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 * Written by Doug Lea and Josh Bloch with assistance from members of 31 * JCP JSR-166 Expert Group and released to the public domain, as explained 32 * at http://creativecommons.org/publicdomain/zero/1.0/ 33 */ 34 35 package java.util; 36 37 // BEGIN android-note 38 // removed link to collections framework docs 39 // END android-note 40 41 /** 42 * A linear collection that supports element insertion and removal at 43 * both ends. The name <i>deque</i> is short for "double ended queue" 44 * and is usually pronounced "deck". Most {@code Deque} 45 * implementations place no fixed limits on the number of elements 46 * they may contain, but this interface supports capacity-restricted 47 * deques as well as those with no fixed size limit. 48 * 49 * <p>This interface defines methods to access the elements at both 50 * ends of the deque. Methods are provided to insert, remove, and 51 * examine the element. Each of these methods exists in two forms: 52 * one throws an exception if the operation fails, the other returns a 53 * special value (either {@code null} or {@code false}, depending on 54 * the operation). The latter form of the insert operation is 55 * designed specifically for use with capacity-restricted 56 * {@code Deque} implementations; in most implementations, insert 57 * operations cannot fail. 58 * 59 * <p>The twelve methods described above are summarized in the 60 * following table: 61 * 62 * <table BORDER CELLPADDING=3 CELLSPACING=1> 63 * <caption>Summary of Deque methods</caption> 64 * <tr> 65 * <td></td> 66 * <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td> 67 * <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td> 68 * </tr> 69 * <tr> 70 * <td></td> 71 * <td ALIGN=CENTER><em>Throws exception</em></td> 72 * <td ALIGN=CENTER><em>Special value</em></td> 73 * <td ALIGN=CENTER><em>Throws exception</em></td> 74 * <td ALIGN=CENTER><em>Special value</em></td> 75 * </tr> 76 * <tr> 77 * <td><b>Insert</b></td> 78 * <td>{@link Deque#addFirst addFirst(e)}</td> 79 * <td>{@link Deque#offerFirst offerFirst(e)}</td> 80 * <td>{@link Deque#addLast addLast(e)}</td> 81 * <td>{@link Deque#offerLast offerLast(e)}</td> 82 * </tr> 83 * <tr> 84 * <td><b>Remove</b></td> 85 * <td>{@link Deque#removeFirst removeFirst()}</td> 86 * <td>{@link Deque#pollFirst pollFirst()}</td> 87 * <td>{@link Deque#removeLast removeLast()}</td> 88 * <td>{@link Deque#pollLast pollLast()}</td> 89 * </tr> 90 * <tr> 91 * <td><b>Examine</b></td> 92 * <td>{@link Deque#getFirst getFirst()}</td> 93 * <td>{@link Deque#peekFirst peekFirst()}</td> 94 * <td>{@link Deque#getLast getLast()}</td> 95 * <td>{@link Deque#peekLast peekLast()}</td> 96 * </tr> 97 * </table> 98 * 99 * <p>This interface extends the {@link Queue} interface. When a deque is 100 * used as a queue, FIFO (First-In-First-Out) behavior results. Elements are 101 * added at the end of the deque and removed from the beginning. The methods 102 * inherited from the {@code Queue} interface are precisely equivalent to 103 * {@code Deque} methods as indicated in the following table: 104 * 105 * <table BORDER CELLPADDING=3 CELLSPACING=1> 106 * <caption>Comparison of Queue and Deque methods</caption> 107 * <tr> 108 * <td ALIGN=CENTER> <b>{@code Queue} Method</b></td> 109 * <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td> 110 * </tr> 111 * <tr> 112 * <td>{@link java.util.Queue#add add(e)}</td> 113 * <td>{@link #addLast addLast(e)}</td> 114 * </tr> 115 * <tr> 116 * <td>{@link java.util.Queue#offer offer(e)}</td> 117 * <td>{@link #offerLast offerLast(e)}</td> 118 * </tr> 119 * <tr> 120 * <td>{@link java.util.Queue#remove remove()}</td> 121 * <td>{@link #removeFirst removeFirst()}</td> 122 * </tr> 123 * <tr> 124 * <td>{@link java.util.Queue#poll poll()}</td> 125 * <td>{@link #pollFirst pollFirst()}</td> 126 * </tr> 127 * <tr> 128 * <td>{@link java.util.Queue#element element()}</td> 129 * <td>{@link #getFirst getFirst()}</td> 130 * </tr> 131 * <tr> 132 * <td>{@link java.util.Queue#peek peek()}</td> 133 * <td>{@link #peek peekFirst()}</td> 134 * </tr> 135 * </table> 136 * 137 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This 138 * interface should be used in preference to the legacy {@link Stack} class. 