Deque.java revision be0e537d8953365ae5a008350461ccf777a719fb
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 35package 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 * <p> 63 * <table BORDER CELLPADDING=3 CELLSPACING=1> 64 * <caption>Summary of Deque methods</caption> 65 * <tr> 66 * <td></td> 67 * <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td> 68 * <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td> 69 * </tr> 70 * <tr> 71 * <td></td> 72 * <td ALIGN=CENTER><em>Throws exception</em></td> 73 * <td ALIGN=CENTER><em>Special value</em></td> 74 * <td ALIGN=CENTER><em>Throws exception</em></td> 75 * <td ALIGN=CENTER><em>Special value</em></td> 76 * </tr> 77 * <tr> 78 * <td><b>Insert</b></td> 79 * <td>{@link Deque#addFirst addFirst(e)}</td> 80 * <td>{@link Deque#offerFirst offerFirst(e)}</td> 81 * <td>{@link Deque#addLast addLast(e)}</td> 82 * <td>{@link Deque#offerLast offerLast(e)}</td> 83 * </tr> 84 * <tr> 85 * <td><b>Remove</b></td> 86 * <td>{@link Deque#removeFirst removeFirst()}</td> 87 * <td>{@link Deque#pollFirst pollFirst()}</td> 88 * <td>{@link Deque#removeLast removeLast()}</td> 89 * <td>{@link Deque#pollLast pollLast()}</td> 90 * </tr> 91 * <tr> 92 * <td><b>Examine</b></td> 93 * <td>{@link Deque#getFirst getFirst()}</td> 94 * <td>{@link Deque#peekFirst peekFirst()}</td> 95 * <td>{@link Deque#getLast getLast()}</td> 96 * <td>{@link Deque#peekLast peekLast()}</td> 97 * </tr> 98 * </table> 99 * 100 * <p>This interface extends the {@link Queue} interface. When a deque is 101 * used as a queue, FIFO (First-In-First-Out) behavior results. Elements are 102 * added at the end of the deque and removed from the beginning. The methods 103 * inherited from the {@code Queue} interface are precisely equivalent to 104 * {@code Deque} methods as indicated in the following table: 105 * 106 * <p> 107 * <table BORDER CELLPADDING=3 CELLSPACING=1> 108 * <caption>Comparison of Queue and Deque methods</caption> 109 * <tr> 110 * <td ALIGN=CENTER> <b>{@code Queue} Method</b></td> 111 * <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td> 112 * </tr> 113 * <tr> 114 * <td>{@link java.util.Queue#add add(e)}</td> 115 * <td>{@link #addLast addLast(e)}</td> 116 * </tr> 117 * <tr> 118 * <td>{@link java.util.Queue#offer offer(e)}</td> 119 * <td>{@link #offerLast offerLast(e)}</td> 120 * </tr> 121 * <tr> 122 * <td>{@link java.util.Queue#remove remove()}</td> 123 * <td>{@link #removeFirst removeFirst()}</td> 124 * </tr> 125 * <tr> 126 * <td>{@link java.util.Queue#poll poll()}</td> 127 * <td>{@link #pollFirst pollFirst()}</td> 128 * </tr> 129 * <tr> 130 * <td>{@link java.util.Queue#element element()}</td> 131 * <td>{@link #getFirst getFirst()}</td> 132 * </tr> 133 * <tr> 134 * <td>{@link java.util.Queue#peek peek()}</td> 135 * <td>{@link #peek peekFirst()}</td> 136 * </tr> 137 * </table> 138 * 139 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This 140 * interface should be used in preference to the legacy {@link Stack} class. 141 * When a deque is used as a stack, elements are pushed and popped from the 142 * beginning of the deque. Stack methods are precisely equivalent to 143 * {@code Deque} methods as indicated in the table below: 144 * 145 * <p> 146 * <table BORDER CELLPADDING=3 CELLSPACING=1> 147 * <caption>Comparison of Stack and Deque methods</caption> 148 * <tr> 149 * <td ALIGN=CENTER> <b>Stack Method</b></td> 150 * <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td> 151 * </tr> 152 * <tr> 153 * <td>{@link #push push(e)}</td> 154 * <td>{@link #addFirst addFirst(e)}</td> 155 * </tr> 156 * <tr> 157 * <td>{@link #pop pop()}</td> 158 * <td>{@link #removeFirst removeFirst()}</td> 159 * </tr> 160 * <tr> 161 * <td>{@link #peek peek()}</td> 162 * <td>{@link #peekFirst peekFirst()}</td> 163 * </tr> 164 * </table> 165 * 166 * <p>Note that the {@link #peek peek} method works equally well when 167 * a deque is used as a queue or a stack; in either case, elements are 168 * drawn from the beginning of the deque. 169 * 170 * <p>This interface provides two methods to remove interior 171 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and 172 * {@link #removeLastOccurrence removeLastOccurrence}. 173 * 174 * <p>Unlike the {@link List} interface, this interface does not 175 * provide support for indexed access to elements. 176 * 177 * <p>While {@code Deque} implementations are not strictly required 178 * to prohibit the insertion of null elements, they are strongly 179 * encouraged to do so. Users of any {@code Deque} implementations 180 * that do allow null elements are strongly encouraged <i>not</i> to 181 * take advantage of the ability to insert nulls. This is so because 182 * {@code null} is used as a special return value by various methods 183 * to indicated that the deque is empty. 184 * 185 * <p>{@code Deque} implementations generally do not define 186 * element-based versions of the {@code equals} and {@code hashCode} 187 * methods, but instead inherit the identity-based versions from class 188 * {@code Object}. 