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}/java/util/package-summary.html#CollectionsFramework">
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 disambiguated 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 iterator.
314 *
315 * @param c the elements to be inserted into this deque
316 * @return {@code true} if this deque changed as a result of the call
317 * @throws NullPointerException if the specified collection or any
318 * of its elements are null
319 */
320 public boolean addAll(Collection<? extends E> c) {
321 final int s, needed;
322 if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0)
323 grow(needed);
324 c.forEach(this::addLast);
325 return size() > s;
326 }
327
328 /**
329 * Inserts the specified element at the front of this deque.
330 *
331 * @param e the element to add
332 * @return {@code true} (as specified by {@link Deque#offerFirst})
333 * @throws NullPointerException if the specified element is null
334 */
335 public boolean offerFirst(E e) {
336 addFirst(e);
337 return true;
338 }
339
340 /**
341 * Inserts the specified element at the end of this deque.
342 *
343 * @param e the element to add
344 * @return {@code true} (as specified by {@link Deque#offerLast})
345 * @throws NullPointerException if the specified element is null
346 */
347 public boolean offerLast(E e) {
348 addLast(e);
349 return true;
350 }
351
352 /**
353 * @throws NoSuchElementException {@inheritDoc}
354 */
355 public E removeFirst() {
356 E e = pollFirst();
357 if (e == null)
358 throw new NoSuchElementException();
359 return e;
360 }
361
362 /**
363 * @throws NoSuchElementException {@inheritDoc}
364 */
365 public E removeLast() {
366 E e = pollLast();
367 if (e == null)
368 throw new NoSuchElementException();
369 return e;
370 }
371
372 public E pollFirst() {
373 final Object[] es;
374 final int h;
375 E e = elementAt(es = elements, h = head);
376 if (e != null) {
377 es[h] = null;
378 head = inc(h, es.length);
379 }
380 return e;
381 }
382
383 public E pollLast() {
384 final Object[] es;
385 final int t;
386 E e = elementAt(es = elements, t = dec(tail, es.length));
387 if (e != null)
388 es[tail = t] = null;
389 return e;
390 }
391
392 /**
393 * @throws NoSuchElementException {@inheritDoc}
394 */
395 public E getFirst() {
396 E e = elementAt(elements, head);
397 if (e == null)
398 throw new NoSuchElementException();
399 return e;
400 }
401
402 /**
403 * @throws NoSuchElementException {@inheritDoc}
404 */
405 public E getLast() {
406 final Object[] es = elements;
407 E e = elementAt(es, dec(tail, es.length));
408 if (e == null)
409 throw new NoSuchElementException();
410 return e;
411 }
412
413 public E peekFirst() {
414 return elementAt(elements, head);
415 }
416
417 public E peekLast() {
418 final Object[] es;
419 return elementAt(es = elements, dec(tail, es.length));
420 }
421
422 /**
423 * Removes the first occurrence of the specified element in this
424 * deque (when traversing the deque from head to tail).
425 * If the deque does not contain the element, it is unchanged.
426 * More formally, removes the first element {@code e} such that
427 * {@code o.equals(e)} (if such an element exists).
428 * Returns {@code true} if this deque contained the specified element
429 * (or equivalently, if this deque changed as a result of the call).
430 *
431 * @param o element to be removed from this deque, if present
432 * @return {@code true} if the deque contained the specified element
433 */
434 public boolean removeFirstOccurrence(Object o) {
435 if (o != null) {
436 final Object[] es = elements;
437 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
438 ; i = 0, to = end) {
439 for (; i < to; i++)
440 if (o.equals(es[i])) {
441 delete(i);
442 return true;
443 }
444 if (to == end) break;
445 }
446 }
447 return false;
448 }
449
450 /**
451 * Removes the last occurrence of the specified element in this
452 * deque (when traversing the deque from head to tail).
453 * If the deque does not contain the element, it is unchanged.
454 * More formally, removes the last element {@code e} such that
455 * {@code o.equals(e)} (if such an element exists).
456 * Returns {@code true} if this deque contained the specified element
457 * (or equivalently, if this deque changed as a result of the call).
