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 java.util.function.Predicate;
40 import java.util.function.UnaryOperator;
41
42 /**
43 * Resizable-array implementation of the {@link Deque} interface. Array
44 * deques have no capacity restrictions; they grow as necessary to support
45 * usage. They are not thread-safe; in the absence of external
46 * synchronization, they do not support concurrent access by multiple threads.
47 * Null elements are prohibited. This class is likely to be faster than
48 * {@link Stack} when used as a stack, and faster than {@link LinkedList}
49 * when used as a queue.
50 *
51 * <p>Most {@code ArrayDeque} operations run in amortized constant time.
52 * Exceptions include
53 * {@link #remove(Object) remove},
54 * {@link #removeFirstOccurrence removeFirstOccurrence},
55 * {@link #removeLastOccurrence removeLastOccurrence},
56 * {@link #contains contains},
57 * {@link #iterator iterator.remove()},
58 * and the bulk operations, all of which run in linear time.
59 *
60 * <p>The iterators returned by this class's {@link #iterator() iterator}
61 * method are <em>fail-fast</em>: If the deque is modified at any time after
62 * the iterator is created, in any way except through the iterator's own
63 * {@code remove} method, the iterator will generally throw a {@link
64 * ConcurrentModificationException}. Thus, in the face of concurrent
65 * modification, the iterator fails quickly and cleanly, rather than risking
66 * arbitrary, non-deterministic behavior at an undetermined time in the
67 * future.
68 *
69 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
70 * as it is, generally speaking, impossible to make any hard guarantees in the
71 * presence of unsynchronized concurrent modification. Fail-fast iterators
72 * throw {@code ConcurrentModificationException} on a best-effort basis.
73 * Therefore, it would be wrong to write a program that depended on this
74 * exception for its correctness: <i>the fail-fast behavior of iterators
75 * should be used only to detect bugs.</i>
76 *
77 * <p>This class and its iterator implement all of the
78 * <em>optional</em> methods of the {@link Collection} and {@link
79 * Iterator} interfaces.
80 *
81 * <p>This class is a member of the
82 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
83 * Java Collections Framework</a>.
84 *
85 * @author Josh Bloch and Doug Lea
86 * @param <E> the type of elements held in this deque
87 * @since 1.6
88 */
89 public class ArrayDeque<E> extends AbstractCollection<E>
90 implements Deque<E>, Cloneable, Serializable
91 {
92 /*
93 * VMs excel at optimizing simple array loops where indices are
94 * incrementing or decrementing over a valid slice, e.g.
95 *
96 * for (int i = start; i < end; i++) ... elements[i]
97 *
98 * Because in a circular array, elements are in general stored in
99 * two disjoint such slices, we help the VM by writing unusual
100 * nested loops for all traversals over the elements. Having only
101 * one hot inner loop body instead of two or three eases human
102 * maintenance and encourages VM loop inlining into the caller.
103 */
104
105 /**
106 * The array in which the elements of the deque are stored.
107 * All array cells not holding deque elements are always null.
108 * The array always has at least one null slot (at tail).
109 */
110 transient Object[] elements;
111
112 /**
113 * The index of the element at the head of the deque (which is the
114 * element that would be removed by remove() or pop()); or an
115 * arbitrary number 0 <= head < elements.length equal to tail if
116 * the deque is empty.
117 */
118 transient int head;
119
120 /**
121 * The index at which the next element would be added to the tail
122 * of the deque (via addLast(E), add(E), or push(E));
123 * elements[tail] is always null.
124 */
125 transient int tail;
126
127 /**
128 * The maximum size of array to allocate.
129 * Some VMs reserve some header words in an array.
130 * Attempts to allocate larger arrays may result in
131 * OutOfMemoryError: Requested array size exceeds VM limit
132 */
133 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
134
135 /**
136 * Increases the capacity of this deque by at least the given amount.
137 *
138 * @param needed the required minimum extra capacity; must be positive
139 */
140 private void grow(int needed) {
141 // overflow-conscious code
142 final int oldCapacity = elements.length;
143 int newCapacity;
144 // Double capacity if small; else grow by 50%
145 int jump = (oldCapacity < 64) ? (oldCapacity + 2) : (oldCapacity >> 1);
146 if (jump < needed
147 || (newCapacity = (oldCapacity + jump)) - MAX_ARRAY_SIZE > 0)
148 newCapacity = newCapacity(needed, jump);
149 final Object[] es = elements = Arrays.copyOf(elements, newCapacity);
150 // Exceptionally, here tail == head needs to be disambiguated
151 if (tail < head || (tail == head && es[head] != null)) {
152 // wrap around; slide first leg forward to end of array
153 int newSpace = newCapacity - oldCapacity;
154 System.arraycopy(es, head,
155 es, head + newSpace,
156 oldCapacity - head);
157 for (int i = head, to = (head += newSpace); i < to; i++)
158 es[i] = null;
159 }
160 }
161
162 /** Capacity calculation for edge conditions, especially overflow. */
163 private int newCapacity(int needed, int jump) {
164 final int oldCapacity = elements.length, minCapacity;
165 if ((minCapacity = oldCapacity + needed) - MAX_ARRAY_SIZE > 0) {
166 if (minCapacity < 0)
167 throw new IllegalStateException("Sorry, deque too big");
168 return Integer.MAX_VALUE;
169 }
170 if (needed > jump)
171 return minCapacity;
172 return (oldCapacity + jump - MAX_ARRAY_SIZE < 0)
173 ? oldCapacity + jump
174 : MAX_ARRAY_SIZE;
175 }
176
177 /**
178 * Constructs an empty array deque with an initial capacity
179 * sufficient to hold 16 elements.
