1 /* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25 /* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 30 * 31 * Written by Josh Bloch of Google Inc. and released to the public domain, 32 * as explained at http://creativecommons.org/publicdomain/zero/1.0/. 33 */ 34 35 package java.util; 36 import java.io.*; 37 38 /** 39 * Resizable-array implementation of the {@link Deque} interface. Array 40 * deques have no capacity restrictions; they grow as necessary to support 41 * usage. They are not thread-safe; in the absence of external 42 * synchronization, they do not support concurrent access by multiple threads. 43 * Null elements are prohibited. This class is likely to be faster than 44 * {@link Stack} when used as a stack, and faster than {@link LinkedList} 45 * when used as a queue. 46 * 47 * <p>Most <tt>ArrayDeque</tt> operations run in amortized constant time. 48 * Exceptions include {@link #remove(Object) remove}, {@link 49 * #removeFirstOccurrence removeFirstOccurrence}, {@link #removeLastOccurrence 50 * removeLastOccurrence}, {@link #contains contains}, {@link #iterator 51 * iterator.remove()}, and the bulk operations, all of which run in linear 52 * time. 53 * 54 * <p>The iterators returned by this class's <tt>iterator</tt> method are 55 * <i>fail-fast</i>: If the deque is modified at any time after the iterator 56 * is created, in any way except through the iterator's own <tt>remove</tt> 57 * method, the iterator will generally throw a {@link 58 * ConcurrentModificationException}. Thus, in the face of concurrent 59 * modification, the iterator fails quickly and cleanly, rather than risking 60 * arbitrary, non-deterministic behavior at an undetermined time in the 61 * future. 62 * 63 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed 64 * as it is, generally speaking, impossible to make any hard guarantees in the 65 * presence of unsynchronized concurrent modification. Fail-fast iterators 66 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. 67 * Therefore, it would be wrong to write a program that depended on this 68 * exception for its correctness: <i>the fail-fast behavior of iterators 69 * should be used only to detect bugs.</i> 70 * 71 * <p>This class and its iterator implement all of the 72 * <em>optional</em> methods of the {@link Collection} and {@link 73 * Iterator} interfaces. 74 * 75 * <p>This class is a member of the 76 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 77 * Java Collections Framework</a>. 78 * 79 * @author Josh Bloch and Doug Lea 80 * @since 1.6 81 * @param <E> the type of elements held in this collection 82 */ 83 public class ArrayDeque<E> extends AbstractCollection<E> 84 implements Deque<E>, Cloneable, Serializable 85 { 86 /** 87 * The array in which the elements of the deque are stored. 88 * The capacity of the deque is the length of this array, which is 89 * always a power of two. The array is never allowed to become 90 * full, except transiently within an addX method where it is 91 * resized (see doubleCapacity) immediately upon becoming full, 92 * thus avoiding head and tail wrapping around to equal each 93 * other. We also guarantee that all array cells not holding 94 * deque elements are always null. 95 */ 96 private transient E[] elements; 97 98 /** 99 * The index of the element at the head of the deque (which is the 100 * element that would be removed by remove() or pop()); or an 101 * arbitrary number equal to tail if the deque is empty. 102 */ 103 private transient int head; 104 105 /** 106 * The index at which the next element would be added to the tail 107 * of the deque (via addLast(E), add(E), or push(E)). 108 */ 109 private transient int tail; 110 111 /** 112 * The minimum capacity that we'll use for a newly created deque. 113 * Must be a power of 2. 114 */ 115 private static final int MIN_INITIAL_CAPACITY = 8; 116 117 // ****** Array allocation and resizing utilities ****** 118 119 /** 120 * Allocate empty array to hold the given number of elements. 121 * 122 * @param numElements the number of elements to hold 123 */ 124 @SuppressWarnings("unchecked") 125 private void allocateElements(int numElements) { 126 int initialCapacity = MIN_INITIAL_CAPACITY; 127 // Find the best power of two to hold elements. 