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 Doug Lea and Josh Bloch with assistance from members of 32 * JCP JSR-166 Expert Group and released to the public domain, as explained 33 * at http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util; 37 38 // Android-changed: removed link to collections framework docs 39 /** 40 * A linear collection that supports element insertion and removal at 41 * both ends. The name <i>deque</i> is short for "double ended queue" 42 * and is usually pronounced "deck". Most {@code Deque} 43 * implementations place no fixed limits on the number of elements 44 * they may contain, but this interface supports capacity-restricted 45 * deques as well as those with no fixed size limit. 46 * 47 * <p>This interface defines methods to access the elements at both 48 * ends of the deque. Methods are provided to insert, remove, and 49 * examine the element. Each of these methods exists in two forms: 50 * one throws an exception if the operation fails, the other returns a 51 * special value (either {@code null} or {@code false}, depending on 52 * the operation). The latter form of the insert operation is 53 * designed specifically for use with capacity-restricted 54 * {@code Deque} implementations; in most implementations, insert 55 * operations cannot fail. 56 * 57 * <p>The twelve methods described above are summarized in the 58 * following table: 59 * 60 * <table class="striped"> 61 * <caption>Summary of Deque methods</caption> 62 * <thead> 63 * <tr> 64 * <td rowspan="2"></td> 65 * <th scope="col" colspan="2"> First Element (Head)</th> 66 * <th scope="col" colspan="2"> Last Element (Tail)</th> 67 * </tr> 68 * <tr> 69 * <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th> 70 * <th scope="col" style="font-weight:normal; font-style:italic">Special value</th> 71 * <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th> 72 * <th scope="col" style="font-weight:normal; font-style:italic">Special value</th> 73 * </tr> 74 * </thead> 75 * <tbody> 76 * <tr> 77 * <th scope="row">Insert</th> 78 * <td>{@link #addFirst(Object) addFirst(e)}</td> 79 * <td>{@link #offerFirst(Object) offerFirst(e)}</td> 80 * <td>{@link #addLast(Object) addLast(e)}</td> 81 * <td>{@link #offerLast(Object) offerLast(e)}</td> 82 * </tr> 83 * <tr> 84 * <th scope="row">Remove</th> 85 * <td>{@link #removeFirst() removeFirst()}</td> 86 * <td>{@link #pollFirst() pollFirst()}</td> 87 * <td>{@link #removeLast() removeLast()}</td> 88 * <td>{@link #pollLast() pollLast()}</td> 89 * </tr> 90 * <tr> 91 * <th scope="row">Examine</th> 92 * <td>{@link #getFirst() getFirst()}</td> 93 * <td>{@link #peekFirst() peekFirst()}</td> 94 * <td>{@link #getLast() getLast()}</td> 95 * <td>{@link #peekLast() peekLast()}</td> 96 * </tr> 97 * </tbody> 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 * <table class="striped"> 107 * <caption>Comparison of Queue and Deque methods</caption> 108 * <thead> 109 * <tr> 110 * <th scope="col"> {@code Queue} Method</th> 111 * <th scope="col"> Equivalent {@code Deque} Method</th> 112 * </tr> 113 * </thead> 114 * <tbody> 115 * <tr> 116 * <th scope="row">{@link #add(Object) add(e)}</th> 117 * <td>{@link #addLast(Object) addLast(e)}</td> 118 * </tr> 119 * <tr> 120 * <th scope="row">{@link #offer(Object) offer(e)}</th> 121 * <td>{@link #offerLast(Object) offerLast(e)}</td> 122 * </tr> 123 * <tr> 124 * <th scope="row">{@link #remove() remove()}</th> 125 * <td>{@link #removeFirst() removeFirst()}</td> 126 * </tr> 127 * <tr> 128 * <th scope="row">{@link #poll() poll()}</th> 129 * <td>{@link #pollFirst() pollFirst()}</td> 130 * </tr> 131 * <tr> 132 * <th scope="row">{@link #element() element()}</th> 133 * <td>{@link #getFirst() getFirst()}</td> 134 * </tr> 135 * <tr> 136 * <th scope="row">{@link #peek() peek()}</th> 137 * <td>{@link #peekFirst() peekFirst()}</td> 138 * </tr> 139 * </tbody> 140 * </table> 141 * 142 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This 143 * interface should be used in preference to the legacy {@link Stack} class. 144 * When a deque is used as a stack, elements are pushed and popped from the 145 * beginning of the deque. Stack methods are equivalent to {@code Deque} 146 * methods as indicated in the table below: 147 * 148 * <table class="striped"> 149 * <caption>Comparison of Stack and Deque methods</caption> 150 * <thead> 151 * <tr> 152 * <th scope="col"> Stack Method</th> 153 * <th scope="col"> Equivalent {@code Deque} Method</th> 154 * </tr> 155 * </thead> 156 * <tbody> 157 * <tr> 158 * <th scope="row">{@link #push(Object) push(e)}</th> 159 * <td>{@link #addFirst(Object) addFirst(e)}</td> 160 * </tr> 161 * <tr> 162 * <th scope="row">{@link #pop() pop()}</th> 163 * <td>{@link #removeFirst() removeFirst()}</td> 164 * </tr> 165 * <tr> 166 * <th scope="row">{@link #peek() peek()}</th> 167 * <td>{@link #getFirst() getFirst()}</td> 168 * </tr> 169 * </tbody> 170 * </table> 171 * 172 * <p>Note that the {@link #peek peek} method works equally well when 173 * a deque is used as a queue or a stack; in either case, elements are 174 * drawn from the beginning of the deque. 