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 * @since 9
321 */
322 public boolean addAll(Collection<? extends E> c) {
323 final int s, needed;
324 if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0)
325 grow(needed);
326 c.forEach(this::addLast);
327 return size() > s;
328 }
329
330 /**
331 * Inserts the specified element at the front of this deque.
332 *
333 * @param e the element to add
334 * @return {@code true} (as specified by {@link Deque#offerFirst})
335 * @throws NullPointerException if the specified element is null
336 */
337 public boolean offerFirst(E e) {
338 addFirst(e);
339 return true;
340 }
341
342 /**
343 * Inserts the specified element at the end of this deque.
344 *
345 * @param e the element to add
346 * @return {@code true} (as specified by {@link Deque#offerLast})
347 * @throws NullPointerException if the specified element is null
348 */
349 public boolean offerLast(E e) {
350 addLast(e);
351 return true;
352 }
353
354 /**
355 * @throws NoSuchElementException {@inheritDoc}
356 */
357 public E removeFirst() {
358 E e = pollFirst();
359 if (e == null)
360 throw new NoSuchElementException();
361 return e;
362 }
363
364 /**
365 * @throws NoSuchElementException {@inheritDoc}
366 */
367 public E removeLast() {
368 E e = pollLast();
369 if (e == null)
370 throw new NoSuchElementException();
371 return e;
372 }
373
374 public E pollFirst() {
375 final Object[] es;
376 final int h;
377 E e = elementAt(es = elements, h = head);
378 if (e != null) {
379 es[h] = null;
380 head = inc(h, es.length);
381 }
382 return e;
383 }
384
385 public E pollLast() {
386 final Object[] es;
387 final int t;
388 E e = elementAt(es = elements, t = dec(tail, es.length));
389 if (e != null)
390 es[tail = t] = null;
391 return e;
392 }
393
394 /**
395 * @throws NoSuchElementException {@inheritDoc}
396 */
397 public E getFirst() {
398 E e = elementAt(elements, head);
399 if (e == null)
400 throw new NoSuchElementException();
401 return e;
402 }
403
404 /**
405 * @throws NoSuchElementException {@inheritDoc}
406 */
407 public E getLast() {
408 final Object[] es = elements;
409 E e = elementAt(es, dec(tail, es.length));
410 if (e == null)
411 throw new NoSuchElementException();
412 return e;
413 }
414
415 public E peekFirst() {
416 return elementAt(elements, head);
417 }
418
419 public E peekLast() {
420 final Object[] es;
421 return elementAt(es = elements, dec(tail, es.length));
422 }
423
424 /**
425 * Removes the first occurrence of the specified element in this
426 * deque (when traversing the deque from head to tail).
427 * If the deque does not contain the element, it is unchanged.
428 * More formally, removes the first element {@code e} such that
429 * {@code o.equals(e)} (if such an element exists).
430 * Returns {@code true} if this deque contained the specified element
431 * (or equivalently, if this deque changed as a result of the call).
432 *
433 * @param o element to be removed from this deque, if present
434 * @return {@code true} if the deque contained the specified element
435 */
436 public boolean removeFirstOccurrence(Object o) {
437 if (o != null) {
438 final Object[] es = elements;
439 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
440 ; i = 0, to = end) {
441 for (; i < to; i++)
442 if (o.equals(es[i])) {
443 delete(i);
444 return true;
445 }
446 if (to == end) break;
447 }
448 }
449 return false;
450 }
451
452 /**
453 * Removes the last occurrence of the specified element in this
454 * deque (when traversing the deque from head to tail).
455 * If the deque does not contain the element, it is unchanged.
456 * More formally, removes the last element {@code e} such that
457 * {@code o.equals(e)} (if such an element exists).
458 * Returns {@code true} if this deque contained the specified element
459 * (or equivalently, if this deque changed as a result of the call).
