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 import jdk.internal.misc.SharedSecrets;
42
43 /**
44 * Resizable-array implementation of the {@link Deque} interface. Array
45 * deques have no capacity restrictions; they grow as necessary to support
46 * usage. They are not thread-safe; in the absence of external
47 * synchronization, they do not support concurrent access by multiple threads.
48 * Null elements are prohibited. This class is likely to be faster than
49 * {@link Stack} when used as a stack, and faster than {@link LinkedList}
50 * when used as a queue.
51 *
52 * <p>Most {@code ArrayDeque} operations run in amortized constant time.
53 * Exceptions include
54 * {@link #remove(Object) remove},
55 * {@link #removeFirstOccurrence removeFirstOccurrence},
56 * {@link #removeLastOccurrence removeLastOccurrence},
57 * {@link #contains contains},
58 * {@link #iterator iterator.remove()},
59 * and the bulk operations, all of which run in linear time.
60 *
61 * <p>The iterators returned by this class's {@link #iterator() iterator}
62 * method are <em>fail-fast</em>: If the deque is modified at any time after
63 * the iterator is created, in any way except through the iterator's own
64 * {@code remove} method, the iterator will generally throw a {@link
65 * ConcurrentModificationException}. Thus, in the face of concurrent
66 * modification, the iterator fails quickly and cleanly, rather than risking
67 * arbitrary, non-deterministic behavior at an undetermined time in the
68 * future.
69 *
70 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
71 * as it is, generally speaking, impossible to make any hard guarantees in the
72 * presence of unsynchronized concurrent modification. Fail-fast iterators
73 * throw {@code ConcurrentModificationException} on a best-effort basis.
74 * Therefore, it would be wrong to write a program that depended on this
75 * exception for its correctness: <i>the fail-fast behavior of iterators
76 * should be used only to detect bugs.</i>
77 *
78 * <p>This class and its iterator implement all of the
79 * <em>optional</em> methods of the {@link Collection} and {@link
80 * Iterator} interfaces.
81 *
82 * <p>This class is a member of the
83 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
84 * Java Collections Framework</a>.
85 *
86 * @author Josh Bloch and Doug Lea
87 * @param <E> the type of elements held in this deque
88 * @since 1.6
89 */
90 public class ArrayDeque<E> extends AbstractCollection<E>
91 implements Deque<E>, Cloneable, Serializable
92 {
93 /*
94 * VMs excel at optimizing simple array loops where indices are
95 * incrementing or decrementing over a valid slice, e.g.
96 *
97 * for (int i = start; i < end; i++) ... elements[i]
98 *
99 * Because in a circular array, elements are in general stored in
100 * two disjoint such slices, we help the VM by writing unusual
101 * nested loops for all traversals over the elements. Having only
102 * one hot inner loop body instead of two or three eases human
103 * maintenance and encourages VM loop inlining into the caller.
104 */
105
106 /**
107 * The array in which the elements of the deque are stored.
108 * All array cells not holding deque elements are always null.
109 * The array always has at least one null slot (at tail).
110 */
111 transient Object[] elements;
112
113 /**
114 * The index of the element at the head of the deque (which is the
115 * element that would be removed by remove() or pop()); or an
116 * arbitrary number 0 <= head < elements.length equal to tail if
117 * the deque is empty.
118 */
119 transient int head;
120
121 /**
122 * The index at which the next element would be added to the tail
123 * of the deque (via addLast(E), add(E), or push(E));
124 * elements[tail] is always null.
125 */
126 transient int tail;
127
128 /**
129 * The maximum size of array to allocate.
130 * Some VMs reserve some header words in an array.
131 * Attempts to allocate larger arrays may result in
132 * OutOfMemoryError: Requested array size exceeds VM limit
133 */
134 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
135
136 /**
137 * Increases the capacity of this deque by at least the given amount.