180 */
181 public ArrayDeque() {
182 elements = new Object[16];
183 }
184
185 /**
186 * Constructs an empty array deque with an initial capacity
187 * sufficient to hold the specified number of elements.
188 *
189 * @param numElements lower bound on initial capacity of the deque
190 */
191 public ArrayDeque(int numElements) {
192 elements =
193 new Object[(numElements < 1) ? 1 :
194 (numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE :
195 numElements + 1];
196 }
197
198 /**
199 * Constructs a deque containing the elements of the specified
200 * collection, in the order they are returned by the collection's
201 * iterator. (The first element returned by the collection's
202 * iterator becomes the first element, or <i>front</i> of the
203 * deque.)
204 *
205 * @param c the collection whose elements are to be placed into the deque
206 * @throws NullPointerException if the specified collection is null
207 */
208 public ArrayDeque(Collection<? extends E> c) {
209 this(c.size());
210 addAll(c);
211 }
212
213 /**
214 * Increments i, mod modulus.
215 * Precondition and postcondition: 0 <= i < modulus.
216 */
217 static final int inc(int i, int modulus) {
218 if (++i >= modulus) i = 0;
219 return i;
220 }
221
222 /**
223 * Decrements i, mod modulus.
224 * Precondition and postcondition: 0 <= i < modulus.
225 */
226 static final int dec(int i, int modulus) {
227 if (--i < 0) i = modulus - 1;
228 return i;
229 }
230
231 /**
232 * Circularly adds the given distance to index i, mod modulus.
233 * Precondition: 0 <= i < modulus, 0 <= distance <= modulus.
234 * @return index 0 <= i < modulus
235 */
236 static final int add(int i, int distance, int modulus) {
237 if ((i += distance) - modulus >= 0) i -= modulus;
238 return i;
239 }
240
241 /**
242 * Subtracts j from i, mod modulus.
243 * Index i must be logically ahead of index j.
244 * Precondition: 0 <= i < modulus, 0 <= j < modulus.
245 * @return the "circular distance" from j to i; corner case i == j
246 * is diambiguated to "empty", returning 0.
247 */
248 static final int sub(int i, int j, int modulus) {
249 if ((i -= j) < 0) i += modulus;
250 return i;
251 }
252
253 /**
254 * Returns element at array index i.
255 * This is a slight abuse of generics, accepted by javac.
256 */
257 @SuppressWarnings("unchecked")
258 static final <E> E elementAt(Object[] es, int i) {
259 return (E) es[i];
260 }
261
262 /**
263 * A version of elementAt that checks for null elements.
264 * This check doesn't catch all possible comodifications,
265 * but does catch ones that corrupt traversal.
266 */
267 static final <E> E nonNullElementAt(Object[] es, int i) {
268 @SuppressWarnings("unchecked") E e = (E) es[i];
269 if (e == null)
270 throw new ConcurrentModificationException();
271 return e;
272 }
273
274 // The main insertion and extraction methods are addFirst,
275 // addLast, pollFirst, pollLast. The other methods are defined in
276 // terms of these.
277
278 /**
279 * Inserts the specified element at the front of this deque.
280 *
281 * @param e the element to add
282 * @throws NullPointerException if the specified element is null
283 */
284 public void addFirst(E e) {
285 if (e == null)
286 throw new NullPointerException();
287 final Object[] es = elements;
288 es[head = dec(head, es.length)] = e;
289 if (head == tail)
290 grow(1);
291 }
292
293 /**
294 * Inserts the specified element at the end of this deque.
295 *
296 * <p>This method is equivalent to {@link #add}.
297 *
298 * @param e the element to add
299 * @throws NullPointerException if the specified element is null
300 */
301 public void addLast(E e) {
302 if (e == null)
303 throw new NullPointerException();
304 final Object[] es = elements;
305 es[tail] = e;
306 if (head == (tail = inc(tail, es.length)))
307 grow(1);
308 }
309
310 /**
311 * Adds all of the elements in the specified collection at the end
312 * of this deque, as if by calling {@link #addLast} on each one,
313 * in the order that they are returned by the collection's
314 * iterator.