460 *
461 * @param o element to be removed from this deque, if present
462 * @return {@code true} if the deque contained the specified element
463 */
464 public boolean removeLastOccurrence(Object o) {
465 if (o != null) {
466 final Object[] es = elements;
467 for (int i = tail, end = head, to = (i >= end) ? end : 0;
468 ; i = es.length, to = end) {
469 for (i--; i > to - 1; i--)
470 if (o.equals(es[i])) {
471 delete(i);
472 return true;
473 }
474 if (to == end) break;
475 }
476 }
477 return false;
478 }
479
480 // *** Queue methods ***
481
482 /**
483 * Inserts the specified element at the end of this deque.
484 *
485 * <p>This method is equivalent to {@link #addLast}.
486 *
487 * @param e the element to add
488 * @return {@code true} (as specified by {@link Collection#add})
489 * @throws NullPointerException if the specified element is null
490 */
491 public boolean add(E e) {
492 addLast(e);
493 return true;
494 }
495
496 /**
497 * Inserts the specified element at the end of this deque.
498 *
499 * <p>This method is equivalent to {@link #offerLast}.
500 *
501 * @param e the element to add
502 * @return {@code true} (as specified by {@link Queue#offer})
503 * @throws NullPointerException if the specified element is null
504 */
505 public boolean offer(E e) {
506 return offerLast(e);
507 }
508
509 /**
510 * Retrieves and removes the head of the queue represented by this deque.
511 *
512 * This method differs from {@link #poll poll} only in that it throws an
513 * exception if this deque is empty.
514 *
515 * <p>This method is equivalent to {@link #removeFirst}.
516 *
517 * @return the head of the queue represented by this deque
518 * @throws NoSuchElementException {@inheritDoc}
519 */
520 public E remove() {
521 return removeFirst();
522 }
523
524 /**
525 * Retrieves and removes the head of the queue represented by this deque
526 * (in other words, the first element of this deque), or returns
527 * {@code null} if this deque is empty.
528 *
529 * <p>This method is equivalent to {@link #pollFirst}.
530 *
531 * @return the head of the queue represented by this deque, or
532 * {@code null} if this deque is empty
533 */
534 public E poll() {
535 return pollFirst();
536 }
537
538 /**
539 * Retrieves, but does not remove, the head of the queue represented by
540 * this deque. This method differs from {@link #peek peek} only in
541 * that it throws an exception if this deque is empty.
542 *
543 * <p>This method is equivalent to {@link #getFirst}.
544 *
545 * @return the head of the queue represented by this deque
546 * @throws NoSuchElementException {@inheritDoc}
547 */
548 public E element() {
549 return getFirst();
550 }
551
552 /**
553 * Retrieves, but does not remove, the head of the queue represented by
554 * this deque, or returns {@code null} if this deque is empty.
555 *
556 * <p>This method is equivalent to {@link #peekFirst}.
557 *
558 * @return the head of the queue represented by this deque, or
559 * {@code null} if this deque is empty
560 */
561 public E peek() {
562 return peekFirst();
563 }
564
565 // *** Stack methods ***
566
567 /**
568 * Pushes an element onto the stack represented by this deque. In other
569 * words, inserts the element at the front of this deque.
570 *
571 * <p>This method is equivalent to {@link #addFirst}.
572 *
573 * @param e the element to push
574 * @throws NullPointerException if the specified element is null
575 */
576 public void push(E e) {
577 addFirst(e);
578 }
579
580 /**
581 * Pops an element from the stack represented by this deque. In other
582 * words, removes and returns the first element of this deque.
583 *
584 * <p>This method is equivalent to {@link #removeFirst()}.
585 *
586 * @return the element at the front of this deque (which is the top
587 * of the stack represented by this deque)
588 * @throws NoSuchElementException {@inheritDoc}
589 */
590 public E pop() {
591 return removeFirst();
592 }
593
594 /**
595 * Removes the element at the specified position in the elements array.
596 * This can result in forward or backwards motion of array elements.
597 * We optimize for least element motion.
598 *
599 * <p>This method is called delete rather than remove to emphasize
600 * that its semantics differ from those of {@link List#remove(int)}.