138 *
139 * @param needed the required minimum extra capacity; must be positive
140 */
141 private void grow(int needed) {
142 // overflow-conscious code
143 final int oldCapacity = elements.length;
144 int newCapacity;
145 // Double capacity if small; else grow by 50%
146 int jump = (oldCapacity < 64) ? (oldCapacity + 2) : (oldCapacity >> 1);
147 if (jump < needed
148 || (newCapacity = (oldCapacity + jump)) - MAX_ARRAY_SIZE > 0)
149 newCapacity = newCapacity(needed, jump);
150 final Object[] es = elements = Arrays.copyOf(elements, newCapacity);
151 // Exceptionally, here tail == head needs to be disambiguated
152 if (tail < head || (tail == head && es[head] != null)) {
153 // wrap around; slide first leg forward to end of array
154 int newSpace = newCapacity - oldCapacity;
155 System.arraycopy(es, head,
156 es, head + newSpace,
157 oldCapacity - head);
158 for (int i = head, to = (head += newSpace); i < to; i++)
159 es[i] = null;
160 }
161 }
162
163 /** Capacity calculation for edge conditions, especially overflow. */
164 private int newCapacity(int needed, int jump) {
165 final int oldCapacity = elements.length, minCapacity;
166 if ((minCapacity = oldCapacity + needed) - MAX_ARRAY_SIZE > 0) {
167 if (minCapacity < 0)
168 throw new IllegalStateException("Sorry, deque too big");
169 return Integer.MAX_VALUE;
170 }
171 if (needed > jump)
172 return minCapacity;
173 return (oldCapacity + jump - MAX_ARRAY_SIZE < 0)
174 ? oldCapacity + jump
175 : MAX_ARRAY_SIZE;
176 }
177
178 /**
179 * Constructs an empty array deque with an initial capacity
180 * sufficient to hold 16 elements.
181 */
182 public ArrayDeque() {
183 elements = new Object[16];
184 }
185
186 /**
187 * Constructs an empty array deque with an initial capacity
188 * sufficient to hold the specified number of elements.
189 *
190 * @param numElements lower bound on initial capacity of the deque
191 */
192 public ArrayDeque(int numElements) {
193 elements =
194 new Object[(numElements < 1) ? 1 :
195 (numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE :
196 numElements + 1];
197 }
198
199 /**
200 * Constructs a deque containing the elements of the specified
201 * collection, in the order they are returned by the collection's
202 * iterator. (The first element returned by the collection's
203 * iterator becomes the first element, or <i>front</i> of the
204 * deque.)
205 *
206 * @param c the collection whose elements are to be placed into the deque
207 * @throws NullPointerException if the specified collection is null
208 */
209 public ArrayDeque(Collection<? extends E> c) {
210 this(c.size());
211 copyElements(c);
212 }
213
214 /**
215 * Circularly increments i, mod modulus.
216 * Precondition and postcondition: 0 <= i < modulus.
217 */
218 static final int inc(int i, int modulus) {
219 if (++i >= modulus) i = 0;
220 return i;
221 }
222
223 /**
224 * Circularly decrements i, mod modulus.
225 * Precondition and postcondition: 0 <= i < modulus.
226 */
227 static final int dec(int i, int modulus) {
228 if (--i < 0) i = modulus - 1;
229 return i;
230 }
231
232 /**
233 * Circularly adds the given distance to index i, mod modulus.
234 * Precondition: 0 <= i < modulus, 0 <= distance <= modulus.
235 * @return index 0 <= i < modulus
236 */
237 static final int inc(int i, int distance, int modulus) {
238 if ((i += distance) - modulus >= 0) i -= modulus;
239 return i;
240 }
241
242 /**
243 * Subtracts j from i, mod modulus.
244 * Index i must be logically ahead of index j.
245 * Precondition: 0 <= i < modulus, 0 <= j < modulus.
246 * @return the "circular distance" from j to i; corner case i == j
247 * is disambiguated to "empty", returning 0.
248 */
249 static final int sub(int i, int j, int modulus) {
250 if ((i -= j) < 0) i += modulus;
251 return i;
252 }
253
254 /**
255 * Returns element at array index i.
256 * This is a slight abuse of generics, accepted by javac.
257 */
258 @SuppressWarnings("unchecked")
259 static final <E> E elementAt(Object[] es, int i) {
260 return (E) es[i];
261 }
262
263 /**
264 * A version of elementAt that checks for null elements.
265 * This check doesn't catch all possible comodifications,
266 * but does catch ones that corrupt traversal.
267 */
268 static final <E> E nonNullElementAt(Object[] es, int i) {
269 @SuppressWarnings("unchecked") E e = (E) es[i];
270 if (e == null)
271 throw new ConcurrentModificationException();
272 return e;
273 }
274
275 // The main insertion and extraction methods are addFirst,
276 // addLast, pollFirst, pollLast. The other methods are defined in
277 // terms of these.
278
279 /**
280 * Inserts the specified element at the front of this deque.
281 *
282 * @param e the element to add
283 * @throws NullPointerException if the specified element is null
284 */
285 public void addFirst(E e) {
286 if (e == null)
287 throw new NullPointerException();
288 final Object[] es = elements;
289 es[head = dec(head, es.length)] = e;
290 if (head == tail)
291 grow(1);
292 }
293
294 /**
295 * Inserts the specified element at the end of this deque.
296 *
297 * <p>This method is equivalent to {@link #add}.