128 // Tests "<=" because arrays aren't kept full. 129 if (numElements >= initialCapacity) { 130 initialCapacity = numElements; 131 initialCapacity |= (initialCapacity >>> 1); 132 initialCapacity |= (initialCapacity >>> 2); 133 initialCapacity |= (initialCapacity >>> 4); 134 initialCapacity |= (initialCapacity >>> 8); 135 initialCapacity |= (initialCapacity >>> 16); 136 initialCapacity++; 137 138 if (initialCapacity < 0) // Too many elements, must back off 139 initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements 140 } 141 elements = (E[]) new Object[initialCapacity]; 142 } 143 144 /** 145 * Double the capacity of this deque. Call only when full, i.e., 146 * when head and tail have wrapped around to become equal. 147 */ 148 private void doubleCapacity() { 149 assert head == tail; 150 int p = head; 151 int n = elements.length; 152 int r = n - p; // number of elements to the right of p 153 int newCapacity = n << 1; 154 if (newCapacity < 0) 155 throw new IllegalStateException("Sorry, deque too big"); 156 @SuppressWarnings("unchecked") 157 E[] a = (E[]) new Object[newCapacity]; 158 System.arraycopy(elements, p, a, 0, r); 159 System.arraycopy(elements, 0, a, r, p); 160 elements = a; 161 head = 0; 162 tail = n; 163 } 164 165 /** 166 * Copies the elements from our element array into the specified array, 167 * in order (from first to last element in the deque). It is assumed 168 * that the array is large enough to hold all elements in the deque. 169 * 170 * @return its argument 171 */ 172 private <T> T[] copyElements(T[] a) { 173 if (head < tail) { 174 System.arraycopy(elements, head, a, 0, size()); 175 } else if (head > tail) { 176 int headPortionLen = elements.length - head; 177 System.arraycopy(elements, head, a, 0, headPortionLen); 178 System.arraycopy(elements, 0, a, headPortionLen, tail); 179 } 180 return a; 181 } 182 183 /** 184 * Constructs an empty array deque with an initial capacity 185 * sufficient to hold 16 elements. 186 */ 187 @SuppressWarnings("unchecked") 188 public ArrayDeque() { 189 elements = (E[]) new Object[16]; 190 } 191 192 /** 193 * Constructs an empty array deque with an initial capacity 194 * sufficient to hold the specified number of elements. 195 * 196 * @param numElements lower bound on initial capacity of the deque 197 */ 198 public ArrayDeque(int numElements) { 199 allocateElements(numElements); 200 } 201 202 /** 203 * Constructs a deque containing the elements of the specified 204 * collection, in the order they are returned by the collection's 205 * iterator. (The first element returned by the collection's 206 * iterator becomes the first element, or <i>front</i> of the 207 * deque.) 208 * 209 * @param c the collection whose elements are to be placed into the deque 210 * @throws NullPointerException if the specified collection is null 211 */ 212 public ArrayDeque(Collection<? extends E> c) { 213 allocateElements(c.size()); 214 addAll(c); 215 } 216 217 // The main insertion and extraction methods are addFirst, 218 // addLast, pollFirst, pollLast. The other methods are defined in 219 // terms of these. 220 221 /** 222 * Inserts the specified element at the front of this deque. 223 * 224 * @param e the element to add 225 * @throws NullPointerException if the specified element is null 226 */ 227 public void addFirst(E e) { 228 if (e == null) 229 throw new NullPointerException(); 230 elements[head = (head - 1) & (elements.length - 1)] = e; 231 if (head == tail) 232 doubleCapacity(); 233 } 234 235 /** 236 * Inserts the specified element at the end of this deque. 237 * 238 * <p>This method is equivalent to {@link #add}. 239 * 240 * @param e the element to add 241 * @throws NullPointerException if the specified element is null 242 */ 243 public void addLast(E e) { 244 if (e == null) 245 throw new NullPointerException(); 246 elements[tail] = e; 247 if ( (tail = (tail + 1) & (elements.length - 1)) == head) 248 doubleCapacity(); 249 } 250 251 /** 252 * Inserts the specified element at the front of this deque. 253 * 254 * @param e the element to add 255 * @return <tt>true</tt> (as specified by {@link Deque#offerFirst}) 256 * @throws NullPointerException if the specified element is null 257 */ 258 public boolean offerFirst(E e) { 259 addFirst(e); 260 return true; 261 } 262 263 /** 264 * Inserts the specified element at the end of this deque. 