139 * When a deque is used as a stack, elements are pushed and popped from the 140 * beginning of the deque. Stack methods are precisely equivalent to 141 * {@code Deque} methods as indicated in the table below: 142 * 143 * <table BORDER CELLPADDING=3 CELLSPACING=1> 144 * <caption>Comparison of Stack and Deque methods</caption> 145 * <tr> 146 * <td ALIGN=CENTER> <b>Stack Method</b></td> 147 * <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td> 148 * </tr> 149 * <tr> 150 * <td>{@link #push push(e)}</td> 151 * <td>{@link #addFirst addFirst(e)}</td> 152 * </tr> 153 * <tr> 154 * <td>{@link #pop pop()}</td> 155 * <td>{@link #removeFirst removeFirst()}</td> 156 * </tr> 157 * <tr> 158 * <td>{@link #peek peek()}</td> 159 * <td>{@link #peekFirst peekFirst()}</td> 160 * </tr> 161 * </table> 162 * 163 * <p>Note that the {@link #peek peek} method works equally well when 164 * a deque is used as a queue or a stack; in either case, elements are 165 * drawn from the beginning of the deque. 166 * 167 * <p>This interface provides two methods to remove interior 168 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and 169 * {@link #removeLastOccurrence removeLastOccurrence}. 170 * 171 * <p>Unlike the {@link List} interface, this interface does not 172 * provide support for indexed access to elements. 173 * 174 * <p>While {@code Deque} implementations are not strictly required 175 * to prohibit the insertion of null elements, they are strongly 176 * encouraged to do so. Users of any {@code Deque} implementations 177 * that do allow null elements are strongly encouraged <i>not</i> to 178 * take advantage of the ability to insert nulls. This is so because 179 * {@code null} is used as a special return value by various methods 180 * to indicated that the deque is empty. 181 * 182 * <p>{@code Deque} implementations generally do not define 183 * element-based versions of the {@code equals} and {@code hashCode} 184 * methods, but instead inherit the identity-based versions from class 185 * {@code Object}. 186 * 187 * @author Doug Lea 188 * @author Josh Bloch 189 * @since 1.6 190 * @param <E> the type of elements held in this deque 191 */ 192 public interface Deque<E> extends Queue<E> { 193 /** 194 * Inserts the specified element at the front of this deque if it is 195 * possible to do so immediately without violating capacity restrictions, 196 * throwing an {@code IllegalStateException} if no space is currently 197 * available. When using a capacity-restricted deque, it is generally 198 * preferable to use method {@link #offerFirst}. 199 * 200 * @param e the element to add 201 * @throws IllegalStateException if the element cannot be added at this 202 * time due to capacity restrictions 203 * @throws ClassCastException if the class of the specified element 204 * prevents it from being added to this deque 205 * @throws NullPointerException if the specified element is null and this 206 * deque does not permit null elements 207 * @throws IllegalArgumentException if some property of the specified 208 * element prevents it from being added to this deque 209 */ addFirst(E e)210 void addFirst(E e); 211 212 /** 213 * Inserts the specified element at the end of this deque if it is 214 * possible to do so immediately without violating capacity restrictions, 215 * throwing an {@code IllegalStateException} if no space is currently 216 * available. When using a capacity-restricted deque, it is generally 217 * preferable to use method {@link #offerLast}. 218 * 219 * <p>This method is equivalent to {@link #add}. 220 * 221 * @param e the element to add 222 * @throws IllegalStateException if the element cannot be added at this 223 * time due to capacity restrictions 224 * @throws ClassCastException if the class of the specified element 225 * prevents it from being added to this deque 226 * @throws NullPointerException if the specified element is null and this 227 * deque does not permit null elements 228 * @throws IllegalArgumentException if some property of the specified 229 * element prevents it from being added to this deque 230 */ addLast(E e)231 void addLast(E e); 232 233 /** 234 * Inserts the specified element at the front of this deque unless it would 235 * violate capacity restrictions. When using a capacity-restricted deque, 236 * this method is generally preferable to the {@link #addFirst} method, 237 * which can fail to insert an element only by throwing an exception. 