298 *
299 * @param e the element to add
300 * @throws NullPointerException if the specified element is null
301 */
302 public void addLast(E e) {
303 if (e == null)
304 throw new NullPointerException();
305 final Object[] es = elements;
306 es[tail] = e;
307 if (head == (tail = inc(tail, es.length)))
308 grow(1);
309 }
310
311 /**
312 * Adds all of the elements in the specified collection at the end
313 * of this deque, as if by calling {@link #addLast} on each one,
314 * in the order that they are returned by the collection's 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 copyElements(c);
326 return size() > s;
327 }
328
329 private void copyElements(Collection<? extends E> c) {
330 c.forEach(this::addLast);
331 }
332
333 /**
334 * Inserts the specified element at the front of this deque.
335 *
336 * @param e the element to add
337 * @return {@code true} (as specified by {@link Deque#offerFirst})
338 * @throws NullPointerException if the specified element is null
339 */
340 public boolean offerFirst(E e) {
341 addFirst(e);
342 return true;
343 }
344
345 /**
346 * Inserts the specified element at the end of this deque.
347 *
348 * @param e the element to add
349 * @return {@code true} (as specified by {@link Deque#offerLast})
350 * @throws NullPointerException if the specified element is null
351 */
352 public boolean offerLast(E e) {
353 addLast(e);
354 return true;
355 }
356
357 /**
358 * @throws NoSuchElementException {@inheritDoc}
359 */
360 public E removeFirst() {
361 E e = pollFirst();
362 if (e == null)
363 throw new NoSuchElementException();
364 return e;
365 }
366
367 /**
368 * @throws NoSuchElementException {@inheritDoc}
369 */
370 public E removeLast() {
371 E e = pollLast();
372 if (e == null)
373 throw new NoSuchElementException();
374 return e;
375 }
376
377 public E pollFirst() {
378 final Object[] es;
379 final int h;
380 E e = elementAt(es = elements, h = head);
381 if (e != null) {
382 es[h] = null;
383 head = inc(h, es.length);
384 }
385 return e;
386 }
387
388 public E pollLast() {
389 final Object[] es;
390 final int t;
391 E e = elementAt(es = elements, t = dec(tail, es.length));
392 if (e != null)
393 es[tail = t] = null;
394 return e;
395 }
396
397 /**
398 * @throws NoSuchElementException {@inheritDoc}
399 */
400 public E getFirst() {
401 E e = elementAt(elements, head);
402 if (e == null)
403 throw new NoSuchElementException();
404 return e;
405 }
406
407 /**
408 * @throws NoSuchElementException {@inheritDoc}
409 */
410 public E getLast() {
411 final Object[] es = elements;
412 E e = elementAt(es, dec(tail, es.length));
413 if (e == null)
414 throw new NoSuchElementException();
415 return e;
416 }
417
418 public E peekFirst() {
419 return elementAt(elements, head);
420 }
421
422 public E peekLast() {
423 final Object[] es;
424 return elementAt(es = elements, dec(tail, es.length));
425 }
426
427 /**
428 * Removes the first occurrence of the specified element in this
429 * deque (when traversing the deque from head to tail).
430 * If the deque does not contain the element, it is unchanged.
431 * More formally, removes the first element {@code e} such that
432 * {@code o.equals(e)} (if such an element exists).
433 * Returns {@code true} if this deque contained the specified element
434 * (or equivalently, if this deque changed as a result of the call).
435 *
436 * @param o element to be removed from this deque, if present
437 * @return {@code true} if the deque contained the specified element
438 */
439 public boolean removeFirstOccurrence(Object o) {
440 if (o != null) {
441 final Object[] es = elements;
442 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
443 ; i = 0, to = end) {
444 for (; i < to; i++)
445 if (o.equals(es[i])) {
446 delete(i);
447 return true;
448 }
449 if (to == end) break;
450 }
451 }
452 return false;
453 }
454
455 /**
456 * Removes the last occurrence of the specified element in this
457 * deque (when traversing the deque from head to tail).
458 * If the deque does not contain the element, it is unchanged.
459 * More formally, removes the last element {@code e} such that
460 * {@code o.equals(e)} (if such an element exists).
461 * Returns {@code true} if this deque contained the specified element
462 * (or equivalently, if this deque changed as a result of the call).
463 *
464 * @param o element to be removed from this deque, if present
465 * @return {@code true} if the deque contained the specified element
466 */
467 public boolean removeLastOccurrence(Object o) {
468 if (o != null) {
469 final Object[] es = elements;
470 for (int i = tail, end = head, to = (i >= end) ? end : 0;
471 ; i = es.length, to = end) {
472 for (i--; i > to - 1; i--)
473 if (o.equals(es[i])) {
474 delete(i);
475 return true;
476 }
477 if (to == end) break;
478 }
479 }
480 return false;
481 }
482
483 // *** Queue methods ***
484
485 /**
486 * Inserts the specified element at the end of this deque.