315 *
316 * @param c the elements to be inserted into this deque
317 * @return {@code true} if this deque changed as a result of the call
318 * @throws NullPointerException if the specified collection or any
319 * of its elements are null
320 */
321 public boolean addAll(Collection<? extends E> c) {
322 final int s, needed;
323 if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0)
324 grow(needed);
325 c.forEach(this::addLast);
326 return size() > s;
327 }
328
329 /**
330 * Inserts the specified element at the front of this deque.
331 *
332 * @param e the element to add
333 * @return {@code true} (as specified by {@link Deque#offerFirst})
334 * @throws NullPointerException if the specified element is null
335 */
336 public boolean offerFirst(E e) {
337 addFirst(e);
338 return true;
339 }
340
341 /**
342 * Inserts the specified element at the end of this deque.
343 *
344 * @param e the element to add
345 * @return {@code true} (as specified by {@link Deque#offerLast})
346 * @throws NullPointerException if the specified element is null
347 */
348 public boolean offerLast(E e) {
349 addLast(e);
350 return true;
351 }
352
353 /**
354 * @throws NoSuchElementException {@inheritDoc}
355 */
356 public E removeFirst() {
357 E e = pollFirst();
358 if (e == null)
359 throw new NoSuchElementException();
360 return e;
361 }
362
363 /**
364 * @throws NoSuchElementException {@inheritDoc}
365 */
366 public E removeLast() {
367 E e = pollLast();
368 if (e == null)
369 throw new NoSuchElementException();
370 return e;
371 }
372
373 public E pollFirst() {
374 final Object[] es;
375 final int h;
376 E e = elementAt(es = elements, h = head);
377 if (e != null) {
378 es[h] = null;
379 head = inc(h, es.length);
380 }
381 return e;
382 }
383
384 public E pollLast() {
385 final Object[] es;
386 final int t;
387 E e = elementAt(es = elements, t = dec(tail, es.length));
388 if (e != null)
389 es[tail = t] = null;
390 return e;
391 }
392
393 /**
394 * @throws NoSuchElementException {@inheritDoc}
395 */
396 public E getFirst() {
397 E e = elementAt(elements, head);
398 if (e == null)
399 throw new NoSuchElementException();
400 return e;
401 }
402
403 /**
404 * @throws NoSuchElementException {@inheritDoc}
405 */
406 public E getLast() {
407 final Object[] es = elements;
408 E e = elementAt(es, dec(tail, es.length));
409 if (e == null)
410 throw new NoSuchElementException();
411 return e;
412 }
413
414 public E peekFirst() {
415 return elementAt(elements, head);
416 }
417
418 public E peekLast() {
419 final Object[] es;
420 return elementAt(es = elements, dec(tail, es.length));
421 }
422
423 /**
424 * Removes the first occurrence of the specified element in this
425 * deque (when traversing the deque from head to tail).
426 * If the deque does not contain the element, it is unchanged.
427 * More formally, removes the first element {@code e} such that
428 * {@code o.equals(e)} (if such an element exists).
429 * Returns {@code true} if this deque contained the specified element
430 * (or equivalently, if this deque changed as a result of the call).
431 *
432 * @param o element to be removed from this deque, if present
433 * @return {@code true} if the deque contained the specified element
434 */
435 public boolean removeFirstOccurrence(Object o) {
436 if (o != null) {
437 final Object[] es = elements;
438 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
439 ; i = 0, to = end) {
440 for (; i < to; i++)
441 if (o.equals(es[i])) {
442 delete(i);
443 return true;
444 }
445 if (to == end) break;
446 }
447 }
448 return false;
449 }
450
451 /**
452 * Removes the last occurrence of the specified element in this
453 * deque (when traversing the deque from head to tail).
454 * If the deque does not contain the element, it is unchanged.
455 * More formally, removes the last element {@code e} such that
456 * {@code o.equals(e)} (if such an element exists).
457 * Returns {@code true} if this deque contained the specified element
458 * (or equivalently, if this deque changed as a result of the call).
459 *
460 * @param o element to be removed from this deque, if present
461 * @return {@code true} if the deque contained the specified element
462 */
463 public boolean removeLastOccurrence(Object o) {
464 if (o != null) {
465 final Object[] es = elements;
466 for (int i = tail, end = head, to = (i >= end) ? end : 0;
467 ; i = es.length, to = end) {
468 for (i--; i > to - 1; i--)
469 if (o.equals(es[i])) {
470 delete(i);
471 return true;
472 }
473 if (to == end) break;
474 }
475 }
476 return false;
477 }
478
479 // *** Queue methods ***
480
481 /**
482 * Inserts the specified element at the end of this deque.