458 *
459 * @param o element to be removed from this deque, if present
460 * @return {@code true} if the deque contained the specified element
461 */
462 public boolean removeLastOccurrence(Object o) {
463 if (o != null) {
464 final Object[] es = elements;
465 for (int i = tail, end = head, to = (i >= end) ? end : 0;
466 ; i = es.length, to = end) {
467 for (i--; i > to - 1; i--)
468 if (o.equals(es[i])) {
469 delete(i);
470 return true;
471 }
472 if (to == end) break;
473 }
474 }
475 return false;
476 }
477
478 // *** Queue methods ***
479
480 /**
481 * Inserts the specified element at the end of this deque.
482 *
483 * <p>This method is equivalent to {@link #addLast}.
484 *
485 * @param e the element to add
486 * @return {@code true} (as specified by {@link Collection#add})
487 * @throws NullPointerException if the specified element is null
488 */
489 public boolean add(E e) {
490 addLast(e);
491 return true;
492 }
493
494 /**
495 * Inserts the specified element at the end of this deque.
496 *
497 * <p>This method is equivalent to {@link #offerLast}.
498 *
499 * @param e the element to add
500 * @return {@code true} (as specified by {@link Queue#offer})
501 * @throws NullPointerException if the specified element is null
502 */
503 public boolean offer(E e) {
504 return offerLast(e);
505 }
506
507 /**
508 * Retrieves and removes the head of the queue represented by this deque.
509 *
510 * This method differs from {@link #poll() poll()} only in that it
511 * throws an exception if this deque is empty.
512 *
513 * <p>This method is equivalent to {@link #removeFirst}.
514 *
515 * @return the head of the queue represented by this deque
516 * @throws NoSuchElementException {@inheritDoc}
517 */
518 public E remove() {
519 return removeFirst();
520 }
521
522 /**
523 * Retrieves and removes the head of the queue represented by this deque
524 * (in other words, the first element of this deque), or returns
525 * {@code null} if this deque is empty.
526 *
527 * <p>This method is equivalent to {@link #pollFirst}.
528 *
529 * @return the head of the queue represented by this deque, or
530 * {@code null} if this deque is empty
531 */
532 public E poll() {
533 return pollFirst();
534 }
535
536 /**
537 * Retrieves, but does not remove, the head of the queue represented by
538 * this deque. This method differs from {@link #peek peek} only in
539 * that it throws an exception if this deque is empty.
540 *
541 * <p>This method is equivalent to {@link #getFirst}.
542 *
543 * @return the head of the queue represented by this deque
544 * @throws NoSuchElementException {@inheritDoc}
545 */
546 public E element() {
547 return getFirst();
548 }
549
550 /**
551 * Retrieves, but does not remove, the head of the queue represented by
552 * this deque, or returns {@code null} if this deque is empty.
553 *
554 * <p>This method is equivalent to {@link #peekFirst}.
555 *
556 * @return the head of the queue represented by this deque, or
557 * {@code null} if this deque is empty
558 */
559 public E peek() {
560 return peekFirst();
561 }
562
563 // *** Stack methods ***
564
565 /**
566 * Pushes an element onto the stack represented by this deque. In other
567 * words, inserts the element at the front of this deque.
568 *
569 * <p>This method is equivalent to {@link #addFirst}.
570 *
571 * @param e the element to push
572 * @throws NullPointerException if the specified element is null
573 */
574 public void push(E e) {
575 addFirst(e);
576 }
577
578 /**
579 * Pops an element from the stack represented by this deque. In other
580 * words, removes and returns the first element of this deque.
581 *
582 * <p>This method is equivalent to {@link #removeFirst()}.
583 *
584 * @return the element at the front of this deque (which is the top
585 * of the stack represented by this deque)
586 * @throws NoSuchElementException {@inheritDoc}
587 */
588 public E pop() {
589 return removeFirst();
590 }
591
592 /**
593 * Removes the element at the specified position in the elements array.
594 * This can result in forward or backwards motion of array elements.
595 * We optimize for least element motion.
596 *
597 * <p>This method is called delete rather than remove to emphasize
598 * that its semantics differ from those of {@link List#remove(int)}.