189 * 190 * @author Doug Lea 191 * @author Josh Bloch 192 * @since 1.6 193 * @param <E> the type of elements held in this collection 194 */ 195public interface Deque<E> extends Queue<E> { 196 /** 197 * Inserts the specified element at the front of this deque if it is 198 * possible to do so immediately without violating capacity restrictions, 199 * throwing an {@code IllegalStateException} if no space is currently 200 * available. When using a capacity-restricted deque, it is generally 201 * preferable to use method {@link #offerFirst}. 202 * 203 * @param e the element to add 204 * @throws IllegalStateException if the element cannot be added at this 205 * time due to capacity restrictions 206 * @throws ClassCastException if the class of the specified element 207 * prevents it from being added to this deque 208 * @throws NullPointerException if the specified element is null and this 209 * deque does not permit null elements 210 * @throws IllegalArgumentException if some property of the specified 211 * element prevents it from being added to this deque 212 */ 213 void addFirst(E e); 214 215 /** 216 * Inserts the specified element at the end of this deque if it is 217 * possible to do so immediately without violating capacity restrictions, 218 * throwing an {@code IllegalStateException} if no space is currently 219 * available. When using a capacity-restricted deque, it is generally 220 * preferable to use method {@link #offerLast}. 221 * 222 * <p>This method is equivalent to {@link #add}. 223 * 224 * @param e the element to add 225 * @throws IllegalStateException if the element cannot be added at this 226 * time due to capacity restrictions 227 * @throws ClassCastException if the class of the specified element 228 * prevents it from being added to this deque 229 * @throws NullPointerException if the specified element is null and this 230 * deque does not permit null elements 231 * @throws IllegalArgumentException if some property of the specified 232 * element prevents it from being added to this deque 233 */ 234 void addLast(E e); 235 236 /** 237 * Inserts the specified element at the front of this deque unless it would 238 * violate capacity restrictions. When using a capacity-restricted deque, 239 * this method is generally preferable to the {@link #addFirst} method, 240 * which can fail to insert an element only by throwing an exception. 241 * 242 * @param e the element to add 243 * @return {@code true} if the element was added to this deque, else 244 * {@code false} 245 * @throws ClassCastException if the class of the specified element 246 * prevents it from being added to this deque 247 * @throws NullPointerException if the specified element is null and this 248 * deque does not permit null elements 249 * @throws IllegalArgumentException if some property of the specified 250 * element prevents it from being added to this deque 251 */ 252 boolean offerFirst(E e); 253 254 /** 255 * Inserts the specified element at the end of this deque unless it would 256 * violate capacity restrictions. When using a capacity-restricted deque, 257 * this method is generally preferable to the {@link #addLast} method, 258 * which can fail to insert an element only by throwing an exception. 259 * 260 * @param e the element to add 261 * @return {@code true} if the element was added to this deque, else 262 * {@code false} 263 * @throws ClassCastException if the class of the specified element 264 * prevents it from being added to this deque 265 * @throws NullPointerException if the specified element is null and this 266 * deque does not permit null elements 267 * @throws IllegalArgumentException if some property of the specified 268 * element prevents it from being added to this deque 269 */ 270 boolean offerLast(E e); 271 272 /** 273 * Retrieves and removes the first element of this deque. This method 274 * differs from {@link #pollFirst pollFirst} only in that it throws an 275 * exception if this deque is empty. 276 * 277 * @return the head of this deque 278 * @throws NoSuchElementException if this deque is empty 279 */ 280 E removeFirst(); 281 282 /** 283 * Retrieves and removes the last element of this deque. This method 284 * differs from {@link #pollLast pollLast} only in that it throws an 285 * exception if this deque is empty. 286 * 287 * @return the tail of this deque 288 * @throws NoSuchElementException if this deque is empty 289 */ 290 E removeLast(); 291 292 /** 293 * Retrieves and removes the first element of this deque, 294 * or returns {@code null} if this deque is empty. 295 * 296 * @return the head of this deque, or {@code null} if this deque is empty 297 */ 298 E pollFirst(); 299 300 /** 301 * Retrieves and removes the last element of this deque, 302 * or returns {@code null} if this deque is empty. 303 * 304 * @return the tail of this deque, or {@code null} if this deque is empty 305 */ 306 E pollLast(); 307 308 /** 309 * Retrieves, but does not remove, the first element of this deque. 310 * 311 * This method differs from {@link #peekFirst peekFirst} only in that it 312 * throws an exception if this deque is empty. 