483 *
484 * <p>This method is equivalent to {@link #addLast}.
485 *
486 * @param e the element to add
487 * @return {@code true} (as specified by {@link Collection#add})
488 * @throws NullPointerException if the specified element is null
489 */
490 public boolean add(E e) {
491 addLast(e);
492 return true;
493 }
494
495 /**
496 * Inserts the specified element at the end of this deque.
497 *
498 * <p>This method is equivalent to {@link #offerLast}.
499 *
500 * @param e the element to add
501 * @return {@code true} (as specified by {@link Queue#offer})
502 * @throws NullPointerException if the specified element is null
503 */
504 public boolean offer(E e) {
505 return offerLast(e);
506 }
507
508 /**
509 * Retrieves and removes the head of the queue represented by this deque.
510 *
511 * This method differs from {@link #poll poll} only in that it throws an
512 * exception if this deque is empty.
513 *
514 * <p>This method is equivalent to {@link #removeFirst}.
515 *
516 * @return the head of the queue represented by this deque
517 * @throws NoSuchElementException {@inheritDoc}
518 */
519 public E remove() {
520 return removeFirst();
521 }
522
523 /**
524 * Retrieves and removes the head of the queue represented by this deque
525 * (in other words, the first element of this deque), or returns
526 * {@code null} if this deque is empty.
527 *
528 * <p>This method is equivalent to {@link #pollFirst}.
529 *
530 * @return the head of the queue represented by this deque, or
531 * {@code null} if this deque is empty
532 */
533 public E poll() {
534 return pollFirst();
535 }
536
537 /**
538 * Retrieves, but does not remove, the head of the queue represented by
539 * this deque. This method differs from {@link #peek peek} only in
540 * that it throws an exception if this deque is empty.
541 *
542 * <p>This method is equivalent to {@link #getFirst}.
543 *
544 * @return the head of the queue represented by this deque
545 * @throws NoSuchElementException {@inheritDoc}
546 */
547 public E element() {
548 return getFirst();
549 }
550
551 /**
552 * Retrieves, but does not remove, the head of the queue represented by
553 * this deque, or returns {@code null} if this deque is empty.
554 *
555 * <p>This method is equivalent to {@link #peekFirst}.
556 *
557 * @return the head of the queue represented by this deque, or
558 * {@code null} if this deque is empty
559 */
560 public E peek() {
561 return peekFirst();
562 }
563
564 // *** Stack methods ***
565
566 /**
567 * Pushes an element onto the stack represented by this deque. In other
568 * words, inserts the element at the front of this deque.
569 *
570 * <p>This method is equivalent to {@link #addFirst}.
571 *
572 * @param e the element to push
573 * @throws NullPointerException if the specified element is null
574 */
575 public void push(E e) {
576 addFirst(e);
577 }
578
579 /**
580 * Pops an element from the stack represented by this deque. In other
581 * words, removes and returns the first element of this deque.
582 *
583 * <p>This method is equivalent to {@link #removeFirst()}.
584 *
585 * @return the element at the front of this deque (which is the top
586 * of the stack represented by this deque)
587 * @throws NoSuchElementException {@inheritDoc}
588 */
589 public E pop() {
590 return removeFirst();
591 }
592
593 /**
594 * Removes the element at the specified position in the elements array.
595 * This can result in forward or backwards motion of array elements.
596 * We optimize for least element motion.
597 *
598 * <p>This method is called delete rather than remove to emphasize
599 * that its semantics differ from those of {@link List#remove(int)}.
600 *
601 * @return true if elements near tail moved backwards
602 */
603 boolean delete(int i) {
604 final Object[] es = elements;
605 final int capacity = es.length;
606 final int h, t;
607 // number of elements before to-be-deleted elt
608 final int front = sub(i, h = head, capacity);
609 // number of elements after to-be-deleted elt
610 final int back = sub(t = tail, i, capacity) - 1;
611 if (front < back) {
612 // move front elements forwards
613 if (h <= i) {
614 System.arraycopy(es, h, es, h + 1, front);
615 } else { // Wrap around
616 System.arraycopy(es, 0, es, 1, i);
617 es[0] = es[capacity - 1];
618 System.arraycopy(es, h, es, h + 1, front - (i + 1));
619 }
620 es[h] = null;
621 head = inc(h, capacity);
622 return false;
623 } else {
624 // move back elements backwards
625 tail = dec(t, capacity);
626 if (i <= tail) {
627 System.arraycopy(es, i + 1, es, i, back);
628 } else { // Wrap around
629 System.arraycopy(es, i + 1, es, i, capacity - (i + 1));
630 es[capacity - 1] = es[0];
631 System.arraycopy(es, 1, es, 0, t - 1);
632 }
633 es[tail] = null;
634 return true;
635 }
636 }
637
638 // *** Collection Methods ***
639
640 /**
641 * Returns the number of elements in this deque.