599 *
600 * @return true if elements near tail moved backwards
601 */
602 boolean delete(int i) {
603 final Object[] es = elements;
604 final int capacity = es.length;
605 final int h, t;
606 // number of elements before to-be-deleted elt
607 final int front = sub(i, h = head, capacity);
608 // number of elements after to-be-deleted elt
609 final int back = sub(t = tail, i, capacity) - 1;
610 if (front < back) {
611 // move front elements forwards
612 if (h <= i) {
613 System.arraycopy(es, h, es, h + 1, front);
614 } else { // Wrap around
615 System.arraycopy(es, 0, es, 1, i);
616 es[0] = es[capacity - 1];
617 System.arraycopy(es, h, es, h + 1, front - (i + 1));
618 }
619 es[h] = null;
620 head = inc(h, capacity);
621 return false;
622 } else {
623 // move back elements backwards
624 tail = dec(t, capacity);
625 if (i <= tail) {
626 System.arraycopy(es, i + 1, es, i, back);
627 } else { // Wrap around
628 System.arraycopy(es, i + 1, es, i, capacity - (i + 1));
629 es[capacity - 1] = es[0];
630 System.arraycopy(es, 1, es, 0, t - 1);
631 }
632 es[tail] = null;
633 return true;
634 }
635 }
636
637 // *** Collection Methods ***
638
639 /**
640 * Returns the number of elements in this deque.
641 *
642 * @return the number of elements in this deque
643 */
644 public int size() {
645 return sub(tail, head, elements.length);
646 }
647
648 /**
649 * Returns {@code true} if this deque contains no elements.
650 *
651 * @return {@code true} if this deque contains no elements
652 */
653 public boolean isEmpty() {
654 return head == tail;
655 }
656
657 /**
658 * Returns an iterator over the elements in this deque. The elements
659 * will be ordered from first (head) to last (tail). This is the same
660 * order that elements would be dequeued (via successive calls to
661 * {@link #remove} or popped (via successive calls to {@link #pop}).
662 *
663 * @return an iterator over the elements in this deque
664 */
665 public Iterator<E> iterator() {
666 return new DeqIterator();
667 }
668
669 public Iterator<E> descendingIterator() {
670 return new DescendingIterator();
671 }
672
673 private class DeqIterator implements Iterator<E> {
674 /** Index of element to be returned by subsequent call to next. */
675 int cursor;
676
677 /** Number of elements yet to be returned. */
678 int remaining = size();
679
680 /**
681 * Index of element returned by most recent call to next.
682 * Reset to -1 if element is deleted by a call to remove.
683 */
684 int lastRet = -1;
685
686 DeqIterator() { cursor = head; }
687
688 public final boolean hasNext() {
689 return remaining > 0;
690 }
691
692 public E next() {
693 if (remaining <= 0)
694 throw new NoSuchElementException();
695 final Object[] es = elements;
696 E e = nonNullElementAt(es, cursor);
697 cursor = inc(lastRet = cursor, es.length);
698 remaining--;
699 return e;
700 }
701
702 void postDelete(boolean leftShifted) {
703 if (leftShifted)
704 cursor = dec(cursor, elements.length);
705 }
706
707 public final void remove() {
708 if (lastRet < 0)
709 throw new IllegalStateException();
710 postDelete(delete(lastRet));
711 lastRet = -1;
712 }
713
714 public void forEachRemaining(Consumer<? super E> action) {
715 Objects.requireNonNull(action);
716 int r;
717 if ((r = remaining) <= 0)
718 return;
719 remaining = 0;
720 final Object[] es = elements;
721 if (es[cursor] == null || sub(tail, cursor, es.length) != r)
722 throw new ConcurrentModificationException();
723 for (int i = cursor, end = tail, to = (i <= end) ? end : es.length;
724 ; i = 0, to = end) {
725 for (; i < to; i++)
726 action.accept(elementAt(es, i));
727 if (to == end) {
728 if (end != tail)
729 throw new ConcurrentModificationException();
730 lastRet = dec(end, es.length);
731 break;
732 }
733 }
734 }
735 }
736
737 private class DescendingIterator extends DeqIterator {
738 DescendingIterator() { cursor = dec(tail, elements.length); }
739
740 public final E next() {
741 if (remaining <= 0)
742 throw new NoSuchElementException();
743 final Object[] es = elements;
744 E e = nonNullElementAt(es, cursor);
745 cursor = dec(lastRet = cursor, es.length);
746 remaining--;
747 return e;
748 }
749
750 void postDelete(boolean leftShifted) {
751 if (!leftShifted)
752 cursor = inc(cursor, elements.length);
753 }
754
755 public final void forEachRemaining(Consumer<? super E> action) {
756 Objects.requireNonNull(action);
757 int r;
758 if ((r = remaining) <= 0)
759 return;
760 remaining = 0;
761 final Object[] es = elements;
762 if (es[cursor] == null || sub(cursor, head, es.length) + 1 != r)
763 throw new ConcurrentModificationException();
764 for (int i = cursor, end = head, to = (i >= end) ? end : 0;
765 ; i = es.length - 1, to = end) {
766 // hotspot generates faster code than for: i >= to !