601 *
602 * @return true if elements near tail moved backwards
603 */
604 boolean delete(int i) {
605 final Object[] es = elements;
606 final int capacity = es.length;
607 final int h, t;
608 // number of elements before to-be-deleted elt
609 final int front = sub(i, h = head, capacity);
610 // number of elements after to-be-deleted elt
611 final int back = sub(t = tail, i, capacity) - 1;
612 if (front < back) {
613 // move front elements forwards
614 if (h <= i) {
615 System.arraycopy(es, h, es, h + 1, front);
616 } else { // Wrap around
617 System.arraycopy(es, 0, es, 1, i);
618 es[0] = es[capacity - 1];
619 System.arraycopy(es, h, es, h + 1, front - (i + 1));
620 }
621 es[h] = null;
622 head = inc(h, capacity);
623 return false;
624 } else {
625 // move back elements backwards
626 tail = dec(t, capacity);
627 if (i <= tail) {
628 System.arraycopy(es, i + 1, es, i, back);
629 } else { // Wrap around
630 System.arraycopy(es, i + 1, es, i, capacity - (i + 1));
631 es[capacity - 1] = es[0];
632 System.arraycopy(es, 1, es, 0, t - 1);
633 }
634 es[tail] = null;
635 return true;
636 }
637 }
638
639 // *** Collection Methods ***
640
641 /**
642 * Returns the number of elements in this deque.
643 *
644 * @return the number of elements in this deque
645 */
646 public int size() {
647 return sub(tail, head, elements.length);
648 }
649
650 /**
651 * Returns {@code true} if this deque contains no elements.
652 *
653 * @return {@code true} if this deque contains no elements
654 */
655 public boolean isEmpty() {
656 return head == tail;
657 }
658
659 /**
660 * Returns an iterator over the elements in this deque. The elements
661 * will be ordered from first (head) to last (tail). This is the same
662 * order that elements would be dequeued (via successive calls to
663 * {@link #remove} or popped (via successive calls to {@link #pop}).
664 *
665 * @return an iterator over the elements in this deque
666 */
667 public Iterator<E> iterator() {
668 return new DeqIterator();
669 }
670
671 public Iterator<E> descendingIterator() {
672 return new DescendingIterator();
673 }
674
675 private class DeqIterator implements Iterator<E> {
676 /** Index of element to be returned by subsequent call to next. */
677 int cursor;
678
679 /** Number of elements yet to be returned. */
680 int remaining = size();
681
682 /**
683 * Index of element returned by most recent call to next.
684 * Reset to -1 if element is deleted by a call to remove.
685 */
686 int lastRet = -1;
687
688 DeqIterator() { cursor = head; }
689
690 public final boolean hasNext() {
691 return remaining > 0;
692 }
693
694 public E next() {
695 if (remaining <= 0)
696 throw new NoSuchElementException();
697 final Object[] es = elements;
698 E e = nonNullElementAt(es, cursor);
699 cursor = inc(lastRet = cursor, es.length);
700 remaining--;
701 return e;
702 }
703
704 void postDelete(boolean leftShifted) {
705 if (leftShifted)
706 cursor = dec(cursor, elements.length);
707 }
708
709 public final void remove() {
710 if (lastRet < 0)
711 throw new IllegalStateException();
712 postDelete(delete(lastRet));
713 lastRet = -1;
714 }
715
716 public void forEachRemaining(Consumer<? super E> action) {
717 Objects.requireNonNull(action);
718 int r;
719 if ((r = remaining) <= 0)
720 return;
721 remaining = 0;
722 final Object[] es = elements;
723 if (es[cursor] == null || sub(tail, cursor, es.length) != r)
724 throw new ConcurrentModificationException();
725 for (int i = cursor, end = tail, to = (i <= end) ? end : es.length;
726 ; i = 0, to = end) {
727 for (; i < to; i++)
728 action.accept(elementAt(es, i));
729 if (to == end) {
730 if (end != tail)
731 throw new ConcurrentModificationException();
732 lastRet = dec(end, es.length);
733 break;
734 }
735 }
736 }
737 }
738
739 private class DescendingIterator extends DeqIterator {
740 DescendingIterator() { cursor = dec(tail, elements.length); }
741
742 public final E next() {
743 if (remaining <= 0)
744 throw new NoSuchElementException();
745 final Object[] es = elements;
746 E e = nonNullElementAt(es, cursor);
747 cursor = dec(lastRet = cursor, es.length);
748 remaining--;
749 return e;
750 }
751
752 void postDelete(boolean leftShifted) {
753 if (!leftShifted)
754 cursor = inc(cursor, elements.length);
755 }
756
757 public final void forEachRemaining(Consumer<? super E> action) {
758 Objects.requireNonNull(action);
759 int r;
760 if ((r = remaining) <= 0)
761 return;
762 remaining = 0;
763 final Object[] es = elements;
764 if (es[cursor] == null || sub(cursor, head, es.length) + 1 != r)
765 throw new ConcurrentModificationException();
766 for (int i = cursor, end = head, to = (i >= end) ? end : 0;
767 ; i = es.length - 1, to = end) {
768 // hotspot generates faster code than for: i >= to !