487 *
488 * <p>This method is equivalent to {@link #addLast}.
489 *
490 * @param e the element to add
491 * @return {@code true} (as specified by {@link Collection#add})
492 * @throws NullPointerException if the specified element is null
493 */
494 public boolean add(E e) {
495 addLast(e);
496 return true;
497 }
498
499 /**
500 * Inserts the specified element at the end of this deque.
501 *
502 * <p>This method is equivalent to {@link #offerLast}.
503 *
504 * @param e the element to add
505 * @return {@code true} (as specified by {@link Queue#offer})
506 * @throws NullPointerException if the specified element is null
507 */
508 public boolean offer(E e) {
509 return offerLast(e);
510 }
511
512 /**
513 * Retrieves and removes the head of the queue represented by this deque.
514 *
515 * This method differs from {@link #poll() poll()} only in that it
516 * throws an exception if this deque is empty.
517 *
518 * <p>This method is equivalent to {@link #removeFirst}.
519 *
520 * @return the head of the queue represented by this deque
521 * @throws NoSuchElementException {@inheritDoc}
522 */
523 public E remove() {
524 return removeFirst();
525 }
526
527 /**
528 * Retrieves and removes the head of the queue represented by this deque
529 * (in other words, the first element of this deque), or returns
530 * {@code null} if this deque is empty.
531 *
532 * <p>This method is equivalent to {@link #pollFirst}.
533 *
534 * @return the head of the queue represented by this deque, or
535 * {@code null} if this deque is empty
536 */
537 public E poll() {
538 return pollFirst();
539 }
540
541 /**
542 * Retrieves, but does not remove, the head of the queue represented by
543 * this deque. This method differs from {@link #peek peek} only in
544 * that it throws an exception if this deque is empty.
545 *
546 * <p>This method is equivalent to {@link #getFirst}.
547 *
548 * @return the head of the queue represented by this deque
549 * @throws NoSuchElementException {@inheritDoc}
550 */
551 public E element() {
552 return getFirst();
553 }
554
555 /**
556 * Retrieves, but does not remove, the head of the queue represented by
557 * this deque, or returns {@code null} if this deque is empty.
558 *
559 * <p>This method is equivalent to {@link #peekFirst}.
560 *
561 * @return the head of the queue represented by this deque, or
562 * {@code null} if this deque is empty
563 */
564 public E peek() {
565 return peekFirst();
566 }
567
568 // *** Stack methods ***
569
570 /**
571 * Pushes an element onto the stack represented by this deque. In other
572 * words, inserts the element at the front of this deque.
573 *
574 * <p>This method is equivalent to {@link #addFirst}.
575 *
576 * @param e the element to push
577 * @throws NullPointerException if the specified element is null
578 */
579 public void push(E e) {
580 addFirst(e);
581 }
582
583 /**
584 * Pops an element from the stack represented by this deque. In other
585 * words, removes and returns the first element of this deque.
586 *
587 * <p>This method is equivalent to {@link #removeFirst()}.
588 *
589 * @return the element at the front of this deque (which is the top
590 * of the stack represented by this deque)
591 * @throws NoSuchElementException {@inheritDoc}
592 */
593 public E pop() {
594 return removeFirst();
595 }
596
597 /**
598 * Removes the element at the specified position in the elements array.
599 * This can result in forward or backwards motion of array elements.
600 * We optimize for least element motion.
601 *
602 * <p>This method is called delete rather than remove to emphasize
603 * that its semantics differ from those of {@link List#remove(int)}.
604 *
605 * @return true if elements near tail moved backwards
606 */
607 boolean delete(int i) {
608 final Object[] es = elements;
609 final int capacity = es.length;
610 final int h, t;
611 // number of elements before to-be-deleted elt
612 final int front = sub(i, h = head, capacity);
613 // number of elements after to-be-deleted elt
614 final int back = sub(t = tail, i, capacity) - 1;
615 if (front < back) {
616 // move front elements forwards
617 if (h <= i) {
618 System.arraycopy(es, h, es, h + 1, front);
619 } else { // Wrap around
620 System.arraycopy(es, 0, es, 1, i);
621 es[0] = es[capacity - 1];
622 System.arraycopy(es, h, es, h + 1, front - (i + 1));
623 }
624 es[h] = null;
625 head = inc(h, capacity);
626 return false;
627 } else {
628 // move back elements backwards
629 tail = dec(t, capacity);
630 if (i <= tail) {
631 System.arraycopy(es, i + 1, es, i, back);
632 } else { // Wrap around
633 System.arraycopy(es, i + 1, es, i, capacity - (i + 1));
634 es[capacity - 1] = es[0];
635 System.arraycopy(es, 1, es, 0, t - 1);
636 }
637 es[tail] = null;
638 return true;
639 }
640 }
641
642 // *** Collection Methods ***
643
644 /**
645 * Returns the number of elements in this deque.