767 for (; i > to - 1; i--)
768 action.accept(elementAt(es, i));
769 if (to == end) {
770 if (end != head)
771 throw new ConcurrentModificationException();
772 lastRet = end;
773 break;
774 }
775 }
776 }
777 }
778
779 /**
780 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
781 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
782 * deque.
783 *
784 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
785 * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
786 * {@link Spliterator#NONNULL}. Overriding implementations should document
787 * the reporting of additional characteristic values.
788 *
789 * @return a {@code Spliterator} over the elements in this deque
790 * @since 1.8
791 */
792 public Spliterator<E> spliterator() {
793 return new DeqSpliterator();
794 }
795
796 final class DeqSpliterator implements Spliterator<E> {
797 private int fence; // -1 until first use
798 private int cursor; // current index, modified on traverse/split
799
800 /** Constructs late-binding spliterator over all elements. */
801 DeqSpliterator() {
802 this.fence = -1;
803 }
804
805 /** Constructs spliterator over the given range. */
806 DeqSpliterator(int origin, int fence) {
807 // assert 0 <= origin && origin < elements.length;
808 // assert 0 <= fence && fence < elements.length;
809 this.cursor = origin;
810 this.fence = fence;
811 }
812
813 /** Ensures late-binding initialization; then returns fence. */
814 private int getFence() { // force initialization
815 int t;
816 if ((t = fence) < 0) {
817 t = fence = tail;
818 cursor = head;
819 }
820 return t;
821 }
822
823 public DeqSpliterator trySplit() {
824 final Object[] es = elements;
825 final int i, n;
826 return ((n = sub(getFence(), i = cursor, es.length) >> 1) <= 0)
827 ? null
828 : new DeqSpliterator(i, cursor = add(i, n, es.length));
829 }
830
831 public void forEachRemaining(Consumer<? super E> action) {
832 if (action == null)
833 throw new NullPointerException();
834 final int end = getFence(), cursor = this.cursor;
835 final Object[] es = elements;
836 if (cursor != end) {
837 this.cursor = end;
838 // null check at both ends of range is sufficient
839 if (es[cursor] == null || es[dec(end, es.length)] == null)
840 throw new ConcurrentModificationException();
841 for (int i = cursor, to = (i <= end) ? end : es.length;
842 ; i = 0, to = end) {
843 for (; i < to; i++)
844 action.accept(elementAt(es, i));
845 if (to == end) break;
846 }
847 }
848 }
849
850 public boolean tryAdvance(Consumer<? super E> action) {
851 Objects.requireNonNull(action);
852 final Object[] es = elements;
853 if (fence < 0) { fence = tail; cursor = head; } // late-binding
854 final int i;
855 if ((i = cursor) == fence)
856 return false;
857 E e = nonNullElementAt(es, i);
858 cursor = inc(i, es.length);
859 action.accept(e);
860 return true;
861 }
862
863 public long estimateSize() {
864 return sub(getFence(), cursor, elements.length);
865 }
866
867 public int characteristics() {
868 return Spliterator.NONNULL
869 | Spliterator.ORDERED
870 | Spliterator.SIZED
871 | Spliterator.SUBSIZED;
872 }
873 }
874
875 /**
876 * @throws NullPointerException {@inheritDoc}
877 */
878 public void forEach(Consumer<? super E> action) {
879 Objects.requireNonNull(action);
880 final Object[] es = elements;
881 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
882 ; i = 0, to = end) {
883 for (; i < to; i++)
884 action.accept(elementAt(es, i));