175 * 176 * <p>This interface provides two methods to remove interior 177 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and 178 * {@link #removeLastOccurrence removeLastOccurrence}. 179 * 180 * <p>Unlike the {@link List} interface, this interface does not 181 * provide support for indexed access to elements. 182 * 183 * <p>While {@code Deque} implementations are not strictly required 184 * to prohibit the insertion of null elements, they are strongly 185 * encouraged to do so. Users of any {@code Deque} implementations 186 * that do allow null elements are strongly encouraged <i>not</i> to 187 * take advantage of the ability to insert nulls. This is so because 188 * {@code null} is used as a special return value by various methods 189 * to indicate that the deque is empty. 190 * 191 * <p>{@code Deque} implementations generally do not define 192 * element-based versions of the {@code equals} and {@code hashCode} 193 * methods, but instead inherit the identity-based versions from class 194 * {@code Object}. 195 * 196 * @author Doug Lea 197 * @author Josh Bloch 198 * @since 1.6 199 * @param <E> the type of elements held in this deque 200 */ 201 public interface Deque<E> extends Queue<E> { 202 // Android-changed: fix framework docs link to "Collection#optional-restrictions" 203 // Several occurrences of the link have been fixed throughout. 204 205 /** 206 * Inserts the specified element at the front of this deque if it is 207 * possible to do so immediately without violating capacity restrictions, 208 * throwing an {@code IllegalStateException} if no space is currently 209 * available. When using a capacity-restricted deque, it is generally 210 * preferable to use method {@link #offerFirst}. 211 * 212 * @param e the element to add 213 * @throws IllegalStateException if the element cannot be added at this 214 * time due to capacity restrictions 215 * @throws ClassCastException if the class of the specified element 216 * prevents it from being added to this deque 217 * @throws NullPointerException if the specified element is null and this 218 * deque does not permit null elements 219 * @throws IllegalArgumentException if some property of the specified 220 * element prevents it from being added to this deque 221 */ addFirst(E e)222 void addFirst(E e); 223 224 /** 225 * Inserts the specified element at the end of this deque if it is 226 * possible to do so immediately without violating capacity restrictions, 227 * throwing an {@code IllegalStateException} if no space is currently 228 * available. When using a capacity-restricted deque, it is generally 229 * preferable to use method {@link #offerLast}. 230 * 231 * <p>This method is equivalent to {@link #add}. 232 * 233 * @param e the element to add 234 * @throws IllegalStateException if the element cannot be added at this 235 * time due to capacity restrictions 236 * @throws ClassCastException if the class of the specified element 237 * prevents it from being added to this deque 238 * @throws NullPointerException if the specified element is null and this 239 * deque does not permit null elements 240 * @throws IllegalArgumentException if some property of the specified 241 * element prevents it from being added to this deque 242 */ addLast(E e)243 void addLast(E e); 244 245 /** 246 * Inserts the specified element at the front of this deque unless it would 247 * violate capacity restrictions. When using a capacity-restricted deque, 248 * this method is generally preferable to the {@link #addFirst} method, 249 * which can fail to insert an element only by throwing an exception. 250 * 251 * @param e the element to add 252 * @return {@code true} if the element was added to this deque, else 253 * {@code false} 254 * @throws ClassCastException if the class of the specified element 255 * prevents it from being added to this deque 256 * @throws NullPointerException if the specified element is null and this 257 * deque does not permit null elements 258 * @throws IllegalArgumentException if some property of the specified 259 * element prevents it from being added to this deque 260 */ offerFirst(E e)261 boolean offerFirst(E e); 262 263 /** 264 * Inserts the specified element at the end of this deque unless it would 265 * violate capacity restrictions. When using a capacity-restricted deque, 266 * this method is generally preferable to the {@link #addLast} method, 267 * which can fail to insert an element only by throwing an exception. 