642 *
643 * @return the number of elements in this deque
644 */
645 public int size() {
646 return sub(tail, head, elements.length);
647 }
648
649 /**
650 * Returns {@code true} if this deque contains no elements.
651 *
652 * @return {@code true} if this deque contains no elements
653 */
654 public boolean isEmpty() {
655 return head == tail;
656 }
657
658 /**
659 * Returns an iterator over the elements in this deque. The elements
660 * will be ordered from first (head) to last (tail). This is the same
661 * order that elements would be dequeued (via successive calls to
662 * {@link #remove} or popped (via successive calls to {@link #pop}).
663 *
664 * @return an iterator over the elements in this deque
665 */
666 public Iterator<E> iterator() {
667 return new DeqIterator();
668 }
669
670 public Iterator<E> descendingIterator() {
671 return new DescendingIterator();
672 }
673
674 private class DeqIterator implements Iterator<E> {
675 /** Index of element to be returned by subsequent call to next. */
676 int cursor;
677
678 /** Number of elements yet to be returned. */
679 int remaining = size();
680
681 /**
682 * Index of element returned by most recent call to next.
683 * Reset to -1 if element is deleted by a call to remove.
684 */
685 int lastRet = -1;
686
687 DeqIterator() { cursor = head; }
688
689 public final boolean hasNext() {
690 return remaining > 0;
691 }
692
693 public E next() {
694 if (remaining <= 0)
695 throw new NoSuchElementException();
696 final Object[] es = elements;
697 E e = nonNullElementAt(es, cursor);
698 cursor = inc(lastRet = cursor, es.length);
699 remaining--;
700 return e;
701 }
702
703 void postDelete(boolean leftShifted) {
704 if (leftShifted)
705 cursor = dec(cursor, elements.length);
706 }
707
708 public final void remove() {
709 if (lastRet < 0)
710 throw new IllegalStateException();
711 postDelete(delete(lastRet));
712 lastRet = -1;
713 }
714
715 public void forEachRemaining(Consumer<? super E> action) {
716 Objects.requireNonNull(action);
717 int r;
718 if ((r = remaining) <= 0)
719 return;
720 remaining = 0;
721 final Object[] es = elements;
722 if (es[cursor] == null || sub(tail, cursor, es.length) != r)
723 throw new ConcurrentModificationException();
724 for (int i = cursor, end = tail, to = (i <= end) ? end : es.length;
725 ; i = 0, to = end) {
726 for (; i < to; i++)
727 action.accept(elementAt(es, i));
728 if (to == end) {
729 if (end != tail)
730 throw new ConcurrentModificationException();
731 lastRet = dec(end, es.length);
732 break;
733 }
734 }
735 }
736 }
737
738 private class DescendingIterator extends DeqIterator {
739 DescendingIterator() { cursor = dec(tail, elements.length); }
740
741 public final E next() {
742 if (remaining <= 0)
743 throw new NoSuchElementException();
744 final Object[] es = elements;
745 E e = nonNullElementAt(es, cursor);
746 cursor = dec(lastRet = cursor, es.length);
747 remaining--;
748 return e;
749 }
750
751 void postDelete(boolean leftShifted) {
752 if (!leftShifted)
753 cursor = inc(cursor, elements.length);
754 }
755
756 public final void forEachRemaining(Consumer<? super E> action) {
757 Objects.requireNonNull(action);
758 int r;
759 if ((r = remaining) <= 0)
760 return;
761 remaining = 0;
762 final Object[] es = elements;
763 if (es[cursor] == null || sub(cursor, head, es.length) + 1 != r)
764 throw new ConcurrentModificationException();
765 for (int i = cursor, end = head, to = (i >= end) ? end : 0;
766 ; i = es.length - 1, to = end) {
767 // hotspot generates faster code than for: i >= to !
768 for (; i > to - 1; i--)
769 action.accept(elementAt(es, i));
770 if (to == end) {
771 if (end != head)
772 throw new ConcurrentModificationException();
773 lastRet = end;
774 break;
775 }
776 }
777 }
778 }
779
780 /**
781 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
782 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
783 * deque.
784 *
785 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
786 * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
787 * {@link Spliterator#NONNULL}. Overriding implementations should document
788 * the reporting of additional characteristic values.