769 for (; i > to - 1; i--)
770 action.accept(elementAt(es, i));
771 if (to == end) {
772 if (end != head)
773 throw new ConcurrentModificationException();
774 lastRet = end;
775 break;
776 }
777 }
778 }
779 }
780
781 /**
782 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
783 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
784 * deque.
785 *
786 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
787 * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
788 * {@link Spliterator#NONNULL}. Overriding implementations should document
789 * the reporting of additional characteristic values.
790 *
791 * @return a {@code Spliterator} over the elements in this deque
792 * @since 1.8
793 */
794 public Spliterator<E> spliterator() {
795 return new DeqSpliterator();
796 }
797
798 final class DeqSpliterator implements Spliterator<E> {
799 private int fence; // -1 until first use
800 private int cursor; // current index, modified on traverse/split
801
802 /** Constructs late-binding spliterator over all elements. */
803 DeqSpliterator() {
804 this.fence = -1;
805 }
806
807 /** Constructs spliterator over the given range. */
808 DeqSpliterator(int origin, int fence) {
809 // assert 0 <= origin && origin < elements.length;
810 // assert 0 <= fence && fence < elements.length;
811 this.cursor = origin;
812 this.fence = fence;
813 }
814
815 /** Ensures late-binding initialization; then returns fence. */
816 private int getFence() { // force initialization
817 int t;
818 if ((t = fence) < 0) {
819 t = fence = tail;
820 cursor = head;
821 }
822 return t;
823 }
824
825 public DeqSpliterator trySplit() {
826 final Object[] es = elements;
827 final int i, n;
828 return ((n = sub(getFence(), i = cursor, es.length) >> 1) <= 0)
829 ? null
830 : new DeqSpliterator(i, cursor = add(i, n, es.length));
831 }
832
833 public void forEachRemaining(Consumer<? super E> action) {
834 if (action == null)
835 throw new NullPointerException();
836 final int end = getFence(), cursor = this.cursor;
837 final Object[] es = elements;
838 if (cursor != end) {
839 this.cursor = end;
840 // null check at both ends of range is sufficient
841 if (es[cursor] == null || es[dec(end, es.length)] == null)
842 throw new ConcurrentModificationException();
843 for (int i = cursor, to = (i <= end) ? end : es.length;
844 ; i = 0, to = end) {
845 for (; i < to; i++)
846 action.accept(elementAt(es, i));
847 if (to == end) break;
848 }
849 }
850 }
851
852 public boolean tryAdvance(Consumer<? super E> action) {
853 Objects.requireNonNull(action);
854 final Object[] es = elements;
855 if (fence < 0) { fence = tail; cursor = head; } // late-binding
856 final int i;
857 if ((i = cursor) == fence)
858 return false;
859 E e = nonNullElementAt(es, i);
860 cursor = inc(i, es.length);
861 action.accept(e);
862 return true;
863 }
864
865 public long estimateSize() {
866 return sub(getFence(), cursor, elements.length);
867 }
868
869 public int characteristics() {
870 return Spliterator.NONNULL
871 | Spliterator.ORDERED
872 | Spliterator.SIZED
873 | Spliterator.SUBSIZED;
874 }
875 }
876
877 /**
878 * @throws NullPointerException {@inheritDoc}
879 * @since 9
880 */
881 public void forEach(Consumer<? super E> action) {
882 Objects.requireNonNull(action);
883 final Object[] es = elements;
884 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
885 ; i = 0, to = end) {
886 for (; i < to; i++)
887 action.accept(elementAt(es, i));
