
433 * @return {@code true} (as specified by {@link Queue#offer})
434 * @throws NullPointerException if the specified element is null
435 */
436 public boolean offer(E e) {
437 return offerLast(e);
438 }
439
440 /**
441 * Retrieves and removes the head of the queue represented by this deque.
442 *
443 * This method differs from {@link #poll poll} only in that it throws an
444 * exception if this deque is empty.
445 *
446 * <p>This method is equivalent to {@link #removeFirst}.
447 *
448 * @return the head of the queue represented by this deque
449 * @throws NoSuchElementException {@inheritDoc}
450 */
451 public E remove() {
452 return removeFirst();
453 }
454
455 /**
456 * Retrieves and removes the head of the queue represented by this deque
457 * (in other words, the first element of this deque), or returns
458 * {@code null} if this deque is empty.
459 *
460 * <p>This method is equivalent to {@link #pollFirst}.
461 *
462 * @return the head of the queue represented by this deque, or
463 * {@code null} if this deque is empty
464 */
465 public E poll() {
466 return pollFirst();
467 }
468
469 /**
470 * Retrieves, but does not remove, the head of the queue represented by
471 * this deque. This method differs from {@link #peek peek} only in
472 * that it throws an exception if this deque is empty.
473 *
474 * <p>This method is equivalent to {@link #getFirst}.
475 *
476 * @return the head of the queue represented by this deque
477 * @throws NoSuchElementException {@inheritDoc}
478 */
479 public E element() {
480 return getFirst();
481 }
482
483 /**
484 * Retrieves, but does not remove, the head of the queue represented by
485 * this deque, or returns {@code null} if this deque is empty.
486 *
487 * <p>This method is equivalent to {@link #peekFirst}.
488 *
489 * @return the head of the queue represented by this deque, or
490 * {@code null} if this deque is empty
491 */
492 public E peek() {
493 return peekFirst();
494 }
495
496 // *** Stack methods ***
497
498 /**
499 * Pushes an element onto the stack represented by this deque. In other
500 * words, inserts the element at the front of this deque.
501 *
502 * <p>This method is equivalent to {@link #addFirst}.
503 *
504 * @param e the element to push
505 * @throws NullPointerException if the specified element is null
506 */
507 public void push(E e) {
508 addFirst(e);
509 }
510
511 /**
512 * Pops an element from the stack represented by this deque. In other
513 * words, removes and returns the first element of this deque.
514 *
515 * <p>This method is equivalent to {@link #removeFirst()}.
516 *
517 * @return the element at the front of this deque (which is the top
518 * of the stack represented by this deque)
519 * @throws NoSuchElementException {@inheritDoc}
520 */
521 public E pop() {
522 return removeFirst();
523 }
524
525 private void checkInvariants() {
526 assert elements[tail] == null;
527 assert head == tail ? elements[head] == null :
528 (elements[head] != null &&
529 elements[(tail - 1) & (elements.length - 1)] != null);
530 assert elements[(head - 1) & (elements.length - 1)] == null;
531 }
532
533 /**
534 * Removes the element at the specified position in the elements array,
535 * adjusting head and tail as necessary. This can result in motion of
536 * elements backwards or forwards in the array.
537 *
538 * <p>This method is called delete rather than remove to emphasize
539 * that its semantics differ from those of {@link List#remove(int)}.
540 *
541 * @return true if elements moved backwards
542 */
543 private boolean delete(int i) {
544 checkInvariants();
545 final Object[] elements = this.elements;
546 final int mask = elements.length - 1;
547 final int h = head;
548 final int t = tail;
549 final int front = (i - h) & mask;
550 final int back = (t - i) & mask;
551
552 // Invariant: head <= i < tail mod circularity
553 if (front >= ((t - h) & mask))
554 throw new ConcurrentModificationException();
555
556 // Optimize for least element motion
557 if (front < back) {
558 if (h <= i) {
559 System.arraycopy(elements, h, elements, h + 1, front);
560 } else { // Wrap around
561 System.arraycopy(elements, 0, elements, 1, i);
562 elements[0] = elements[mask];
563 System.arraycopy(elements, h, elements, h + 1, mask - h);
564 }
565 elements[h] = null;
566 head = (h + 1) & mask;
567 return false;
568 } else {
569 if (i < t) { // Copy the null tail as well
570 System.arraycopy(elements, i + 1, elements, i, back);
571 tail = t - 1;
572 } else { // Wrap around
573 System.arraycopy(elements, i + 1, elements, i, mask - i);
574 elements[mask] = elements[0];
575 System.arraycopy(elements, 1, elements, 0, t);
576 tail = (t - 1) & mask;
577 }
578 return true;
579 }
580 }
581
582 // *** Collection Methods ***
583
584 /**
585 * Returns the number of elements in this deque.