885 if (to == end) {
886 if (end != tail) throw new ConcurrentModificationException();
887 break;
888 }
889 }
890 }
891
892 /**
893 * @throws NullPointerException {@inheritDoc}
894 */
895 public boolean removeIf(Predicate<? super E> filter) {
896 Objects.requireNonNull(filter);
897 return bulkRemove(filter);
898 }
899
900 /**
901 * @throws NullPointerException {@inheritDoc}
902 */
903 public boolean removeAll(Collection<?> c) {
904 Objects.requireNonNull(c);
905 return bulkRemove(e -> c.contains(e));
906 }
907
908 /**
909 * @throws NullPointerException {@inheritDoc}
910 */
911 public boolean retainAll(Collection<?> c) {
912 Objects.requireNonNull(c);
913 return bulkRemove(e -> !c.contains(e));
914 }
915
916 /** Implementation of bulk remove methods. */
917 private boolean bulkRemove(Predicate<? super E> filter) {
918 final Object[] es = elements;
919 // Optimize for initial run of survivors
920 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
921 ; i = 0, to = end) {
922 for (; i < to; i++)
923 if (filter.test(elementAt(es, i)))
924 return bulkRemoveModified(filter, i);
925 if (to == end) {
926 if (end != tail) throw new ConcurrentModificationException();
927 break;
928 }
929 }
930 return false;
931 }
932
933 // A tiny bit set implementation
934
935 private static long[] nBits(int n) {
936 return new long[((n - 1) >> 6) + 1];
937 }
938 private static void setBit(long[] bits, int i) {
939 bits[i >> 6] |= 1L << i;
940 }
941 private static boolean isClear(long[] bits, int i) {
942 return (bits[i >> 6] & (1L << i)) == 0;
943 }
944
945 /**
946 * Helper for bulkRemove, in case of at least one deletion.
947 * Tolerate predicates that reentrantly access the collection for
948 * read (but writers still get CME), so traverse once to find
949 * elements to delete, a second pass to physically expunge.
950 *
951 * @param beg valid index of first element to be deleted
952 */
953 private boolean bulkRemoveModified(
954 Predicate<? super E> filter, final int beg) {
955 final Object[] es = elements;
956 final int capacity = es.length;
957 final int end = tail;
958 final long[] deathRow = nBits(sub(end, beg, capacity));
959 deathRow[0] = 1L; // set bit 0
960 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
961 ; i = 0, to = end, k -= capacity) {
962 for (; i < to; i++)
963 if (filter.test(elementAt(es, i)))
964 setBit(deathRow, i - k);
965 if (to == end) break;
966 }
967 // a two-finger traversal, with hare i reading, tortoise w writing
968 int w = beg;
969 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
970 ; w = 0) { // w rejoins i on second leg
971 // In this loop, i and w are on the same leg, with i > w
972 for (; i < to; i++)
973 if (isClear(deathRow, i - k))
974 es[w++] = es[i];
975 if (to == end) break;
976 // In this loop, w is on the first leg, i on the second
977 for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++)
978 if (isClear(deathRow, i - k))
979 es[w++] = es[i];
980 if (i >= to) {
981 if (w == capacity) w = 0; // "corner" case
982 break;
983 }
984 }
985 if (end != tail) throw new ConcurrentModificationException();
986 circularClear(es, tail = w, end);
987 return true;
988 }
989
990 /**
991 * Returns {@code true} if this deque contains the specified element.
992 * More formally, returns {@code true} if and only if this deque contains
993 * at least one element {@code e} such that {@code o.equals(e)}.