265 * 266 * @param e the element to add 267 * @return <tt>true</tt> (as specified by {@link Deque#offerLast}) 268 * @throws NullPointerException if the specified element is null 269 */ 270 public boolean offerLast(E e) { 271 addLast(e); 272 return true; 273 } 274 275 /** 276 * @throws NoSuchElementException {@inheritDoc} 277 */ 278 public E removeFirst() { 279 E x = pollFirst(); 280 if (x == null) 281 throw new NoSuchElementException(); 282 return x; 283 } 284 285 /** 286 * @throws NoSuchElementException {@inheritDoc} 287 */ 288 public E removeLast() { 289 E x = pollLast(); 290 if (x == null) 291 throw new NoSuchElementException(); 292 return x; 293 } 294 295 public E pollFirst() { 296 int h = head; 297 E result = elements[h]; // Element is null if deque empty 298 if (result == null) 299 return null; 300 elements[h] = null; // Must null out slot 301 head = (h + 1) & (elements.length - 1); 302 return result; 303 } 304 305 public E pollLast() { 306 int t = (tail - 1) & (elements.length - 1); 307 E result = elements[t]; 308 if (result == null) 309 return null; 310 elements[t] = null; 311 tail = t; 312 return result; 313 } 314 315 /** 316 * @throws NoSuchElementException {@inheritDoc} 317 */ 318 public E getFirst() { 319 E x = elements[head]; 320 if (x == null) 321 throw new NoSuchElementException(); 322 return x; 323 } 324 325 /** 326 * @throws NoSuchElementException {@inheritDoc} 327 */ 328 public E getLast() { 329 E x = elements[(tail - 1) & (elements.length - 1)]; 330 if (x == null) 331 throw new NoSuchElementException(); 332 return x; 333 } 334 335 public E peekFirst() { 336 return elements[head]; // elements[head] is null if deque empty 337 } 338 339 public E peekLast() { 340 return elements[(tail - 1) & (elements.length - 1)]; 341 } 342 343 /** 344 * Removes the first occurrence of the specified element in this 345 * deque (when traversing the deque from head to tail). 346 * If the deque does not contain the element, it is unchanged. 347 * More formally, removes the first element <tt>e</tt> such that 348 * <tt>o.equals(e)</tt> (if such an element exists). 349 * Returns <tt>true</tt> if this deque contained the specified element 350 * (or equivalently, if this deque changed as a result of the call). 351 * 352 * @param o element to be removed from this deque, if present 353 * @return <tt>true</tt> if the deque contained the specified element 354 */ 355 public boolean removeFirstOccurrence(Object o) { 356 if (o == null) 357 return false; 358 int mask = elements.length - 1; 359 int i = head; 360 E x; 361 while ( (x = elements[i]) != null) { 362 if (o.equals(x)) { 363 delete(i); 364 return true; 365 } 366 i = (i + 1) & mask; 367 } 368 return false; 369 } 370 371 /** 372 * Removes the last occurrence of the specified element in this 373 * deque (when traversing the deque from head to tail). 374 * If the deque does not contain the element, it is unchanged. 375 * More formally, removes the last element <tt>e</tt> such that 376 * <tt>o.equals(e)</tt> (if such an element exists). 377 * Returns <tt>true</tt> if this deque contained the specified element 378 * (or equivalently, if this deque changed as a result of the call). 379 * 380 * @param o element to be removed from this deque, if present 381 * @return <tt>true</tt> if the deque contained the specified element 382 */ 383 public boolean removeLastOccurrence(Object o) { 384 if (o == null) 385 return false; 386 int mask = elements.length - 1; 387 int i = (tail - 1) & mask; 388 E x; 389 while ( (x = elements[i]) != null) { 390 if (o.equals(x)) { 391 delete(i); 392 return true; 393 } 394 i = (i - 1) & mask; 395 } 396 return false; 397 } 398 399 // *** Queue methods *** 400 401 /** 402 * Inserts the specified element at the end of this deque. 403 * 404 * <p>This method is equivalent to {@link #addLast}. 405 * 406 * @param e the element to add 407 * @return <tt>true</tt> (as specified by {@link Collection#add}) 408 * @throws NullPointerException if the specified element is null 409 */ 410 public boolean add(E e) { 411 addLast(e); 412 return true; 413 } 414 415 /** 416 * Inserts the specified element at the end of this deque. 417 * 418 * <p>This method is equivalent to {@link #offerLast}. 419 * 420 * @param e the element to add 421 * @return <tt>true</tt> (as specified by {@link Queue#offer}) 422 * @throws NullPointerException if the specified element is null 423 */ 424 public boolean offer(E e) { 425 return offerLast(e); 426 } 427 428 /** 429 * Retrieves and removes the head of the queue represented by this deque. 430 * 431 * This method differs from {@link #poll poll} only in that it throws an 432 * exception if this deque is empty. 433 * 434 * <p>This method is equivalent to {@link #removeFirst}. 