586 *
587 * @return the number of elements in this deque
588 */
589 public int size() {
590 return (tail - head) & (elements.length - 1);
591 }
592
593 /**
594 * Returns {@code true} if this deque contains no elements.
595 *
596 * @return {@code true} if this deque contains no elements
597 */
598 public boolean isEmpty() {
599 return head == tail;
600 }
601
602 /**
603 * Returns an iterator over the elements in this deque. The elements
604 * will be ordered from first (head) to last (tail). This is the same
605 * order that elements would be dequeued (via successive calls to
606 * {@link #remove} or popped (via successive calls to {@link #pop}).
607 *
608 * @return an iterator over the elements in this deque
609 */
610 public Iterator<E> iterator() {
611 return new DeqIterator();
612 }
613
614 public Iterator<E> descendingIterator() {
615 return new DescendingIterator();
616 }
617
618 private class DeqIterator implements Iterator<E> {
619 /**
620 * Index of element to be returned by subsequent call to next.
621 */
622 private int cursor = head;
623
624 /**
625 * Tail recorded at construction (also in remove), to stop
626 * iterator and also to check for comodification.
627 */
628 private int fence = tail;
629
630 /**
631 * Index of element returned by most recent call to next.
632 * Reset to -1 if element is deleted by a call to remove.
633 */
634 private int lastRet = -1;
635
636 public boolean hasNext() {
637 return cursor != fence;
638 }
639
640 public E next() {
641 if (cursor == fence)
642 throw new NoSuchElementException();
643 @SuppressWarnings("unchecked")
644 E result = (E) elements[cursor];
645 // This check doesn't catch all possible comodifications,
646 // but does catch the ones that corrupt traversal
647 if (tail != fence || result == null)
648 throw new ConcurrentModificationException();
649 lastRet = cursor;
650 cursor = (cursor + 1) & (elements.length - 1);
651 return result;
652 }
653
654 public void remove() {
655 if (lastRet < 0)
656 throw new IllegalStateException();
657 if (delete(lastRet)) { // if left-shifted, undo increment in next()
658 cursor = (cursor - 1) & (elements.length - 1);
659 fence = tail;
660 }
661 lastRet = -1;
662 }
663
664 public void forEachRemaining(Consumer<? super E> action) {
665 Objects.requireNonNull(action);
666 Object[] a = elements;
667 int m = a.length - 1, f = fence, i = cursor;
668 cursor = f;
669 while (i != f) {
670 @SuppressWarnings("unchecked") E e = (E)a[i];
671 i = (i + 1) & m;
672 if (e == null)
673 throw new ConcurrentModificationException();
674 action.accept(e);
675 }
676 }
677 }
678
679 private class DescendingIterator implements Iterator<E> {
680 /*
681 * This class is nearly a mirror-image of DeqIterator, using
682 * tail instead of head for initial cursor, and head instead of
683 * tail for fence.
684 */
685 private int cursor = tail;
686 private int fence = head;
687 private int lastRet = -1;
688
689 public boolean hasNext() {
690 return cursor != fence;
691 }
692
693 public E next() {
694 if (cursor == fence)
695 throw new NoSuchElementException();
696 cursor = (cursor - 1) & (elements.length - 1);
697 @SuppressWarnings("unchecked")
698 E result = (E) elements[cursor];
699 if (head != fence || result == null)
700 throw new ConcurrentModificationException();
701 lastRet = cursor;
702 return result;
703 }
704
705 public void remove() {
706 if (lastRet < 0)
707 throw new IllegalStateException();
708 if (!delete(lastRet)) {
709 cursor = (cursor + 1) & (elements.length - 1);
710 fence = head;
711 }
712 lastRet = -1;
713 }
714 }
715
716 /**
717 * Returns {@code true} if this deque contains the specified element.