313 * 314 * @return the head of this deque 315 * @throws NoSuchElementException if this deque is empty 316 */ 317 E getFirst(); 318 319 /** 320 * Retrieves, but does not remove, the last element of this deque. 321 * This method differs from {@link #peekLast peekLast} only in that it 322 * throws an exception if this deque is empty. 323 * 324 * @return the tail of this deque 325 * @throws NoSuchElementException if this deque is empty 326 */ 327 E getLast(); 328 329 /** 330 * Retrieves, but does not remove, the first element of this deque, 331 * or returns {@code null} if this deque is empty. 332 * 333 * @return the head of this deque, or {@code null} if this deque is empty 334 */ 335 E peekFirst(); 336 337 /** 338 * Retrieves, but does not remove, the last element of this deque, 339 * or returns {@code null} if this deque is empty. 340 * 341 * @return the tail of this deque, or {@code null} if this deque is empty 342 */ 343 E peekLast(); 344 345 /** 346 * Removes the first occurrence of the specified element from this deque. 347 * If the deque does not contain the element, it is unchanged. 348 * More formally, removes the first element {@code e} such that 349 * <tt>(o==null ? e==null : o.equals(e))</tt> 350 * (if such an element exists). 351 * Returns {@code true} if this deque contained the specified element 352 * (or equivalently, if this deque changed as a result of the call). 353 * 354 * @param o element to be removed from this deque, if present 355 * @return {@code true} if an element was removed as a result of this call 356 * @throws ClassCastException if the class of the specified element 357 * is incompatible with this deque (optional) 358 * @throws NullPointerException if the specified element is null and this 359 * deque does not permit null elements (optional) 360 */ 361 boolean removeFirstOccurrence(Object o); 362 363 /** 364 * Removes the last occurrence of the specified element from this deque. 365 * If the deque does not contain the element, it is unchanged. 366 * More formally, removes the last element {@code e} such that 367 * <tt>(o==null ? e==null : o.equals(e))</tt> 368 * (if such an element exists). 369 * Returns {@code true} if this deque contained the specified element 370 * (or equivalently, if this deque changed as a result of the call). 371 * 372 * @param o element to be removed from this deque, if present 373 * @return {@code true} if an element was removed as a result of this call 374 * @throws ClassCastException if the class of the specified element 375 * is incompatible with this deque (optional) 376 * @throws NullPointerException if the specified element is null and this 377 * deque does not permit null elements (optional) 378 */ 379 boolean removeLastOccurrence(Object o); 380 381 // *** Queue methods *** 382 383 /** 384 * Inserts the specified element into the queue represented by this deque 385 * (in other words, at the tail of this deque) if it is possible to do so 386 * immediately without violating capacity restrictions, returning 387 * {@code true} upon success and throwing an 388 * {@code IllegalStateException} if no space is currently available. 389 * When using a capacity-restricted deque, it is generally preferable to 390 * use {@link #offer(Object) offer}. 391 * 392 * <p>This method is equivalent to {@link #addLast}. 393 * 394 * @param e the element to add 395 * @return {@code true} (as specified by {@link Collection#add}) 396 * @throws IllegalStateException if the element cannot be added at this 397 * time due to capacity restrictions 398 * @throws ClassCastException if the class of the specified element 399 * prevents it from being added to this deque 400 * @throws NullPointerException if the specified element is null and this 401 * deque does not permit null elements 402 * @throws IllegalArgumentException if some property of the specified 403 * element prevents it from being added to this deque 404 */ 405 boolean add(E e); 406 407 /** 408 * Inserts the specified element into the queue represented by this deque 409 * (in other words, at the tail of this deque) if it is possible to do so 410 * immediately without violating capacity restrictions, returning 411 * {@code true} upon success and {@code false} if no space is currently 412 * available. When using a capacity-restricted deque, this method is 413 * generally preferable to the {@link #add} method, which can fail to 414 * insert an element only by throwing an exception. 415 * 416 * <p>This method is equivalent to {@link #offerLast}. 417 * 418 * @param e the element to add 419 * @return {@code true} if the element was added to this deque, else 420 * {@code false} 421 * @throws ClassCastException if the class of the specified element 422 * prevents it from being added to this deque 423 * @throws NullPointerException if the specified element is null and this 424 * deque does not permit null elements 425 * @throws IllegalArgumentException if some property of the specified 426 * element prevents it from being added to this deque 427 */ 428 boolean offer(E e); 429 430 /** 431 * Retrieves and removes the head of the queue represented by this deque 432 * (in other words, the first element of this deque). 