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