888 if (to == end) {
889 if (end != tail) throw new ConcurrentModificationException();
890 break;
891 }
892 }
893 }
894
895 /**
896 * @throws NullPointerException {@inheritDoc}
897 * @since 9
898 */
899 public boolean removeIf(Predicate<? super E> filter) {
900 Objects.requireNonNull(filter);
901 return bulkRemove(filter);
902 }
903
904 /**
905 * @throws NullPointerException {@inheritDoc}
906 * @since 9
907 */
908 public boolean removeAll(Collection<?> c) {
909 Objects.requireNonNull(c);
910 return bulkRemove(e -> c.contains(e));
911 }
912
913 /**
914 * @throws NullPointerException {@inheritDoc}
915 * @since 9
916 */
917 public boolean retainAll(Collection<?> c) {
918 Objects.requireNonNull(c);
919 return bulkRemove(e -> !c.contains(e));
920 }
921
922 /** Implementation of bulk remove methods. */
923 private boolean bulkRemove(Predicate<? super E> filter) {
924 final Object[] es = elements;
925 // Optimize for initial run of survivors
926 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
927 ; i = 0, to = end) {
928 for (; i < to; i++)
929 if (filter.test(elementAt(es, i)))
930 return bulkRemoveModified(filter, i);
931 if (to == end) {
932 if (end != tail) throw new ConcurrentModificationException();
933 break;
934 }
935 }
936 return false;
937 }
938
939 // A tiny bit set implementation
940
941 private static long[] nBits(int n) {
942 return new long[((n - 1) >> 6) + 1];
943 }
944 private static void setBit(long[] bits, int i) {
945 bits[i >> 6] |= 1L << i;
946 }
947 private static boolean isClear(long[] bits, int i) {
948 return (bits[i >> 6] & (1L << i)) == 0;
949 }
950
951 /**
952 * Helper for bulkRemove, in case of at least one deletion.
953 * Tolerate predicates that reentrantly access the collection for
954 * read (but writers still get CME), so traverse once to find
955 * elements to delete, a second pass to physically expunge.
956 *
957 * @param beg valid index of first element to be deleted
958 */
959 private boolean bulkRemoveModified(
960 Predicate<? super E> filter, final int beg) {
961 final Object[] es = elements;
962 final int capacity = es.length;
963 final int end = tail;
964 final long[] deathRow = nBits(sub(end, beg, capacity));
965 deathRow[0] = 1L; // set bit 0
966 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
967 ; i = 0, to = end, k -= capacity) {
968 for (; i < to; i++)
969 if (filter.test(elementAt(es, i)))
970 setBit(deathRow, i - k);
971 if (to == end) break;
972 }
973 // a two-finger traversal, with hare i reading, tortoise w writing
974 int w = beg;
975 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
976 ; w = 0) { // w rejoins i on second leg
977 // In this loop, i and w are on the same leg, with i > w
978 for (; i < to; i++)
979 if (isClear(deathRow, i - k))
980 es[w++] = es[i];
981 if (to == end) break;
982 // In this loop, w is on the first leg, i on the second
983 for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++)
984 if (isClear(deathRow, i - k))
985 es[w++] = es[i];
986 if (i >= to) {
987 if (w == capacity) w = 0; // "corner" case
988 break;
989 }
990 }
991 if (end != tail) throw new ConcurrentModificationException();
992 circularClear(es, tail = w, end);
993 return true;
994 }
995
996 /**
997 * Returns {@code true} if this deque contains the specified element.
998 * More formally, returns {@code true} if and only if this deque contains
999 * at least one element {@code e} such that {@code o.equals(e)}.