268 * 269 * @param e the element to add 270 * @return {@code true} if the element was added to this deque, else 271 * {@code false} 272 * @throws ClassCastException if the class of the specified element 273 * prevents it from being added to this deque 274 * @throws NullPointerException if the specified element is null and this 275 * deque does not permit null elements 276 * @throws IllegalArgumentException if some property of the specified 277 * element prevents it from being added to this deque 278 */ offerLast(E e)279 boolean offerLast(E e); 280 281 /** 282 * Retrieves and removes the first element of this deque. This method 283 * differs from {@link #pollFirst pollFirst} only in that it throws an 284 * exception if this deque is empty. 285 * 286 * @return the head of this deque 287 * @throws NoSuchElementException if this deque is empty 288 */ removeFirst()289 E removeFirst(); 290 291 /** 292 * Retrieves and removes the last element of this deque. This method 293 * differs from {@link #pollLast pollLast} only in that it throws an 294 * exception if this deque is empty. 295 * 296 * @return the tail of this deque 297 * @throws NoSuchElementException if this deque is empty 298 */ removeLast()299 E removeLast(); 300 301 /** 302 * Retrieves and removes the first element of this deque, 303 * or returns {@code null} if this deque is empty. 304 * 305 * @return the head of this deque, or {@code null} if this deque is empty 306 */ pollFirst()307 E pollFirst(); 308 309 /** 310 * Retrieves and removes the last element of this deque, 311 * or returns {@code null} if this deque is empty. 312 * 313 * @return the tail of this deque, or {@code null} if this deque is empty 314 */ pollLast()315 E pollLast(); 316 317 /** 318 * Retrieves, but does not remove, the first element of this deque. 319 * 320 * This method differs from {@link #peekFirst peekFirst} only in that it 321 * throws an exception if this deque is empty. 322 * 323 * @return the head of this deque 324 * @throws NoSuchElementException if this deque is empty 325 */ getFirst()326 E getFirst(); 327 328 /** 329 * Retrieves, but does not remove, the last element of this deque. 330 * This method differs from {@link #peekLast peekLast} only in that it 331 * throws an exception if this deque is empty. 332 * 333 * @return the tail of this deque 334 * @throws NoSuchElementException if this deque is empty 335 */ getLast()336 E getLast(); 337 338 /** 339 * Retrieves, but does not remove, the first element of this deque, 340 * or returns {@code null} if this deque is empty. 341 * 342 * @return the head of this deque, or {@code null} if this deque is empty 343 */ peekFirst()344 E peekFirst(); 345 346 /** 347 * Retrieves, but does not remove, the last element of this deque, 348 * or returns {@code null} if this deque is empty. 349 * 350 * @return the tail of this deque, or {@code null} if this deque is empty 351 */ peekLast()352 E peekLast(); 353 354 /** 355 * Removes the first occurrence of the specified element from this deque. 356 * If the deque does not contain the element, it is unchanged. 357 * More formally, removes the first element {@code e} such that 358 * {@code Objects.equals(o, e)} (if such an element exists). 359 * Returns {@code true} if this deque contained the specified element 360 * (or equivalently, if this deque changed as a result of the call). 361 * 362 * @param o element to be removed from this deque, if present 363 * @return {@code true} if an element was removed as a result of this call 364 * @throws ClassCastException if the class of the specified element 365 * is incompatible with this deque 366 * (<a href="Collection.html#optional-restrictions">optional</a>) 367 * @throws NullPointerException if the specified element is null and this 368 * deque does not permit null elements 369 * (<a href="Collection.html#optional-restrictions">optional</a>) 370 */ removeFirstOccurrence(Object o)371 boolean removeFirstOccurrence(Object o); 372 373 /** 374 * Removes the last occurrence of the specified element from this deque. 375 * If the deque does not contain the element, it is unchanged. 376 * More formally, removes the last element {@code e} such that 377 * {@code Objects.equals(o, e)} (if such an element exists). 378 * Returns {@code true} if this deque contained the specified element 379 * (or equivalently, if this deque changed as a result of the call). 380 * 381 * @param o element to be removed from this deque, if present 382 * @return {@code true} if an element was removed as a result of this call 383 * @throws ClassCastException if the class of the specified element 384 * is incompatible with this deque 385 * (<a href="Collection.html#optional-restrictions">optional</a>) 386 * @throws NullPointerException if the specified element is null and this 387 * deque does not permit null elements 388 * (<a href="Collection.html#optional-restrictions">optional</a>) 389 */ removeLastOccurrence(Object o)390 boolean removeLastOccurrence(Object o); 391 392 // *** Queue methods *** 393 394 /** 395 * Inserts the specified element into the queue represented by this deque 396 * (in other words, at the tail of this deque) if it is possible to do so 397 * immediately without violating capacity restrictions, returning 398 * {@code true} upon success and throwing an 399 * {@code IllegalStateException} if no space is currently available. 