435 * 436 * @return the head of the queue represented by this deque 437 * @throws NoSuchElementException {@inheritDoc} 438 */ 439 public E remove() { 440 return removeFirst(); 441 } 442 443 /** 444 * Retrieves and removes the head of the queue represented by this deque 445 * (in other words, the first element of this deque), or returns 446 * <tt>null</tt> if this deque is empty. 447 * 448 * <p>This method is equivalent to {@link #pollFirst}. 449 * 450 * @return the head of the queue represented by this deque, or 451 * <tt>null</tt> if this deque is empty 452 */ 453 public E poll() { 454 return pollFirst(); 455 } 456 457 /** 458 * Retrieves, but does not remove, the head of the queue represented by 459 * this deque. This method differs from {@link #peek peek} only in 460 * that it throws an exception if this deque is empty. 461 * 462 * <p>This method is equivalent to {@link #getFirst}. 463 * 464 * @return the head of the queue represented by this deque 465 * @throws NoSuchElementException {@inheritDoc} 466 */ 467 public E element() { 468 return getFirst(); 469 } 470 471 /** 472 * Retrieves, but does not remove, the head of the queue represented by 473 * this deque, or returns <tt>null</tt> if this deque is empty. 474 * 475 * <p>This method is equivalent to {@link #peekFirst}. 476 * 477 * @return the head of the queue represented by this deque, or 478 * <tt>null</tt> if this deque is empty 479 */ 480 public E peek() { 481 return peekFirst(); 482 } 483 484 // *** Stack methods *** 485 486 /** 487 * Pushes an element onto the stack represented by this deque. In other 488 * words, inserts the element at the front of this deque. 489 * 490 * <p>This method is equivalent to {@link #addFirst}. 491 * 492 * @param e the element to push 493 * @throws NullPointerException if the specified element is null 494 */ 495 public void push(E e) { 496 addFirst(e); 497 } 498 499 /** 500 * Pops an element from the stack represented by this deque. In other 501 * words, removes and returns the first element of this deque. 502 * 503 * <p>This method is equivalent to {@link #removeFirst()}. 504 * 505 * @return the element at the front of this deque (which is the top 506 * of the stack represented by this deque) 507 * @throws NoSuchElementException {@inheritDoc} 508 */ 509 public E pop() { 510 return removeFirst(); 511 } 512 513 private void checkInvariants() { 514 assert elements[tail] == null; 515 assert head == tail ? elements[head] == null : 516 (elements[head] != null && 517 elements[(tail - 1) & (elements.length - 1)] != null); 518 assert elements[(head - 1) & (elements.length - 1)] == null; 519 } 520 521 /** 522 * Removes the element at the specified position in the elements array, 523 * adjusting head and tail as necessary. This can result in motion of 524 * elements backwards or forwards in the array. 525 * 526 * <p>This method is called delete rather than remove to emphasize 527 * that its semantics differ from those of {@link List#remove(int)}. 528 * 529 * @return true if elements moved backwards 530 */ 531 private boolean delete(int i) { 532 checkInvariants(); 533 final E[] elements = this.elements; 534 final int mask = elements.length - 1; 535 final int h = head; 536 final int t = tail; 537 final int front = (i - h) & mask; 538 final int back = (t - i) & mask; 539 540 // Invariant: head <= i < tail mod circularity 541 if (front >= ((t - h) & mask)) 542 throw new ConcurrentModificationException(); 543 544 // Optimize for least element motion 545 if (front < back) { 546 if (h <= i) { 547 System.arraycopy(elements, h, elements, h + 1, front); 548 } else { // Wrap around 549 System.arraycopy(elements, 0, elements, 1, i); 550 elements[0] = elements[mask]; 551 System.arraycopy(elements, h, elements, h + 1, mask - h); 552 } 553 elements[h] = null; 554 head = (h + 1) & mask; 555 return false; 556 } else { 557 if (i < t) { // Copy the null tail as well 558 System.arraycopy(elements, i + 1, elements, i, back); 559 tail = t - 1; 560 } else { // Wrap around 561 System.arraycopy(elements, i + 1, elements, i, mask - i); 562 elements[mask] = elements[0]; 563 System.arraycopy(elements, 1, elements, 0, t); 564 tail = (t - 1) & mask; 565 } 566 return true; 567 } 568 } 569 570 // *** Collection Methods *** 571 572 /** 573 * Returns the number of elements in this deque. 574 * 575 * @return the number of elements in this deque 576 */ 577 public int size() { 578 return (tail - head) & (elements.length - 1); 579 } 580 581 /** 582 * Returns <tt>true</tt> if this deque contains no elements. 