994 *
995 * @param o object to be checked for containment in this deque
996 * @return {@code true} if this deque contains the specified element
997 */
998 public boolean contains(Object o) {
999 if (o != null) {
1000 final Object[] es = elements;
1001 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1002 ; i = 0, to = end) {
1003 for (; i < to; i++)
1004 if (o.equals(es[i]))
1005 return true;
1006 if (to == end) break;
1007 }
1008 }
1009 return false;
1010 }
1011
1012 /**
1013 * Removes a single instance of the specified element from this deque.
1014 * If the deque does not contain the element, it is unchanged.
1015 * More formally, removes the first element {@code e} such that
1016 * {@code o.equals(e)} (if such an element exists).
1017 * Returns {@code true} if this deque contained the specified element
1018 * (or equivalently, if this deque changed as a result of the call).
1019 *
1020 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
1021 *
1022 * @param o element to be removed from this deque, if present
1023 * @return {@code true} if this deque contained the specified element
1024 */
1025 public boolean remove(Object o) {
1026 return removeFirstOccurrence(o);
1027 }
1028
1029 /**
1030 * Removes all of the elements from this deque.
1031 * The deque will be empty after this call returns.
1032 */
1033 public void clear() {
1034 circularClear(elements, head, tail);
1035 head = tail = 0;
1036 }
1037
1038 /**
1039 * Nulls out slots starting at array index i, upto index end.
1040 * Condition i == end means "empty" - nothing to do.
1041 */
1042 private static void circularClear(Object[] es, int i, int end) {
1043 // assert 0 <= i && i < es.length;
1044 // assert 0 <= end && end < es.length;
1045 for (int to = (i <= end) ? end : es.length;
1046 ; i = 0, to = end) {
1047 for (; i < to; i++) es[i] = null;
1048 if (to == end) break;
1049 }
1050 }
1051
1052 /**
1053 * Returns an array containing all of the elements in this deque
1054 * in proper sequence (from first to last element).
1055 *
1056 * <p>The returned array will be "safe" in that no references to it are
1057 * maintained by this deque. (In other words, this method must allocate
1058 * a new array). The caller is thus free to modify the returned array.
1059 *
1060 * <p>This method acts as bridge between array-based and collection-based
1061 * APIs.
1062 *
1063 * @return an array containing all of the elements in this deque
1064 */
1065 public Object[] toArray() {
1066 return toArray(Object[].class);
1067 }
1068
1069 private <T> T[] toArray(Class<T[]> klazz) {
1070 final Object[] es = elements;
1071 final T[] a;
1072 final int head = this.head, tail = this.tail, end;
1073 if ((end = tail + ((head <= tail) ? 0 : es.length)) >= 0) {
1074 // Uses null extension feature of copyOfRange
1075 a = Arrays.copyOfRange(es, head, end, klazz);
1076 } else {
1077 // integer overflow!
1078 a = Arrays.copyOfRange(es, 0, end - head, klazz);
1079 System.arraycopy(es, head, a, 0, es.length - head);
1080 }
1081 if (end != tail)
1082 System.arraycopy(es, 0, a, es.length - head, tail);
1083 return a;
1084 }
1085
1086 /**
1087 * Returns an array containing all of the elements in this deque in
1088 * proper sequence (from first to last element); the runtime type of the
1089 * returned array is that of the specified array. If the deque fits in
1090 * the specified array, it is returned therein. Otherwise, a new array
1091 * is allocated with the runtime type of the specified array and the
1092 * size of this deque.
1093 *
1094 * <p>If this deque fits in the specified array with room to spare
1095 * (i.e., the array has more elements than this deque), the element in
1096 * the array immediately following the end of the deque is set to
1097 * {@code null}.
1098 *
1099 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1100 * array-based and collection-based APIs. Further, this method allows
1101 * precise control over the runtime type of the output array, and may,
1102 * under certain circumstances, be used to save allocation costs.
1103 *
1104 * <p>Suppose {@code x} is a deque known to contain only strings.