646 *
647 * @return the number of elements in this deque
648 */
649 public int size() {
650 return sub(tail, head, elements.length);
651 }
652
653 /**
654 * Returns {@code true} if this deque contains no elements.
655 *
656 * @return {@code true} if this deque contains no elements
657 */
658 public boolean isEmpty() {
659 return head == tail;
660 }
661
662 /**
663 * Returns an iterator over the elements in this deque. The elements
664 * will be ordered from first (head) to last (tail). This is the same
665 * order that elements would be dequeued (via successive calls to
666 * {@link #remove} or popped (via successive calls to {@link #pop}).
667 *
668 * @return an iterator over the elements in this deque
669 */
670 public Iterator<E> iterator() {
671 return new DeqIterator();
672 }
673
674 public Iterator<E> descendingIterator() {
675 return new DescendingIterator();
676 }
677
678 private class DeqIterator implements Iterator<E> {
679 /** Index of element to be returned by subsequent call to next. */
680 int cursor;
681
682 /** Number of elements yet to be returned. */
683 int remaining = size();
684
685 /**
686 * Index of element returned by most recent call to next.
687 * Reset to -1 if element is deleted by a call to remove.
688 */
689 int lastRet = -1;
690
691 DeqIterator() { cursor = head; }
692
693 public final boolean hasNext() {
694 return remaining > 0;
695 }
696
697 public E next() {
698 if (remaining <= 0)
699 throw new NoSuchElementException();
700 final Object[] es = elements;
701 E e = nonNullElementAt(es, cursor);
702 cursor = inc(lastRet = cursor, es.length);
703 remaining--;
704 return e;
705 }
706
707 void postDelete(boolean leftShifted) {
708 if (leftShifted)
709 cursor = dec(cursor, elements.length);
710 }
711
712 public final void remove() {
713 if (lastRet < 0)
714 throw new IllegalStateException();
715 postDelete(delete(lastRet));
716 lastRet = -1;
717 }
718
719 public void forEachRemaining(Consumer<? super E> action) {
720 Objects.requireNonNull(action);
721 int r;
722 if ((r = remaining) <= 0)
723 return;
724 remaining = 0;
725 final Object[] es = elements;
726 if (es[cursor] == null || sub(tail, cursor, es.length) != r)
727 throw new ConcurrentModificationException();
728 for (int i = cursor, end = tail, to = (i <= end) ? end : es.length;
729 ; i = 0, to = end) {
730 for (; i < to; i++)
731 action.accept(elementAt(es, i));
732 if (to == end) {
733 if (end != tail)
734 throw new ConcurrentModificationException();
735 lastRet = dec(end, es.length);
736 break;
737 }
738 }
739 }
740 }
741
742 private class DescendingIterator extends DeqIterator {
743 DescendingIterator() { cursor = dec(tail, elements.length); }
744
745 public final E next() {
746 if (remaining <= 0)
747 throw new NoSuchElementException();
748 final Object[] es = elements;
749 E e = nonNullElementAt(es, cursor);
750 cursor = dec(lastRet = cursor, es.length);
751 remaining--;
752 return e;
753 }
754
755 void postDelete(boolean leftShifted) {
756 if (!leftShifted)
757 cursor = inc(cursor, elements.length);
758 }
759
760 public final void forEachRemaining(Consumer<? super E> action) {
761 Objects.requireNonNull(action);
762 int r;
763 if ((r = remaining) <= 0)
764 return;
765 remaining = 0;
766 final Object[] es = elements;
767 if (es[cursor] == null || sub(cursor, head, es.length) + 1 != r)
768 throw new ConcurrentModificationException();
769 for (int i = cursor, end = head, to = (i >= end) ? end : 0;
770 ; i = es.length - 1, to = end) {
771 // hotspot generates faster code than for: i >= to !
772 for (; i > to - 1; i--)
773 action.accept(elementAt(es, i));
774 if (to == end) {
775 if (end != head)
776 throw new ConcurrentModificationException();
777 lastRet = end;
778 break;
779 }
780 }
781 }
782 }
783
784 /**
785 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
786 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
787 * deque.