718 * More formally, returns {@code true} if and only if this deque contains
719 * at least one element {@code e} such that {@code o.equals(e)}.
720 *
721 * @param o object to be checked for containment in this deque
722 * @return {@code true} if this deque contains the specified element
723 */
724 public boolean contains(Object o) {
725 if (o == null)
726 return false;
727 int mask = elements.length - 1;
728 int i = head;
729 Object x;
730 while ( (x = elements[i]) != null) {
731 if (o.equals(x))
732 return true;
733 i = (i + 1) & mask;
734 }
735 return false;
736 }
737
738 /**
739 * Removes a single instance of the specified element from this deque.
740 * If the deque does not contain the element, it is unchanged.
741 * More formally, removes the first element {@code e} such that
742 * {@code o.equals(e)} (if such an element exists).
743 * Returns {@code true} if this deque contained the specified element
744 * (or equivalently, if this deque changed as a result of the call).
745 *
746 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
747 *
748 * @param o element to be removed from this deque, if present
749 * @return {@code true} if this deque contained the specified element
750 */
751 public boolean remove(Object o) {
752 return removeFirstOccurrence(o);
753 }
754
755 /**
756 * Removes all of the elements from this deque.
757 * The deque will be empty after this call returns.
758 */
759 public void clear() {
760 int h = head;
761 int t = tail;
762 if (h != t) { // clear all cells
763 head = tail = 0;
764 int i = h;
765 int mask = elements.length - 1;
766 do {
767 elements[i] = null;
768 i = (i + 1) & mask;
769 } while (i != t);
770 }
771 }
772
773 /**
774 * Returns an array containing all of the elements in this deque
775 * in proper sequence (from first to last element).
776 *
777 * <p>The returned array will be "safe" in that no references to it are
778 * maintained by this deque. (In other words, this method must allocate
779 * a new array). The caller is thus free to modify the returned array.
780 *
781 * <p>This method acts as bridge between array-based and collection-based
782 * APIs.
783 *
784 * @return an array containing all of the elements in this deque
785 */
786 public Object[] toArray() {
787 return copyElements(new Object[size()]);
788 }
789
790 /**
791 * Returns an array containing all of the elements in this deque in
792 * proper sequence (from first to last element); the runtime type of the
793 * returned array is that of the specified array. If the deque fits in
794 * the specified array, it is returned therein. Otherwise, a new array
795 * is allocated with the runtime type of the specified array and the
796 * size of this deque.
797 *
798 * <p>If this deque fits in the specified array with room to spare
799 * (i.e., the array has more elements than this deque), the element in
800 * the array immediately following the end of the deque is set to
801 * {@code null}.
802 *
803 * <p>Like the {@link #toArray()} method, this method acts as bridge between
804 * array-based and collection-based APIs. Further, this method allows
805 * precise control over the runtime type of the output array, and may,
806 * under certain circumstances, be used to save allocation costs.
807 *
808 * <p>Suppose {@code x} is a deque known to contain only strings.
809 * The following code can be used to dump the deque into a newly
810 * allocated array of {@code String}:
811 *
812 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
813 *
814 * Note that {@code toArray(new Object[0])} is identical in function to
815 * {@code toArray()}.
816 *
817 * @param a the array into which the elements of the deque are to
818 * be stored, if it is big enough; otherwise, a new array of the
819 * same runtime type is allocated for this purpose
820 * @return an array containing all of the elements in this deque
821 * @throws ArrayStoreException if the runtime type of the specified array
822 * is not a supertype of the runtime type of every element in
823 * this deque
824 * @throws NullPointerException if the specified array is null
825 */
826 @SuppressWarnings("unchecked")
827 public <T> T[] toArray(T[] a) {
828 int size = size();
829 if (a.length < size)
830 a = (T[])java.lang.reflect.Array.newInstance(
831 a.getClass().getComponentType(), size);
832 copyElements(a);
833 if (a.length > size)
834 a[size] = null;
835 return a;
836 }
837
838 // *** Object methods ***
839
840 /**
841 * Returns a copy of this deque.