789 *
790 * @return a {@code Spliterator} over the elements in this deque
791 * @since 1.8
792 */
793 public Spliterator<E> spliterator() {
794 return new DeqSpliterator();
795 }
796
797 final class DeqSpliterator implements Spliterator<E> {
798 private int fence; // -1 until first use
799 private int cursor; // current index, modified on traverse/split
800
801 /** Constructs late-binding spliterator over all elements. */
802 DeqSpliterator() {
803 this.fence = -1;
804 }
805
806 /** Constructs spliterator over the given range. */
807 DeqSpliterator(int origin, int fence) {
808 // assert 0 <= origin && origin < elements.length;
809 // assert 0 <= fence && fence < elements.length;
810 this.cursor = origin;
811 this.fence = fence;
812 }
813
814 /** Ensures late-binding initialization; then returns fence. */
815 private int getFence() { // force initialization
816 int t;
817 if ((t = fence) < 0) {
818 t = fence = tail;
819 cursor = head;
820 }
821 return t;
822 }
823
824 public DeqSpliterator trySplit() {
825 final Object[] es = elements;
826 final int i, n;
827 return ((n = sub(getFence(), i = cursor, es.length) >> 1) <= 0)
828 ? null
829 : new DeqSpliterator(i, cursor = add(i, n, es.length));
830 }
831
832 public void forEachRemaining(Consumer<? super E> action) {
833 if (action == null)
834 throw new NullPointerException();
835 final int end = getFence(), cursor = this.cursor;
836 final Object[] es = elements;
837 if (cursor != end) {
838 this.cursor = end;
839 // null check at both ends of range is sufficient
840 if (es[cursor] == null || es[dec(end, es.length)] == null)
841 throw new ConcurrentModificationException();
842 for (int i = cursor, to = (i <= end) ? end : es.length;
843 ; i = 0, to = end) {
844 for (; i < to; i++)
845 action.accept(elementAt(es, i));
846 if (to == end) break;
847 }
848 }
849 }
850
851 public boolean tryAdvance(Consumer<? super E> action) {
852 Objects.requireNonNull(action);
853 final Object[] es = elements;
854 if (fence < 0) { fence = tail; cursor = head; } // late-binding
855 final int i;
856 if ((i = cursor) == fence)
857 return false;
858 E e = nonNullElementAt(es, i);
859 cursor = inc(i, es.length);
860 action.accept(e);
861 return true;
862 }
863
864 public long estimateSize() {
865 return sub(getFence(), cursor, elements.length);
866 }
867
868 public int characteristics() {
869 return Spliterator.NONNULL
870 | Spliterator.ORDERED
871 | Spliterator.SIZED
872 | Spliterator.SUBSIZED;
873 }
874 }
875
876 /**
877 * @throws NullPointerException {@inheritDoc}
878 */
879 public void forEach(Consumer<? super E> action) {
880 Objects.requireNonNull(action);
881 final Object[] es = elements;
882 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
883 ; i = 0, to = end) {
884 for (; i < to; i++)
885 action.accept(elementAt(es, i));
886 if (to == end) {
887 if (end != tail) throw new ConcurrentModificationException();
888 break;
889 }
890 }
891 }
892
893 /**
894 * @throws NullPointerException {@inheritDoc}
895 */
896 public boolean removeIf(Predicate<? super E> filter) {
897 Objects.requireNonNull(filter);
898 return bulkRemove(filter);
899 }
900
901 /**
902 * @throws NullPointerException {@inheritDoc}
903 */
904 public boolean removeAll(Collection<?> c) {
905 Objects.requireNonNull(c);
906 return bulkRemove(e -> c.contains(e));
907 }
908
909 /**
910 * @throws NullPointerException {@inheritDoc}
911 */
912 public boolean retainAll(Collection<?> c) {
913 Objects.requireNonNull(c);
914 return bulkRemove(e -> !c.contains(e));
915 }
916
917 /** Implementation of bulk remove methods. */
918 private boolean bulkRemove(Predicate<? super E> filter) {
919 final Object[] es = elements;
920 // Optimize for initial run of survivors
921 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
922 ; i = 0, to = end) {
923 for (; i < to; i++)
924 if (filter.test(elementAt(es, i)))
925 return bulkRemoveModified(filter, i);
926 if (to == end) {
927 if (end != tail) throw new ConcurrentModificationException();
928 break;
929 }
930 }
931 return false;
932 }
933
934 // A tiny bit set implementation
935
936 private static long[] nBits(int n) {
937 return new long[((n - 1) >> 6) + 1];
938 }
939 private static void setBit(long[] bits, int i) {
940 bits[i >> 6] |= 1L << i;
941 }
942 private static boolean isClear(long[] bits, int i) {
943 return (bits[i >> 6] & (1L << i)) == 0;
944 }
945
946 /**
947 * Helper for bulkRemove, in case of at least one deletion.
948 * Tolerate predicates that reentrantly access the collection for
949 * read (but writers still get CME), so traverse once to find
950 * elements to delete, a second pass to physically expunge.