788 *
789 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
790 * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
791 * {@link Spliterator#NONNULL}. Overriding implementations should document
792 * the reporting of additional characteristic values.
793 *
794 * @return a {@code Spliterator} over the elements in this deque
795 * @since 1.8
796 */
797 public Spliterator<E> spliterator() {
798 return new DeqSpliterator();
799 }
800
801 final class DeqSpliterator implements Spliterator<E> {
802 private int fence; // -1 until first use
803 private int cursor; // current index, modified on traverse/split
804
805 /** Constructs late-binding spliterator over all elements. */
806 DeqSpliterator() {
807 this.fence = -1;
808 }
809
810 /** Constructs spliterator over the given range. */
811 DeqSpliterator(int origin, int fence) {
812 // assert 0 <= origin && origin < elements.length;
813 // assert 0 <= fence && fence < elements.length;
814 this.cursor = origin;
815 this.fence = fence;
816 }
817
818 /** Ensures late-binding initialization; then returns fence. */
819 private int getFence() { // force initialization
820 int t;
821 if ((t = fence) < 0) {
822 t = fence = tail;
823 cursor = head;
824 }
825 return t;
826 }
827
828 public DeqSpliterator trySplit() {
829 final Object[] es = elements;
830 final int i, n;
831 return ((n = sub(getFence(), i = cursor, es.length) >> 1) <= 0)
832 ? null
833 : new DeqSpliterator(i, cursor = inc(i, n, es.length));
834 }
835
836 public void forEachRemaining(Consumer<? super E> action) {
837 if (action == null)
838 throw new NullPointerException();
839 final int end = getFence(), cursor = this.cursor;
840 final Object[] es = elements;
841 if (cursor != end) {
842 this.cursor = end;
843 // null check at both ends of range is sufficient
844 if (es[cursor] == null || es[dec(end, es.length)] == null)
845 throw new ConcurrentModificationException();
846 for (int i = cursor, to = (i <= end) ? end : es.length;
847 ; i = 0, to = end) {
848 for (; i < to; i++)
849 action.accept(elementAt(es, i));
850 if (to == end) break;
851 }
852 }
853 }
854
855 public boolean tryAdvance(Consumer<? super E> action) {
856 Objects.requireNonNull(action);
857 final Object[] es = elements;
858 if (fence < 0) { fence = tail; cursor = head; } // late-binding
859 final int i;
860 if ((i = cursor) == fence)
861 return false;
862 E e = nonNullElementAt(es, i);
863 cursor = inc(i, es.length);
864 action.accept(e);
865 return true;
866 }
867
868 public long estimateSize() {
869 return sub(getFence(), cursor, elements.length);
870 }
871
872 public int characteristics() {
873 return Spliterator.NONNULL
874 | Spliterator.ORDERED
875 | Spliterator.SIZED
876 | Spliterator.SUBSIZED;
877 }
878 }
879
880 /**
881 * @throws NullPointerException {@inheritDoc}
882 */
883 public void forEach(Consumer<? super E> action) {
884 Objects.requireNonNull(action);
885 final Object[] es = elements;
886 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
887 ; i = 0, to = end) {
888 for (; i < to; i++)
889 action.accept(elementAt(es, i));
890 if (to == end) {
891 if (end != tail) throw new ConcurrentModificationException();
892 break;
893 }
894 }
895 }
896
897 /**
898 * @throws NullPointerException {@inheritDoc}
899 */
900 public boolean removeIf(Predicate<? super E> filter) {
901 Objects.requireNonNull(filter);
902 return bulkRemove(filter);
903 }
904
905 /**
906 * @throws NullPointerException {@inheritDoc}
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 */
916 public boolean retainAll(Collection<?> c) {
917 Objects.requireNonNull(c);
918 return bulkRemove(e -> !c.contains(e));
919 }
920
921 /** Implementation of bulk remove methods. */
922 private boolean bulkRemove(Predicate<? super E> filter) {
923 final Object[] es = elements;
924 // Optimize for initial run of survivors
925 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
926 ; i = 0, to = end) {
927 for (; i < to; i++)
928 if (filter.test(elementAt(es, i)))
929 return bulkRemoveModified(filter, i);
930 if (to == end) {
931 if (end != tail) throw new ConcurrentModificationException();
932 break;
933 }
934 }
935 return false;
936 }
937
938 // A tiny bit set implementation
939
940 private static long[] nBits(int n) {
941 return new long[((n - 1) >> 6) + 1];
942 }
943 private static void setBit(long[] bits, int i) {
944 bits[i >> 6] |= 1L << i;
945 }
946 private static boolean isClear(long[] bits, int i) {
947 return (bits[i >> 6] & (1L << i)) == 0;
948 }
949
950 /**
951 * Helper for bulkRemove, in case of at least one deletion.