238 * 239 * @param e the element to add 240 * @return {@code true} if the element was added to this deque, else 241 * {@code false} 242 * @throws ClassCastException if the class of the specified element 243 * prevents it from being added to this deque 244 * @throws NullPointerException if the specified element is null and this 245 * deque does not permit null elements 246 * @throws IllegalArgumentException if some property of the specified 247 * element prevents it from being added to this deque 248 */ offerFirst(E e)249 boolean offerFirst(E e); 250 251 /** 252 * Inserts the specified element at the end of this deque unless it would 253 * violate capacity restrictions. When using a capacity-restricted deque, 254 * this method is generally preferable to the {@link #addLast} method, 255 * which can fail to insert an element only by throwing an exception. 256 * 257 * @param e the element to add 258 * @return {@code true} if the element was added to this deque, else 259 * {@code false} 260 * @throws ClassCastException if the class of the specified element 261 * prevents it from being added to this deque 262 * @throws NullPointerException if the specified element is null and this 263 * deque does not permit null elements 264 * @throws IllegalArgumentException if some property of the specified 265 * element prevents it from being added to this deque 266 */ offerLast(E e)267 boolean offerLast(E e); 268 269 /** 270 * Retrieves and removes the first element of this deque. This method 271 * differs from {@link #pollFirst pollFirst} only in that it throws an 272 * exception if this deque is empty. 273 * 274 * @return the head of this deque 275 * @throws NoSuchElementException if this deque is empty 276 */ removeFirst()277 E removeFirst(); 278 279 /** 280 * Retrieves and removes the last element of this deque. This method 281 * differs from {@link #pollLast pollLast} only in that it throws an 282 * exception if this deque is empty. 283 * 284 * @return the tail of this deque 285 * @throws NoSuchElementException if this deque is empty 286 */ removeLast()287 E removeLast(); 288 289 /** 290 * Retrieves and removes the first element of this deque, 291 * or returns {@code null} if this deque is empty. 292 * 293 * @return the head of this deque, or {@code null} if this deque is empty 294 */ pollFirst()295 E pollFirst(); 296 297 /** 298 * Retrieves and removes the last element of this deque, 299 * or returns {@code null} if this deque is empty. 300 * 301 * @return the tail of this deque, or {@code null} if this deque is empty 302 */ pollLast()303 E pollLast(); 304 305 /** 306 * Retrieves, but does not remove, the first element of this deque. 307 * 308 * This method differs from {@link #peekFirst peekFirst} only in that it 309 * throws an exception if this deque is empty. 310 * 311 * @return the head of this deque 312 * @throws NoSuchElementException if this deque is empty 313 */ getFirst()314 E getFirst(); 315 316 /** 317 * Retrieves, but does not remove, the last element of this deque. 318 * This method differs from {@link #peekLast peekLast} only in that it 319 * throws an exception if this deque is empty. 320 * 321 * @return the tail of this deque 322 * @throws NoSuchElementException if this deque is empty 323 */ getLast()324 E getLast(); 325 326 /** 327 * Retrieves, but does not remove, the first element of this deque, 328 * or returns {@code null} if this deque is empty. 329 * 330 * @return the head of this deque, or {@code null} if this deque is empty 331 */ peekFirst()332 E peekFirst(); 333 334 /** 335 * Retrieves, but does not remove, the last element of this deque, 336 * or returns {@code null} if this deque is empty. 337 * 338 * @return the tail of this deque, or {@code null} if this deque is empty 339 */ peekLast()340 E peekLast(); 341 342 /** 343 * Removes the first occurrence of the specified element from this deque. 344 * If the deque does not contain the element, it is unchanged. 345 * More formally, removes the first element {@code e} such that 346 * {@code Objects.equals(o, e)} (if such an element exists). 347 * Returns {@code true} if this deque contained the specified element 348 * (or equivalently, if this deque changed as a result of the call). 349 * 350 * @param o element to be removed from this deque, if present 351 * @return {@code true} if an element was removed as a result of this call 352 * @throws ClassCastException if the class of the specified element 353 * is incompatible with this deque 354 * (<a href="Collection.html#optional-restrictions">optional</a>) 355 * @throws NullPointerException if the specified element is null and this 356 * deque does not permit null elements 357 * (<a href="Collection.html#optional-restrictions">optional</a>) 358 */ removeFirstOccurrence(Object o)359 boolean removeFirstOccurrence(Object o); 360 361 /** 362 * Removes the last occurrence of the specified element from this deque. 363 * If the deque does not contain the element, it is unchanged. 