951 *
952 * @param beg valid index of first element to be deleted
953 */
954 private boolean bulkRemoveModified(
955 Predicate<? super E> filter, final int beg) {
956 final Object[] es = elements;
957 final int capacity = es.length;
958 final int end = tail;
959 final long[] deathRow = nBits(sub(end, beg, capacity));
960 deathRow[0] = 1L; // set bit 0
961 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
962 ; i = 0, to = end, k -= capacity) {
963 for (; i < to; i++)
964 if (filter.test(elementAt(es, i)))
965 setBit(deathRow, i - k);
966 if (to == end) break;
967 }
968 // a two-finger traversal, with hare i reading, tortoise w writing
969 int w = beg;
970 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
971 ; w = 0) { // w rejoins i on second leg
972 // In this loop, i and w are on the same leg, with i > w
973 for (; i < to; i++)
974 if (isClear(deathRow, i - k))
975 es[w++] = es[i];
976 if (to == end) break;
977 // In this loop, w is on the first leg, i on the second
978 for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++)
979 if (isClear(deathRow, i - k))
980 es[w++] = es[i];
981 if (i >= to) {
982 if (w == capacity) w = 0; // "corner" case
983 break;
984 }
985 }
986 if (end != tail) throw new ConcurrentModificationException();
987 circularClear(es, tail = w, end);
988 return true;
989 }
990
991 /**
992 * Returns {@code true} if this deque contains the specified element.
993 * More formally, returns {@code true} if and only if this deque contains
994 * at least one element {@code e} such that {@code o.equals(e)}.
995 *
996 * @param o object to be checked for containment in this deque
997 * @return {@code true} if this deque contains the specified element
998 */
999 public boolean contains(Object o) {
1000 if (o != null) {
1001 final Object[] es = elements;
1002 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1003 ; i = 0, to = end) {
1004 for (; i < to; i++)
1005 if (o.equals(es[i]))
1006 return true;
1007 if (to == end) break;
1008 }
1009 }
1010 return false;
1011 }
1012
1013 /**
1014 * Removes a single instance of the specified element from this deque.
1015 * If the deque does not contain the element, it is unchanged.
1016 * More formally, removes the first element {@code e} such that
1017 * {@code o.equals(e)} (if such an element exists).
1018 * Returns {@code true} if this deque contained the specified element
1019 * (or equivalently, if this deque changed as a result of the call).
1020 *
1021 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
1022 *
1023 * @param o element to be removed from this deque, if present
1024 * @return {@code true} if this deque contained the specified element
1025 */
1026 public boolean remove(Object o) {
1027 return removeFirstOccurrence(o);
1028 }
1029
1030 /**
1031 * Removes all of the elements from this deque.
1032 * The deque will be empty after this call returns.
1033 */
1034 public void clear() {
1035 circularClear(elements, head, tail);
1036 head = tail = 0;
1037 }
1038
1039 /**
1040 * Nulls out slots starting at array index i, upto index end.
1041 * Condition i == end means "empty" - nothing to do.
1042 */
1043 private static void circularClear(Object[] es, int i, int end) {
1044 // assert 0 <= i && i < es.length;
1045 // assert 0 <= end && end < es.length;
1046 for (int to = (i <= end) ? end : es.length;
1047 ; i = 0, to = end) {
1048 for (; i < to; i++) es[i] = null;
1049 if (to == end) break;
1050 }
1051 }
1052
1053 /**
1054 * Returns an array containing all of the elements in this deque
1055 * in proper sequence (from first to last element).
1056 *
1057 * <p>The returned array will be "safe" in that no references to it are
1058 * maintained by this deque. (In other words, this method must allocate
1059 * a new array). The caller is thus free to modify the returned array.
1060 *
1061 * <p>This method acts as bridge between array-based and collection-based
1062 * APIs.
1063 *
1064 * @return an array containing all of the elements in this deque
1065 */
1066 public Object[] toArray() {
1067 return toArray(Object[].class);
1068 }
1069
1070 private <T> T[] toArray(Class<T[]> klazz) {
1071 final Object[] es = elements;
1072 final T[] a;
1073 final int head = this.head, tail = this.tail, end;
1074 if ((end = tail + ((head <= tail) ? 0 : es.length)) >= 0) {
1075 // Uses null extension feature of copyOfRange
1076 a = Arrays.copyOfRange(es, head, end, klazz);
1077 } else {
1078 // integer overflow!
1079 a = Arrays.copyOfRange(es, 0, end - head, klazz);
1080 System.arraycopy(es, head, a, 0, es.length - head);
1081 }
1082 if (end != tail)
1083 System.arraycopy(es, 0, a, es.length - head, tail);
1084 return a;
1085 }
1086
1087 /**
1088 * Returns an array containing all of the elements in this deque in
1089 * proper sequence (from first to last element); the runtime type of the
1090 * returned array is that of the specified array. If the deque fits in
1091 * the specified array, it is returned therein. Otherwise, a new array
1092 * is allocated with the runtime type of the specified array and the
1093 * size of this deque.