583 * 584 * @return <tt>true</tt> if this deque contains no elements 585 */ 586 public boolean isEmpty() { 587 return head == tail; 588 } 589 590 /** 591 * Returns an iterator over the elements in this deque. The elements 592 * will be ordered from first (head) to last (tail). This is the same 593 * order that elements would be dequeued (via successive calls to 594 * {@link #remove} or popped (via successive calls to {@link #pop}). 595 * 596 * @return an iterator over the elements in this deque 597 */ 598 public Iterator<E> iterator() { 599 return new DeqIterator(); 600 } 601 602 public Iterator<E> descendingIterator() { 603 return new DescendingIterator(); 604 } 605 606 private class DeqIterator implements Iterator<E> { 607 /** 608 * Index of element to be returned by subsequent call to next. 609 */ 610 private int cursor = head; 611 612 /** 613 * Tail recorded at construction (also in remove), to stop 614 * iterator and also to check for comodification. 615 */ 616 private int fence = tail; 617 618 /** 619 * Index of element returned by most recent call to next. 620 * Reset to -1 if element is deleted by a call to remove. 621 */ 622 private int lastRet = -1; 623 624 public boolean hasNext() { 625 return cursor != fence; 626 } 627 628 public E next() { 629 if (cursor == fence) 630 throw new NoSuchElementException(); 631 E result = elements[cursor]; 632 // This check doesn't catch all possible comodifications, 633 // but does catch the ones that corrupt traversal 634 if (tail != fence || result == null) 635 throw new ConcurrentModificationException(); 636 lastRet = cursor; 637 cursor = (cursor + 1) & (elements.length - 1); 638 return result; 639 } 640 641 public void remove() { 642 if (lastRet < 0) 643 throw new IllegalStateException(); 644 if (delete(lastRet)) { // if left-shifted, undo increment in next() 645 cursor = (cursor - 1) & (elements.length - 1); 646 fence = tail; 647 } 648 lastRet = -1; 649 } 650 } 651 652 private class DescendingIterator implements Iterator<E> { 653 /* 654 * This class is nearly a mirror-image of DeqIterator, using 655 * tail instead of head for initial cursor, and head instead of 656 * tail for fence. 657 */ 658 private int cursor = tail; 659 private int fence = head; 660 private int lastRet = -1; 661 662 public boolean hasNext() { 663 return cursor != fence; 664 } 665 666 public E next() { 667 if (cursor == fence) 668 throw new NoSuchElementException(); 669 cursor = (cursor - 1) & (elements.length - 1); 670 E result = elements[cursor]; 671 if (head != fence || result == null) 672 throw new ConcurrentModificationException(); 673 lastRet = cursor; 674 return result; 675 } 676 677 public void remove() { 678 if (lastRet < 0) 679 throw new IllegalStateException(); 680 if (!delete(lastRet)) { 681 cursor = (cursor + 1) & (elements.length - 1); 682 fence = head; 683 } 684 lastRet = -1; 685 } 686 } 687 688 /** 689 * Returns <tt>true</tt> if this deque contains the specified element. 690 * More formally, returns <tt>true</tt> if and only if this deque contains 691 * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>. 692 * 693 * @param o object to be checked for containment in this deque 694 * @return <tt>true</tt> if this deque contains the specified element 695 */ 696 public boolean contains(Object o) { 697 if (o == null) 698 return false; 699 int mask = elements.length - 1; 700 int i = head; 701 E x; 702 while ( (x = elements[i]) != null) { 703 if (o.equals(x)) 704 return true; 705 i = (i + 1) & mask; 706 } 707 return false; 708 } 709 710 /** 711 * Removes a single instance of the specified element from this deque. 712 * If the deque does not contain the element, it is unchanged. 713 * More formally, removes the first element <tt>e</tt> such that 714 * <tt>o.equals(e)</tt> (if such an element exists). 715 * Returns <tt>true</tt> if this deque contained the specified element 716 * (or equivalently, if this deque changed as a result of the call). 717 * 718 * <p>This method is equivalent to {@link #removeFirstOccurrence}. 719 * 720 * @param o element to be removed from this deque, if present 721 * @return <tt>true</tt> if this deque contained the specified element 722 */ 723 public boolean remove(Object o) { 724 return removeFirstOccurrence(o); 725 } 726 727 /** 728 * Removes all of the elements from this deque. 729 * The deque will be empty after this call returns. 