400 * When using a capacity-restricted deque, it is generally preferable to 401 * use {@link #offer(Object) offer}. 402 * 403 * <p>This method is equivalent to {@link #addLast}. 404 * 405 * @param e the element to add 406 * @return {@code true} (as specified by {@link Collection#add}) 407 * @throws IllegalStateException if the element cannot be added at this 408 * time due to capacity restrictions 409 * @throws ClassCastException if the class of the specified element 410 * prevents it from being added to this deque 411 * @throws NullPointerException if the specified element is null and this 412 * deque does not permit null elements 413 * @throws IllegalArgumentException if some property of the specified 414 * element prevents it from being added to this deque 415 */ add(E e)416 boolean add(E e); 417 418 /** 419 * Inserts the specified element into the queue represented by this deque 420 * (in other words, at the tail of this deque) if it is possible to do so 421 * immediately without violating capacity restrictions, returning 422 * {@code true} upon success and {@code false} if no space is currently 423 * available. When using a capacity-restricted deque, this method is 424 * generally preferable to the {@link #add} method, which can fail to 425 * insert an element only by throwing an exception. 426 * 427 * <p>This method is equivalent to {@link #offerLast}. 428 * 429 * @param e the element to add 430 * @return {@code true} if the element was added to this deque, else 431 * {@code false} 432 * @throws ClassCastException if the class of the specified element 433 * prevents it from being added to this deque 434 * @throws NullPointerException if the specified element is null and this 435 * deque does not permit null elements 436 * @throws IllegalArgumentException if some property of the specified 437 * element prevents it from being added to this deque 438 */ offer(E e)439 boolean offer(E e); 440 441 /** 442 * Retrieves and removes the head of the queue represented by this deque 443 * (in other words, the first element of this deque). 444 * This method differs from {@link #poll() poll()} only in that it 445 * throws an exception if this deque is empty. 446 * 447 * <p>This method is equivalent to {@link #removeFirst()}. 448 * 449 * @return the head of the queue represented by this deque 450 * @throws NoSuchElementException if this deque is empty 451 */ remove()452 E remove(); 453 454 /** 455 * Retrieves and removes the head of the queue represented by this deque 456 * (in other words, the first element of this deque), or returns 457 * {@code null} if this deque is empty. 458 * 459 * <p>This method is equivalent to {@link #pollFirst()}. 460 * 461 * @return the first element of this deque, or {@code null} if 462 * this deque is empty 463 */ poll()464 E poll(); 465 466 /** 467 * Retrieves, but does not remove, the head of the queue represented by 468 * this deque (in other words, the first element of this deque). 469 * This method differs from {@link #peek peek} only in that it throws an 470 * exception if this deque is empty. 471 * 472 * <p>This method is equivalent to {@link #getFirst()}. 473 * 474 * @return the head of the queue represented by this deque 475 * @throws NoSuchElementException if this deque is empty 476 */ element()477 E element(); 478 479 /** 480 * Retrieves, but does not remove, the head of the queue represented by 481 * this deque (in other words, the first element of this deque), or 482 * returns {@code null} if this deque is empty. 483 * 484 * <p>This method is equivalent to {@link #peekFirst()}. 485 * 486 * @return the head of the queue represented by this deque, or 487 * {@code null} if this deque is empty 488 */ peek()489 E peek(); 490 491 /** 492 * Adds all of the elements in the specified collection at the end 493 * of this deque, as if by calling {@link #addLast} on each one, 494 * in the order that they are returned by the collection's iterator. 495 * 496 * <p>When using a capacity-restricted deque, it is generally preferable 497 * to call {@link #offer(Object) offer} separately on each element. 498 * 499 * <p>An exception encountered while trying to add an element may result 500 * in only some of the elements having been successfully added when 501 * the associated exception is thrown. 