952 * Tolerate predicates that reentrantly access the collection for
953 * read (but writers still get CME), so traverse once to find
954 * elements to delete, a second pass to physically expunge.
955 *
956 * @param beg valid index of first element to be deleted
957 */
958 private boolean bulkRemoveModified(
959 Predicate<? super E> filter, final int beg) {
960 final Object[] es = elements;
961 final int capacity = es.length;
962 final int end = tail;
963 final long[] deathRow = nBits(sub(end, beg, capacity));
964 deathRow[0] = 1L; // set bit 0
965 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
966 ; i = 0, to = end, k -= capacity) {
967 for (; i < to; i++)
968 if (filter.test(elementAt(es, i)))
969 setBit(deathRow, i - k);
970 if (to == end) break;
971 }
972 // a two-finger traversal, with hare i reading, tortoise w writing
973 int w = beg;
974 for (int i = beg + 1, to = (i <= end) ? end : es.length, k = beg;
975 ; w = 0) { // w rejoins i on second leg
976 // In this loop, i and w are on the same leg, with i > w
977 for (; i < to; i++)
978 if (isClear(deathRow, i - k))
979 es[w++] = es[i];
980 if (to == end) break;
981 // In this loop, w is on the first leg, i on the second
982 for (i = 0, to = end, k -= capacity; i < to && w < capacity; i++)
983 if (isClear(deathRow, i - k))
984 es[w++] = es[i];
985 if (i >= to) {
986 if (w == capacity) w = 0; // "corner" case
987 break;
988 }
989 }
990 if (end != tail) throw new ConcurrentModificationException();
991 circularClear(es, tail = w, end);
992 return true;
993 }
994
995 /**
996 * Returns {@code true} if this deque contains the specified element.
997 * More formally, returns {@code true} if and only if this deque contains
998 * at least one element {@code e} such that {@code o.equals(e)}.
999 *
1000 * @param o object to be checked for containment in this deque
1001 * @return {@code true} if this deque contains the specified element
1002 */
1003 public boolean contains(Object o) {
1004 if (o != null) {
1005 final Object[] es = elements;
1006 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1007 ; i = 0, to = end) {
1008 for (; i < to; i++)
1009 if (o.equals(es[i]))
1010 return true;
1011 if (to == end) break;
1012 }
1013 }
1014 return false;
1015 }
1016
1017 /**
1018 * Removes a single instance of the specified element from this deque.
1019 * If the deque does not contain the element, it is unchanged.
1020 * More formally, removes the first element {@code e} such that
1021 * {@code o.equals(e)} (if such an element exists).
1022 * Returns {@code true} if this deque contained the specified element
1023 * (or equivalently, if this deque changed as a result of the call).
1024 *
1025 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
1026 *
1027 * @param o element to be removed from this deque, if present
1028 * @return {@code true} if this deque contained the specified element
1029 */
1030 public boolean remove(Object o) {
1031 return removeFirstOccurrence(o);
1032 }
1033
1034 /**
1035 * Removes all of the elements from this deque.
1036 * The deque will be empty after this call returns.
1037 */
1038 public void clear() {
1039 circularClear(elements, head, tail);
1040 head = tail = 0;
1041 }
1042
1043 /**
1044 * Nulls out slots starting at array index i, upto index end.
1045 * Condition i == end means "empty" - nothing to do.
1046 */
1047 private static void circularClear(Object[] es, int i, int end) {
1048 // assert 0 <= i && i < es.length;
1049 // assert 0 <= end && end < es.length;
1050 for (int to = (i <= end) ? end : es.length;
1051 ; i = 0, to = end) {
1052 for (; i < to; i++) es[i] = null;
1053 if (to == end) break;
1054 }
1055 }
1056
1057 /**
1058 * Returns an array containing all of the elements in this deque
1059 * in proper sequence (from first to last element).
1060 *
1061 * <p>The returned array will be "safe" in that no references to it are
1062 * maintained by this deque. (In other words, this method must allocate
1063 * a new array). The caller is thus free to modify the returned array.
1064 *
1065 * <p>This method acts as bridge between array-based and collection-based
1066 * APIs.
1067 *
1068 * @return an array containing all of the elements in this deque
1069 */
1070 public Object[] toArray() {
1071 return toArray(Object[].class);
1072 }
1073
1074 private <T> T[] toArray(Class<T[]> klazz) {
1075 final Object[] es = elements;
1076 final T[] a;
1077 final int head = this.head, tail = this.tail, end;
1078 if ((end = tail + ((head <= tail) ? 0 : es.length)) >= 0) {
1079 // Uses null extension feature of copyOfRange
1080 a = Arrays.copyOfRange(es, head, end, klazz);
1081 } else {
1082 // integer overflow!