433 * This method differs from {@link #poll poll} only in that it throws an 434 * exception if this deque is empty. 435 * 436 * <p>This method is equivalent to {@link #removeFirst()}. 437 * 438 * @return the head of the queue represented by this deque 439 * @throws NoSuchElementException if this deque is empty 440 */ 441 E remove(); 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 * {@code null} if this deque is empty. 447 * 448 * <p>This method is equivalent to {@link #pollFirst()}. 449 * 450 * @return the first element of this deque, or {@code null} if 451 * this deque is empty 452 */ 453 E poll(); 454 455 /** 456 * Retrieves, but does not remove, the head of the queue represented by 457 * this deque (in other words, the first element of this deque). 458 * This method differs from {@link #peek peek} only in that it throws an 459 * exception if this deque is empty. 460 * 461 * <p>This method is equivalent to {@link #getFirst()}. 462 * 463 * @return the head of the queue represented by this deque 464 * @throws NoSuchElementException if this deque is empty 465 */ 466 E element(); 467 468 /** 469 * Retrieves, but does not remove, the head of the queue represented by 470 * this deque (in other words, the first element of this deque), or 471 * returns {@code null} if this deque is empty. 472 * 473 * <p>This method is equivalent to {@link #peekFirst()}. 474 * 475 * @return the head of the queue represented by this deque, or 476 * {@code null} if this deque is empty 477 */ 478 E peek(); 479 480 481 // *** Stack methods *** 482 483 /** 484 * Pushes an element onto the stack represented by this deque (in other 485 * words, at the head of this deque) if it is possible to do so 486 * immediately without violating capacity restrictions, throwing an 487 * {@code IllegalStateException} if no space is currently available. 488 * 489 * <p>This method is equivalent to {@link #addFirst}. 490 * 491 * @param e the element to push 492 * @throws IllegalStateException if the element cannot be added at this 493 * time due to capacity restrictions 494 * @throws ClassCastException if the class of the specified element 495 * prevents it from being added to this deque 496 * @throws NullPointerException if the specified element is null and this 497 * deque does not permit null elements 498 * @throws IllegalArgumentException if some property of the specified 499 * element prevents it from being added to this deque 500 */ 501 void push(E e); 502 503 /** 504 * Pops an element from the stack represented by this deque. In other 505 * words, removes and returns the first element of this deque. 506 * 507 * <p>This method is equivalent to {@link #removeFirst()}. 508 * 509 * @return the element at the front of this deque (which is the top 510 * of the stack represented by this deque) 511 * @throws NoSuchElementException if this deque is empty 512 */ 513 E pop(); 514 515 516 // *** Collection methods *** 517 518 /** 519 * Removes the first occurrence of the specified element from this deque. 520 * If the deque does not contain the element, it is unchanged. 521 * More formally, removes the first element {@code e} such that 522 * <tt>(o==null ? e==null : o.equals(e))</tt> 523 * (if such an element exists). 524 * Returns {@code true} if this deque contained the specified element 525 * (or equivalently, if this deque changed as a result of the call). 526 * 527 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}. 528 * 529 * @param o element to be removed from this deque, if present 530 * @return {@code true} if an element was removed as a result of this call 531 * @throws ClassCastException if the class of the specified element 532 * is incompatible with this deque (optional) 533 * @throws NullPointerException if the specified element is null and this 534 * deque does not permit null elements (optional) 535 */ 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 542 * <tt>(o==null ? e==null : o.equals(e))</tt>. 543 * 544 * @param o element whose presence in this deque is to be tested 545 * @return {@code true} if this deque contains the specified element 546 * @throws ClassCastException if the type of the specified element 547 * is incompatible with this deque (optional) 548 * @throws NullPointerException if the specified element is null and this 549 * deque does not permit null elements (optional) 550 */ 551 boolean contains(Object o); 552 553 /** 554 * Returns the number of elements in this deque. 555 * 556 * @return the number of elements in this deque 557 */ 558 public int size(); 559 560 /** 561 * Returns an iterator over the elements in this deque in proper sequence. 562 * The elements will be returned in order from first (head) to last (tail). 563 * 564 * @return an iterator over the elements in this deque in proper sequence 565 */ 566 Iterator<E> iterator(); 567 568 /** 569 * Returns an iterator over the elements in this deque in reverse 570 * sequential order. The elements will be returned in order from 571 * last (tail) to first (head). 572 * 573 * @return an iterator over the elements in this deque in reverse 574 * sequence 575 */ 576 Iterator<E> descendingIterator(); 577 578} 579