1094 *
1095 * <p>If this deque fits in the specified array with room to spare
1096 * (i.e., the array has more elements than this deque), the element in
1097 * the array immediately following the end of the deque is set to
1098 * {@code null}.
1099 *
1100 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1101 * array-based and collection-based APIs. Further, this method allows
1102 * precise control over the runtime type of the output array, and may,
1103 * under certain circumstances, be used to save allocation costs.
1104 *
1105 * <p>Suppose {@code x} is a deque known to contain only strings.
1106 * The following code can be used to dump the deque into a newly
1107 * allocated array of {@code String}:
1108 *
1109 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
1110 *
1111 * Note that {@code toArray(new Object[0])} is identical in function to
1112 * {@code toArray()}.
1113 *
1114 * @param a the array into which the elements of the deque are to
1115 * be stored, if it is big enough; otherwise, a new array of the
1116 * same runtime type is allocated for this purpose
1117 * @return an array containing all of the elements in this deque
1118 * @throws ArrayStoreException if the runtime type of the specified array
1119 * is not a supertype of the runtime type of every element in
1120 * this deque
1121 * @throws NullPointerException if the specified array is null
1122 */
1123 @SuppressWarnings("unchecked")
1124 public <T> T[] toArray(T[] a) {
1125 final int size;
1126 if ((size = size()) > a.length)
1127 return toArray((Class<T[]>) a.getClass());
1128 final Object[] es = elements;
1129 for (int i = head, j = 0, len = Math.min(size, es.length - i);
1130 ; i = 0, len = tail) {
1131 System.arraycopy(es, i, a, j, len);
1132 if ((j += len) == size) break;
1133 }
1134 if (size < a.length)
1135 a[size] = null;
1136 return a;
1137 }
1138
1139 // *** Object methods ***
1140
1141 /**
1142 * Returns a copy of this deque.
1143 *
1144 * @return a copy of this deque
1145 */
1146 public ArrayDeque<E> clone() {
1147 try {
1148 @SuppressWarnings("unchecked")
1149 ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
1150 result.elements = Arrays.copyOf(elements, elements.length);
1151 return result;
1152 } catch (CloneNotSupportedException e) {
1153 throw new AssertionError();
1154 }
1155 }
1156
1157 private static final long serialVersionUID = 2340985798034038923L;
1158
1159 /**
1160 * Saves this deque to a stream (that is, serializes it).
1161 *
1162 * @param s the stream
1163 * @throws java.io.IOException if an I/O error occurs
1164 * @serialData The current size ({@code int}) of the deque,
1165 * followed by all of its elements (each an object reference) in
1166 * first-to-last order.
1167 */
1168 private void writeObject(java.io.ObjectOutputStream s)
1169 throws java.io.IOException {
1170 s.defaultWriteObject();
1171
1172 // Write out size
1173 s.writeInt(size());
1174
1175 // Write out elements in order.
1176 final Object[] es = elements;
1177 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1178 ; i = 0, to = end) {
1179 for (; i < to; i++)
1180 s.writeObject(es[i]);
1181 if (to == end) break;
1182 }
1183 }
1184
1185 /**
1186 * Reconstitutes this deque from a stream (that is, deserializes it).
1187 * @param s the stream
1188 * @throws ClassNotFoundException if the class of a serialized object
1189 * could not be found
1190 * @throws java.io.IOException if an I/O error occurs
1191 */
1192 private void readObject(java.io.ObjectInputStream s)
1193 throws java.io.IOException, ClassNotFoundException {
1194 s.defaultReadObject();
1195
1196 // Read in size and allocate array
1197 int size = s.readInt();
1198 elements = new Object[size + 1];
1199 this.tail = size;
1200
1201 // Read in all elements in the proper order.
1202 for (int i = 0; i < size; i++)
1203 elements[i] = s.readObject();
1204 }
1205
1206 /** debugging */
1207 void checkInvariants() {
1208 // Use head and tail fields with empty slot at tail strategy.
1209 // head == tail disambiguates to "empty".
1210 try {
1211 int capacity = elements.length;
1212 // assert 0 <= head && head < capacity;
1213 // assert 0 <= tail && tail < capacity;
1214 // assert capacity > 0;
1215 // assert size() < capacity;
1216 // assert head == tail || elements[head] != null;
1217 // assert elements[tail] == null;
1218 // assert head == tail || elements[dec(tail, capacity)] != null;
1219 } catch (Throwable t) {
1220 System.err.printf("head=%d tail=%d capacity=%d%n",
1221 head, tail, elements.length);
1222 System.err.printf("elements=%s%n",
1223 Arrays.toString(elements));
1224 throw t;
1225 }
1226 }
1227
1228 }