1000 *
1001 * @param o object to be checked for containment in this deque
1002 * @return {@code true} if this deque contains the specified element
1003 */
1004 public boolean contains(Object o) {
1005 if (o != null) {
1006 final Object[] es = elements;
1007 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1008 ; i = 0, to = end) {
1009 for (; i < to; i++)
1010 if (o.equals(es[i]))
1011 return true;
1012 if (to == end) break;
1013 }
1014 }
1015 return false;
1016 }
1017
1018 /**
1019 * Removes a single instance of the specified element from this deque.
1020 * If the deque does not contain the element, it is unchanged.
1021 * More formally, removes the first element {@code e} such that
1022 * {@code o.equals(e)} (if such an element exists).
1023 * Returns {@code true} if this deque contained the specified element
1024 * (or equivalently, if this deque changed as a result of the call).
1025 *
1026 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
1027 *
1028 * @param o element to be removed from this deque, if present
1029 * @return {@code true} if this deque contained the specified element
1030 */
1031 public boolean remove(Object o) {
1032 return removeFirstOccurrence(o);
1033 }
1034
1035 /**
1036 * Removes all of the elements from this deque.
1037 * The deque will be empty after this call returns.
1038 */
1039 public void clear() {
1040 circularClear(elements, head, tail);
1041 head = tail = 0;
1042 }
1043
1044 /**
1045 * Nulls out slots starting at array index i, upto index end.
1046 * Condition i == end means "empty" - nothing to do.
1047 */
1048 private static void circularClear(Object[] es, int i, int end) {
1049 // assert 0 <= i && i < es.length;
1050 // assert 0 <= end && end < es.length;
1051 for (int to = (i <= end) ? end : es.length;
1052 ; i = 0, to = end) {
1053 for (; i < to; i++) es[i] = null;
1054 if (to == end) break;
1055 }
1056 }
1057
1058 /**
1059 * Returns an array containing all of the elements in this deque
1060 * in proper sequence (from first to last element).
1061 *
1062 * <p>The returned array will be "safe" in that no references to it are
1063 * maintained by this deque. (In other words, this method must allocate
1064 * a new array). The caller is thus free to modify the returned array.
1065 *
1066 * <p>This method acts as bridge between array-based and collection-based
1067 * APIs.
1068 *
1069 * @return an array containing all of the elements in this deque
1070 */
1071 public Object[] toArray() {
1072 return toArray(Object[].class);
1073 }
1074
1075 private <T> T[] toArray(Class<T[]> klazz) {
1076 final Object[] es = elements;
1077 final T[] a;
1078 final int head = this.head, tail = this.tail, end;
1079 if ((end = tail + ((head <= tail) ? 0 : es.length)) >= 0) {
1080 // Uses null extension feature of copyOfRange
1081 a = Arrays.copyOfRange(es, head, end, klazz);
1082 } else {
1083 // integer overflow!
1084 a = Arrays.copyOfRange(es, 0, end - head, klazz);
1085 System.arraycopy(es, head, a, 0, es.length - head);
1086 }
1087 if (end != tail)
1088 System.arraycopy(es, 0, a, es.length - head, tail);
1089 return a;
1090 }
1091
1092 /**
1093 * Returns an array containing all of the elements in this deque in
1094 * proper sequence (from first to last element); the runtime type of the
1095 * returned array is that of the specified array. If the deque fits in
1096 * the specified array, it is returned therein. Otherwise, a new array
1097 * is allocated with the runtime type of the specified array and the
1098 * size of this deque.
1099 *
1100 * <p>If this deque fits in the specified array with room to spare
1101 * (i.e., the array has more elements than this deque), the element in
1102 * the array immediately following the end of the deque is set to
1103 * {@code null}.
1104 *
1105 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1106 * array-based and collection-based APIs. Further, this method allows
1107 * precise control over the runtime type of the output array, and may,
1108 * under certain circumstances, be used to save allocation costs.
1109 *
1110 * <p>Suppose {@code x} is a deque known to contain only strings.
1111 * The following code can be used to dump the deque into a newly
1112 * allocated array of {@code String}:
1113 *
1114 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
1115 *
1116 * Note that {@code toArray(new Object[0])} is identical in function to
1117 * {@code toArray()}.