502 * 503 * @param c the elements to be inserted into this deque 504 * @return {@code true} if this deque changed as a result of the call 505 * @throws IllegalStateException if not all the elements can be added at 506 * this time due to insertion restrictions 507 * @throws ClassCastException if the class of an element of the specified 508 * collection prevents it from being added to this deque 509 * @throws NullPointerException if the specified collection contains a 510 * null element and this deque does not permit null elements, 511 * or if the specified collection is null 512 * @throws IllegalArgumentException if some property of an element of the 513 * specified collection prevents it from being added to this deque 514 */ addAll(Collection<? extends E> c)515 boolean addAll(Collection<? extends E> c); 516 517 // *** Stack methods *** 518 519 /** 520 * Pushes an element onto the stack represented by this deque (in other 521 * words, at the head of this deque) if it is possible to do so 522 * immediately without violating capacity restrictions, throwing an 523 * {@code IllegalStateException} if no space is currently available. 524 * 525 * <p>This method is equivalent to {@link #addFirst}. 526 * 527 * @param e the element to push 528 * @throws IllegalStateException if the element cannot be added at this 529 * time due to capacity restrictions 530 * @throws ClassCastException if the class of the specified element 531 * prevents it from being added to this deque 532 * @throws NullPointerException if the specified element is null and this 533 * deque does not permit null elements 534 * @throws IllegalArgumentException if some property of the specified 535 * element prevents it from being added to this deque 536 */ push(E e)537 void push(E e); 538 539 /** 540 * Pops an element from the stack represented by this deque. In other 541 * words, removes and returns the first element of this deque. 542 * 543 * <p>This method is equivalent to {@link #removeFirst()}. 544 * 545 * @return the element at the front of this deque (which is the top 546 * of the stack represented by this deque) 547 * @throws NoSuchElementException if this deque is empty 548 */ pop()549 E pop(); 550 551 552 // *** Collection methods *** 553 554 /** 555 * Removes the first occurrence of the specified element from this deque. 556 * If the deque does not contain the element, it is unchanged. 557 * More formally, removes the first element {@code e} such that 558 * {@code Objects.equals(o, e)} (if such an element exists). 559 * Returns {@code true} if this deque contained the specified element 560 * (or equivalently, if this deque changed as a result of the call). 561 * 562 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}. 563 * 564 * @param o element to be removed from this deque, if present 565 * @return {@code true} if an element was removed as a result of this call 566 * @throws ClassCastException if the class of the specified element 567 * is incompatible with this deque 568 * (<a href="Collection.html#optional-restrictions">optional</a>) 569 * @throws NullPointerException if the specified element is null and this 570 * deque does not permit null elements 571 * (<a href="Collection.html#optional-restrictions">optional</a>) 572 */ remove(Object o)573 boolean remove(Object o); 574 575 /** 576 * Returns {@code true} if this deque contains the specified element. 577 * More formally, returns {@code true} if and only if this deque contains 578 * at least one element {@code e} such that {@code Objects.equals(o, e)}. 579 * 580 * @param o element whose presence in this deque is to be tested 581 * @return {@code true} if this deque contains the specified element 582 * @throws ClassCastException if the class of the specified element 583 * is incompatible with this deque 584 * (<a href="Collection.html#optional-restrictions">optional</a>) 585 * @throws NullPointerException if the specified element is null and this 586 * deque does not permit null elements 587 * (<a href="Collection.html#optional-restrictions">optional</a>) 588 */ contains(Object o)589 boolean contains(Object o); 590 591 /** 592 * Returns the number of elements in this deque. 593 * 594 * @return the number of elements in this deque 595 */ size()596 int size(); 597 598 /** 599 * Returns an iterator over the elements in this deque in proper sequence. 600 * The elements will be returned in order from first (head) to last (tail). 601 * 602 * @return an iterator over the elements in this deque in proper sequence 603 */ iterator()604 Iterator<E> iterator(); 605 606 /** 607 * Returns an iterator over the elements in this deque in reverse 608 * sequential order. The elements will be returned in order from 609 * last (tail) to first (head). 610 * 611 * @return an iterator over the elements in this deque in reverse 612 * sequence 613 */ descendingIterator()614 Iterator<E> descendingIterator(); 615 616 } 617