364 * More formally, removes the last element {@code e} such that 365 * {@code Objects.equals(o, e)} (if such an element exists). 366 * Returns {@code true} if this deque contained the specified element 367 * (or equivalently, if this deque changed as a result of the call). 368 * 369 * @param o element to be removed from this deque, if present 370 * @return {@code true} if an element was removed as a result of this call 371 * @throws ClassCastException if the class of the specified element 372 * is incompatible with this deque 373 * (<a href="Collection.html#optional-restrictions">optional</a>) 374 * @throws NullPointerException if the specified element is null and this 375 * deque does not permit null elements 376 * (<a href="Collection.html#optional-restrictions">optional</a>) 377 */ removeLastOccurrence(Object o)378 boolean removeLastOccurrence(Object o); 379 380 // *** Queue methods *** 381 382 /** 383 * Inserts the specified element into the queue represented by this deque 384 * (in other words, at the tail of this deque) if it is possible to do so 385 * immediately without violating capacity restrictions, returning 386 * {@code true} upon success and throwing an 387 * {@code IllegalStateException} if no space is currently available. 388 * When using a capacity-restricted deque, it is generally preferable to 389 * use {@link #offer(Object) offer}. 390 * 391 * <p>This method is equivalent to {@link #addLast}. 392 * 393 * @param e the element to add 394 * @return {@code true} (as specified by {@link Collection#add}) 395 * @throws IllegalStateException if the element cannot be added at this 396 * time due to capacity restrictions 397 * @throws ClassCastException if the class of the specified element 398 * prevents it from being added to this deque 399 * @throws NullPointerException if the specified element is null and this 400 * deque does not permit null elements 401 * @throws IllegalArgumentException if some property of the specified 402 * element prevents it from being added to this deque 403 */ add(E e)404 boolean add(E e); 405 406 /** 407 * Inserts the specified element into the queue represented by this deque 408 * (in other words, at the tail of this deque) if it is possible to do so 409 * immediately without violating capacity restrictions, returning 410 * {@code true} upon success and {@code false} if no space is currently 411 * available. When using a capacity-restricted deque, this method is 412 * generally preferable to the {@link #add} method, which can fail to 413 * insert an element only by throwing an exception. 414 * 415 * <p>This method is equivalent to {@link #offerLast}. 416 * 417 * @param e the element to add 418 * @return {@code true} if the element was added to this deque, else 419 * {@code false} 420 * @throws ClassCastException if the class of the specified element 421 * prevents it from being added to this deque 422 * @throws NullPointerException if the specified element is null and this 423 * deque does not permit null elements 424 * @throws IllegalArgumentException if some property of the specified 425 * element prevents it from being added to this deque 426 */ offer(E e)427 boolean offer(E e); 428 429 /** 430 * Retrieves and removes the head of the queue represented by this deque 431 * (in other words, the first element of this deque). 432 * This method differs from {@link #poll poll} only in that it throws an 433 * exception if this deque is empty. 434 * 435 * <p>This method is equivalent to {@link #removeFirst()}. 436 * 437 * @return the head of the queue represented by this deque 438 * @throws NoSuchElementException if this deque is empty 439 */ remove()440 E remove(); 441 442 /** 443 * Retrieves and removes the head of the queue represented by this deque 444 * (in other words, the first element of this deque), or returns 445 * {@code null} if this deque is empty. 446 * 447 * <p>This method is equivalent to {@link #pollFirst()}. 448 * 449 * @return the first element of this deque, or {@code null} if 450 * this deque is empty 451 */ poll()452 E poll(); 453 454 /** 455 * Retrieves, but does not remove, the head of the queue represented by 456 * this deque (in other words, the first element of this deque). 457 * This method differs from {@link #peek peek} only in that it throws an 458 * exception if this deque is empty. 459 * 460 * <p>This method is equivalent to {@link #getFirst()}. 