1083 a = Arrays.copyOfRange(es, 0, end - head, klazz);
1084 System.arraycopy(es, head, a, 0, es.length - head);
1085 }
1086 if (end != tail)
1087 System.arraycopy(es, 0, a, es.length - head, tail);
1088 return a;
1089 }
1090
1091 /**
1092 * Returns an array containing all of the elements in this deque in
1093 * proper sequence (from first to last element); the runtime type of the
1094 * returned array is that of the specified array. If the deque fits in
1095 * the specified array, it is returned therein. Otherwise, a new array
1096 * is allocated with the runtime type of the specified array and the
1097 * size of this deque.
1098 *
1099 * <p>If this deque fits in the specified array with room to spare
1100 * (i.e., the array has more elements than this deque), the element in
1101 * the array immediately following the end of the deque is set to
1102 * {@code null}.
1103 *
1104 * <p>Like the {@link #toArray()} method, this method acts as bridge between
1105 * array-based and collection-based APIs. Further, this method allows
1106 * precise control over the runtime type of the output array, and may,
1107 * under certain circumstances, be used to save allocation costs.
1108 *
1109 * <p>Suppose {@code x} is a deque known to contain only strings.
1110 * The following code can be used to dump the deque into a newly
1111 * allocated array of {@code String}:
1112 *
1113 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
1114 *
1115 * Note that {@code toArray(new Object[0])} is identical in function to
1116 * {@code toArray()}.
1117 *
1118 * @param a the array into which the elements of the deque are to
1119 * be stored, if it is big enough; otherwise, a new array of the
1120 * same runtime type is allocated for this purpose
1121 * @return an array containing all of the elements in this deque
1122 * @throws ArrayStoreException if the runtime type of the specified array
1123 * is not a supertype of the runtime type of every element in
1124 * this deque
1125 * @throws NullPointerException if the specified array is null
1126 */
1127 @SuppressWarnings("unchecked")
1128 public <T> T[] toArray(T[] a) {
1129 final int size;
1130 if ((size = size()) > a.length)
1131 return toArray((Class<T[]>) a.getClass());
1132 final Object[] es = elements;
1133 for (int i = head, j = 0, len = Math.min(size, es.length - i);
1134 ; i = 0, len = tail) {
1135 System.arraycopy(es, i, a, j, len);
1136 if ((j += len) == size) break;
1137 }
1138 if (size < a.length)
1139 a[size] = null;
1140 return a;
1141 }
1142
1143 // *** Object methods ***
1144
1145 /**
1146 * Returns a copy of this deque.
1147 *
1148 * @return a copy of this deque
1149 */
1150 public ArrayDeque<E> clone() {
1151 try {
1152 @SuppressWarnings("unchecked")
1153 ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
1154 result.elements = Arrays.copyOf(elements, elements.length);
1155 return result;
1156 } catch (CloneNotSupportedException e) {
1157 throw new AssertionError();
1158 }
1159 }
1160
1161 private static final long serialVersionUID = 2340985798034038923L;
1162
1163 /**
1164 * Saves this deque to a stream (that is, serializes it).
1165 *
1166 * @param s the stream
1167 * @throws java.io.IOException if an I/O error occurs
1168 * @serialData The current size ({@code int}) of the deque,
1169 * followed by all of its elements (each an object reference) in
1170 * first-to-last order.
1171 */
1172 private void writeObject(java.io.ObjectOutputStream s)
1173 throws java.io.IOException {
1174 s.defaultWriteObject();
1175
1176 // Write out size
1177 s.writeInt(size());
1178
1179 // Write out elements in order.
1180 final Object[] es = elements;
1181 for (int i = head, end = tail, to = (i <= end) ? end : es.length;
1182 ; i = 0, to = end) {
1183 for (; i < to; i++)
1184 s.writeObject(es[i]);
1185 if (to == end) break;
1186 }
1187 }
1188
1189 /**
1190 * Reconstitutes this deque from a stream (that is, deserializes it).
1191 * @param s the stream
1192 * @throws ClassNotFoundException if the class of a serialized object
1193 * could not be found
1194 * @throws java.io.IOException if an I/O error occurs
1195 */
1196 private void readObject(java.io.ObjectInputStream s)
1197 throws java.io.IOException, ClassNotFoundException {
1198 s.defaultReadObject();
1199
1200 // Read in size and allocate array
1201 int size = s.readInt();
1202 SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size + 1);
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 }
1234