842 *
843 * @return a copy of this deque
844 */
845 public ArrayDeque<E> clone() {
846 try {
847 @SuppressWarnings("unchecked")
848 ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
849 result.elements = Arrays.copyOf(elements, elements.length);
850 return result;
851 } catch (CloneNotSupportedException e) {
852 throw new AssertionError();
853 }
854 }
855
856 private static final long serialVersionUID = 2340985798034038923L;
857
858 /**
859 * Saves this deque to a stream (that is, serializes it).
860 *
861 * @serialData The current size ({@code int}) of the deque,
862 * followed by all of its elements (each an object reference) in
863 * first-to-last order.
864 */
865 private void writeObject(java.io.ObjectOutputStream s)
866 throws java.io.IOException {
867 s.defaultWriteObject();
868
869 // Write out size
870 s.writeInt(size());
871
872 // Write out elements in order.
873 int mask = elements.length - 1;
874 for (int i = head; i != tail; i = (i + 1) & mask)
875 s.writeObject(elements[i]);
876 }
877
878 /**
879 * Reconstitutes this deque from a stream (that is, deserializes it).
880 */
881 private void readObject(java.io.ObjectInputStream s)
882 throws java.io.IOException, ClassNotFoundException {
883 s.defaultReadObject();
884
885 // Read in size and allocate array
886 int size = s.readInt();
887 int capacity = calculateSize(size);
888 SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
889 allocateElements(size);
890 head = 0;
891 tail = size;
892
893 // Read in all elements in the proper order.
894 for (int i = 0; i < size; i++)
895 elements[i] = s.readObject();
896 }
897
898 /**
899 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
900 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
901 * deque.
902 *
903 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
904 * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
905 * {@link Spliterator#NONNULL}. Overriding implementations should document
906 * the reporting of additional characteristic values.
907 *
908 * @return a {@code Spliterator} over the elements in this deque
909 * @since 1.8
910 */
911 public Spliterator<E> spliterator() {
912 return new DeqSpliterator<E>(this, -1, -1);
913 }
914
915 static final class DeqSpliterator<E> implements Spliterator<E> {
916 private final ArrayDeque<E> deq;
917 private int fence; // -1 until first use
918 private int index; // current index, modified on traverse/split
919
920 /** Creates new spliterator covering the given array and range */
921 DeqSpliterator(ArrayDeque<E> deq, int origin, int fence) {
922 this.deq = deq;
923 this.index = origin;
924 this.fence = fence;
925 }
926
927 private int getFence() { // force initialization
928 int t;
929 if ((t = fence) < 0) {
930 t = fence = deq.tail;
931 index = deq.head;
932 }
933 return t;
934 }
935
936 public DeqSpliterator<E> trySplit() {
937 int t = getFence(), h = index, n = deq.elements.length;
938 if (h != t && ((h + 1) & (n - 1)) != t) {
939 if (h > t)
940 t += n;
941 int m = ((h + t) >>> 1) & (n - 1);
942 return new DeqSpliterator<>(deq, h, index = m);
943 }
944 return null;
945 }
946
947 public void forEachRemaining(Consumer<? super E> consumer) {
948 if (consumer == null)
949 throw new NullPointerException();
950 Object[] a = deq.elements;
951 int m = a.length - 1, f = getFence(), i = index;
952 index = f;
953 while (i != f) {
954 @SuppressWarnings("unchecked") E e = (E)a[i];
955 i = (i + 1) & m;
956 if (e == null)
957 throw new ConcurrentModificationException();
958 consumer.accept(e);
959 }
960 }
961
962 public boolean tryAdvance(Consumer<? super E> consumer) {
963 if (consumer == null)
964 throw new NullPointerException();
965 Object[] a = deq.elements;
966 int m = a.length - 1, f = getFence(), i = index;
967 if (i != fence) {
968 @SuppressWarnings("unchecked") E e = (E)a[i];
969 index = (i + 1) & m;
970 if (e == null)
971 throw new ConcurrentModificationException();
972 consumer.accept(e);
973 return true;
974 }
975 return false;
976 }
977
978 public long estimateSize() {
979 int n = getFence() - index;
980 if (n < 0)
981 n += deq.elements.length;
982 return (long) n;
983 }
984
985 @Override
986 public int characteristics() {
987 return Spliterator.ORDERED | Spliterator.SIZED |
988 Spliterator.NONNULL | Spliterator.SUBSIZED;
989 }
990 }
991
992 }