1105 * The following code can be used to dump the deque into a newly
1106 * allocated array of {@code String}:
1107 *
1108 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
1109 *
1110 * Note that {@code toArray(new Object[0])} is identical in function to
1111 * {@code toArray()}.
1112 *
1113 * @param a the array into which the elements of the deque are to
1114 * be stored, if it is big enough; otherwise, a new array of the
1115 * same runtime type is allocated for this purpose
1116 * @return an array containing all of the elements in this deque
1117 * @throws ArrayStoreException if the runtime type of the specified array
1118 * is not a supertype of the runtime type of every element in
1119 * this deque
1120 * @throws NullPointerException if the specified array is null
1121 */
1122 @SuppressWarnings("unchecked")
1123 public <T> T[] toArray(T[] a) {
1124 final int size;
1125 if ((size = size()) > a.length)
1126 return toArray((Class<T[]>) a.getClass());
1127 final Object[] es = elements;
1128 for (int i = head, j = 0, len = Math.min(size, es.length - i);
1129 ; i = 0, len = tail) {
1130 System.arraycopy(es, i, a, j, len);
1131 if ((j += len) == size) break;
1132 }
1133 if (size < a.length)
1134 a[size] = null;
1135 return a;
1136 }
1137
1138 // *** Object methods ***
1139
1140 /**
1141 * Returns a copy of this deque.
1142 *
1143 * @return a copy of this deque
1144 */
1145 public ArrayDeque<E> clone() {
1146 try {
1147 @SuppressWarnings("unchecked")
1148 ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
1149 result.elements = Arrays.copyOf(elements, elements.length);
1150 return result;
1151 } catch (CloneNotSupportedException e) {
1152 throw new AssertionError();
1153 }
1154 }
1155
1156 private static final long serialVersionUID = 2340985798034038923L;
1157
1158 /**
1159 * Saves this deque to a stream (that is, serializes it).
1160 *
1161 * @param s the stream
1162 * @throws java.io.IOException if an I/O error occurs
1163 * @serialData The current size ({@code int}) of the deque,
1164 * followed by all of its elements (each an object reference) in
1165 * first-to-last order.
1166 */
1167 private void writeObject(java.io.ObjectOutputStream s)
1168 throws java.io.IOException {
1169 s.defaultWriteObject();
1170
1171 // Write out size
1172 s.writeInt(size());
1173
1174 // Write out elements in order.
1175 final Object[] es = elements;
1176 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1177 ; i = 0, to = end) {
1178 for (; i < to; i++)
1179 s.writeObject(es[i]);
1180 if (to == end) break;
1181 }
1182 }
1183
1184 /**
1185 * Reconstitutes this deque from a stream (that is, deserializes it).
1186 * @param s the stream
1187 * @throws ClassNotFoundException if the class of a serialized object
1188 * could not be found
1189 * @throws java.io.IOException if an I/O error occurs
1190 */
1191 private void readObject(java.io.ObjectInputStream s)
1192 throws java.io.IOException, ClassNotFoundException {
1193 s.defaultReadObject();
1194
1195 // Read in size and allocate array
1196 int size = s.readInt();
1197 elements = new Object[size + 1];
1198 this.tail = size;
1199
1200 // Read in all elements in the proper order.
1201 for (int i = 0; i < size; i++)
1202 elements[i] = s.readObject();
1203 }
1204
1205 /** debugging */
1206 void checkInvariants() {
1207 // Use head and tail fields with empty slot at tail strategy.
1208 // head == tail disambiguates to "empty".
1209 try {
1210 int capacity = elements.length;
1211 // assert 0 <= head && head < capacity;
1212 // assert 0 <= tail && tail < capacity;
1213 // assert capacity > 0;
1214 // assert size() < capacity;
1215 // assert head == tail || elements[head] != null;
1216 // assert elements[tail] == null;
1217 // assert head == tail || elements[dec(tail, capacity)] != null;
1218 } catch (Throwable t) {
1219 System.err.printf("head=%d tail=%d capacity=%d%n",
1220 head, tail, elements.length);
1221 System.err.printf("elements=%s%n",
1222 Arrays.toString(elements));
1223 throw t;
1224 }
1225 }
1226
1227 }