730 */ 731 public void clear() { 732 int h = head; 733 int t = tail; 734 if (h != t) { // clear all cells 735 head = tail = 0; 736 int i = h; 737 int mask = elements.length - 1; 738 do { 739 elements[i] = null; 740 i = (i + 1) & mask; 741 } while (i != t); 742 } 743 } 744 745 /** 746 * Returns an array containing all of the elements in this deque 747 * in proper sequence (from first to last element). 748 * 749 * <p>The returned array will be "safe" in that no references to it are 750 * maintained by this deque. (In other words, this method must allocate 751 * a new array). The caller is thus free to modify the returned array. 752 * 753 * <p>This method acts as bridge between array-based and collection-based 754 * APIs. 755 * 756 * @return an array containing all of the elements in this deque 757 */ 758 public Object[] toArray() { 759 return copyElements(new Object[size()]); 760 } 761 762 /** 763 * Returns an array containing all of the elements in this deque in 764 * proper sequence (from first to last element); the runtime type of the 765 * returned array is that of the specified array. If the deque fits in 766 * the specified array, it is returned therein. Otherwise, a new array 767 * is allocated with the runtime type of the specified array and the 768 * size of this deque. 769 * 770 * <p>If this deque fits in the specified array with room to spare 771 * (i.e., the array has more elements than this deque), the element in 772 * the array immediately following the end of the deque is set to 773 * <tt>null</tt>. 774 * 775 * <p>Like the {@link #toArray()} method, this method acts as bridge between 776 * array-based and collection-based APIs. Further, this method allows 777 * precise control over the runtime type of the output array, and may, 778 * under certain circumstances, be used to save allocation costs. 779 * 780 * <p>Suppose <tt>x</tt> is a deque known to contain only strings. 781 * The following code can be used to dump the deque into a newly 782 * allocated array of <tt>String</tt>: 783 * 784 * <pre> 785 * String[] y = x.toArray(new String[0]);</pre> 786 * 787 * Note that <tt>toArray(new Object[0])</tt> is identical in function to 788 * <tt>toArray()</tt>. 789 * 790 * @param a the array into which the elements of the deque are to 791 * be stored, if it is big enough; otherwise, a new array of the 792 * same runtime type is allocated for this purpose 793 * @return an array containing all of the elements in this deque 794 * @throws ArrayStoreException if the runtime type of the specified array 795 * is not a supertype of the runtime type of every element in 796 * this deque 797 * @throws NullPointerException if the specified array is null 798 */ 799 @SuppressWarnings("unchecked") 800 public <T> T[] toArray(T[] a) { 801 int size = size(); 802 if (a.length < size) 803 a = (T[])java.lang.reflect.Array.newInstance( 804 a.getClass().getComponentType(), size); 805 copyElements(a); 806 if (a.length > size) 807 a[size] = null; 808 return a; 809 } 810 811 // *** Object methods *** 812 813 /** 814 * Returns a copy of this deque. 815 * 816 * @return a copy of this deque 817 */ 818 public ArrayDeque<E> clone() { 819 try { 820 @SuppressWarnings("unchecked") 821 ArrayDeque<E> result = (ArrayDeque<E>) super.clone(); 822 result.elements = Arrays.copyOf(elements, elements.length); 823 return result; 824 825 } catch (CloneNotSupportedException e) { 826 throw new AssertionError(); 827 } 828 } 829 830 /** 831 * Appease the serialization gods. 832 */ 833 private static final long serialVersionUID = 2340985798034038923L; 834 835 /** 836 * Serialize this deque. 837 * 838 * @serialData The current size (<tt>int</tt>) of the deque, 839 * followed by all of its elements (each an object reference) in 840 * first-to-last order. 841 */ 842 private void writeObject(ObjectOutputStream s) throws IOException { 843 s.defaultWriteObject(); 844 845 // Write out size 846 s.writeInt(size()); 847 848 // Write out elements in order. 849 int mask = elements.length - 1; 850 for (int i = head; i != tail; i = (i + 1) & mask) 851 s.writeObject(elements[i]); 852 } 853 854 /** 855 * Deserialize this deque. 856 */ 857 @SuppressWarnings("unchecked") 858 private void readObject(ObjectInputStream s) 859 throws IOException, ClassNotFoundException { 860 s.defaultReadObject(); 861 862 // Read in size and allocate array 863 int size = s.readInt(); 864 allocateElements(size); 865 head = 0; 866 tail = size; 867 868 // Read in all elements in the proper order. 869 for (int i = 0; i < size; i++) 870 elements[i] = (E)s.readObject(); 871 } 872 }