1118 *
1119 * @param a the array into which the elements of the deque are to
1120 * be stored, if it is big enough; otherwise, a new array of the
1121 * same runtime type is allocated for this purpose
1122 * @return an array containing all of the elements in this deque
1123 * @throws ArrayStoreException if the runtime type of the specified array
1124 * is not a supertype of the runtime type of every element in
1125 * this deque
1126 * @throws NullPointerException if the specified array is null
1127 */
1128 @SuppressWarnings("unchecked")
1129 public <T> T[] toArray(T[] a) {
1130 final int size;
1131 if ((size = size()) > a.length)
1132 return toArray((Class<T[]>) a.getClass());
1133 final Object[] es = elements;
1134 for (int i = head, j = 0, len = Math.min(size, es.length - i);
1135 ; i = 0, len = tail) {
1136 System.arraycopy(es, i, a, j, len);
1137 if ((j += len) == size) break;
1138 }
1139 if (size < a.length)
1140 a[size] = null;
1141 return a;
1142 }
1143
1144 // *** Object methods ***
1145
1146 /**
1147 * Returns a copy of this deque.
1148 *
1149 * @return a copy of this deque
1150 */
1151 public ArrayDeque<E> clone() {
1152 try {
1153 @SuppressWarnings("unchecked")
1154 ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
1155 result.elements = Arrays.copyOf(elements, elements.length);
1156 return result;
1157 } catch (CloneNotSupportedException e) {
1158 throw new AssertionError();
1159 }
1160 }
1161
1162 private static final long serialVersionUID = 2340985798034038923L;
1163
1164 /**
1165 * Saves this deque to a stream (that is, serializes it).
1166 *
1167 * @param s the stream
1168 * @throws java.io.IOException if an I/O error occurs
1169 * @serialData The current size ({@code int}) of the deque,
1170 * followed by all of its elements (each an object reference) in
1171 * first-to-last order.
1172 */
1173 private void writeObject(java.io.ObjectOutputStream s)
1174 throws java.io.IOException {
1175 s.defaultWriteObject();
1176
1177 // Write out size
1178 s.writeInt(size());
1179
1180 // Write out elements in order.
1181 final Object[] es = elements;
1182 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1183 ; i = 0, to = end) {
1184 for (; i < to; i++)
1185 s.writeObject(es[i]);
1186 if (to == end) break;
1187 }
1188 }
1189
1190 /**
1191 * Reconstitutes this deque from a stream (that is, deserializes it).
1192 * @param s the stream
1193 * @throws ClassNotFoundException if the class of a serialized object
1194 * could not be found
1195 * @throws java.io.IOException if an I/O error occurs
1196 */
1197 private void readObject(java.io.ObjectInputStream s)
1198 throws java.io.IOException, ClassNotFoundException {
1199 s.defaultReadObject();
1200
1201 // Read in size and allocate array
1202 int size = s.readInt();
1203 elements = new Object[size + 1];
1204 this.tail = size;
1205
1206 // Read in all elements in the proper order.
1207 for (int i = 0; i < size; i++)
1208 elements[i] = s.readObject();
1209 }
1210
1211 /** debugging */
1212 void checkInvariants() {
1213 // Use head and tail fields with empty slot at tail strategy.
1214 // head == tail disambiguates to "empty".
1215 try {
1216 int capacity = elements.length;
1217 // assert 0 <= head && head < capacity;
1218 // assert 0 <= tail && tail < capacity;
1219 // assert capacity > 0;
1220 // assert size() < capacity;
1221 // assert head == tail || elements[head] != null;
1222 // assert elements[tail] == null;
1223 // assert head == tail || elements[dec(tail, capacity)] != null;
1224 } catch (Throwable t) {
1225 System.err.printf("head=%d tail=%d capacity=%d%n",
1226 head, tail, elements.length);
1227 System.err.printf("elements=%s%n",
1228 Arrays.toString(elements));
1229 throw t;
1230 }
1231 }
1232
1233 }