461 * 462 * @return the head of the queue represented by this deque 463 * @throws NoSuchElementException if this deque is empty 464 */ element()465 E element(); 466 467 /** 468 * Retrieves, but does not remove, the head of the queue represented by 469 * this deque (in other words, the first element of this deque), or 470 * returns {@code null} if this deque is empty. 471 * 472 * <p>This method is equivalent to {@link #peekFirst()}. 473 * 474 * @return the head of the queue represented by this deque, or 475 * {@code null} if this deque is empty 476 */ peek()477 E peek(); 478 479 480 // *** Stack methods *** 481 482 /** 483 * Pushes an element onto the stack represented by this deque (in other 484 * words, at the head of this deque) if it is possible to do so 485 * immediately without violating capacity restrictions, throwing an 486 * {@code IllegalStateException} if no space is currently available. 487 * 488 * <p>This method is equivalent to {@link #addFirst}. 489 * 490 * @param e the element to push 491 * @throws IllegalStateException if the element cannot be added at this 492 * time due to capacity restrictions 493 * @throws ClassCastException if the class of the specified element 494 * prevents it from being added to this deque 495 * @throws NullPointerException if the specified element is null and this 496 * deque does not permit null elements 497 * @throws IllegalArgumentException if some property of the specified 498 * element prevents it from being added to this deque 499 */ push(E e)500 void push(E e); 501 502 /** 503 * Pops an element from the stack represented by this deque. In other 504 * words, removes and returns the first element of this deque. 505 * 506 * <p>This method is equivalent to {@link #removeFirst()}. 507 * 508 * @return the element at the front of this deque (which is the top 509 * of the stack represented by this deque) 510 * @throws NoSuchElementException if this deque is empty 511 */ pop()512 E pop(); 513 514 515 // *** Collection methods *** 516 517 /** 518 * Removes the first occurrence of the specified element from this deque. 519 * If the deque does not contain the element, it is unchanged. 520 * More formally, removes the first element {@code e} such that 521 * {@code Objects.equals(o, e)} (if such an element exists). 522 * Returns {@code true} if this deque contained the specified element 523 * (or equivalently, if this deque changed as a result of the call). 524 * 525 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}. 526 * 527 * @param o element to be removed from this deque, if present 528 * @return {@code true} if an element was removed as a result of this call 529 * @throws ClassCastException if the class of the specified element 530 * is incompatible with this deque 531 * (<a href="Collection.html#optional-restrictions">optional</a>) 532 * @throws NullPointerException if the specified element is null and this 533 * deque does not permit null elements 534 * (<a href="Collection.html#optional-restrictions">optional</a>) 535 */ remove(Object o)536 boolean remove(Object o); 537 538 /** 539 * Returns {@code true} if this deque contains the specified element. 540 * More formally, returns {@code true} if and only if this deque contains 541 * at least one element {@code e} such that {@code Objects.equals(o, e)}. 542 * 543 * @param o element whose presence in this deque is to be tested 544 * @return {@code true} if this deque contains the specified element 545 * @throws ClassCastException if the class of the specified element 546 * is incompatible with this deque 547 * (<a href="Collection.html#optional-restrictions">optional</a>) 548 * @throws NullPointerException if the specified element is null and this 549 * deque does not permit null elements 550 * (<a href="Collection.html#optional-restrictions">optional</a>) 551 */ contains(Object o)552 boolean contains(Object o); 553 554 /** 555 * Returns the number of elements in this deque. 556 * 557 * @return the number of elements in this deque 558 */ size()559 int size(); 560 561 /** 562 * Returns an iterator over the elements in this deque in proper sequence. 563 * The elements will be returned in order from first (head) to last (tail). 564 * 565 * @return an iterator over the elements in this deque in proper sequence 566 */ iterator()567 Iterator<E> iterator(); 568 569 /** 570 * Returns an iterator over the elements in this deque in reverse 571 * sequential order. The elements will be returned in order from 572 * last (tail) to first (head). 573 * 574 * @return an iterator over the elements in this deque in reverse 575 * sequence 576 */ descendingIterator()577 Iterator<E> descendingIterator(); 578 579 } 580