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 /** 39 * A linear collection that supports element insertion and removal at 40 * both ends. The name <i>deque</i> is short for "double ended queue" 41 * and is usually pronounced "deck". Most {@code Deque} 42 * implementations place no fixed limits on the number of elements 43 * they may contain, but this interface supports capacity-restricted 44 * deques as well as those with no fixed size limit. 45 * 46 * <p>This interface defines methods to access the elements at both 47 * ends of the deque. Methods are provided to insert, remove, and 48 * examine the element. Each of these methods exists in two forms: 49 * one throws an exception if the operation fails, the other returns a 50 * special value (either {@code null} or {@code false}, depending on 51 * the operation). The latter form of the insert operation is 52 * designed specifically for use with capacity-restricted 53 * {@code Deque} implementations; in most implementations, insert 54 * operations cannot fail. 55 * 56 * <p>The twelve methods described above are summarized in the 57 * following table: 58 * 59 * <table BORDER CELLPADDING=3 CELLSPACING=1> 60 * <caption>Summary of Deque methods</caption> 61 * <tr> 62 * <td></td> 63 * <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td> 64 * <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td> 65 * </tr> 66 * <tr> 67 * <td></td> 68 * <td ALIGN=CENTER><em>Throws exception</em></td> 69 * <td ALIGN=CENTER><em>Special value</em></td> 70 * <td ALIGN=CENTER><em>Throws exception</em></td> 71 * <td ALIGN=CENTER><em>Special value</em></td> 72 * </tr> 73 * <tr> 74 * <td><b>Insert</b></td> 75 * <td>{@link Deque#addFirst addFirst(e)}</td> 76 * <td>{@link Deque#offerFirst offerFirst(e)}</td> 77 * <td>{@link Deque#addLast addLast(e)}</td> 78 * <td>{@link Deque#offerLast offerLast(e)}</td> 79 * </tr> 80 * <tr> 81 * <td><b>Remove</b></td> 82 * <td>{@link Deque#removeFirst removeFirst()}</td> 83 * <td>{@link Deque#pollFirst pollFirst()}</td> 84 * <td>{@link Deque#removeLast removeLast()}</td> 85 * <td>{@link Deque#pollLast pollLast()}</td> 86 * </tr> 87 * <tr> 88 * <td><b>Examine</b></td> 89 * <td>{@link Deque#getFirst getFirst()}</td> 90 * <td>{@link Deque#peekFirst peekFirst()}</td> 91 * <td>{@link Deque#getLast getLast()}</td> 92 * <td>{@link Deque#peekLast peekLast()}</td> 93 * </tr> 94 * </table> 95 * 96 * <p>This interface extends the {@link Queue} interface. When a deque is 97 * used as a queue, FIFO (First-In-First-Out) behavior results. Elements are 98 * added at the end of the deque and removed from the beginning. The methods 99 * inherited from the {@code Queue} interface are precisely equivalent to 100 * {@code Deque} methods as indicated in the following table: 101 * 102 * <table BORDER CELLPADDING=3 CELLSPACING=1> 103 * <caption>Comparison of Queue and Deque methods</caption> 104 * <tr> 105 * <td ALIGN=CENTER> <b>{@code Queue} Method</b></td> 106 * <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td> 107 * </tr> 108 * <tr> 109 * <td>{@link java.util.Queue#add add(e)}</td> 110 * <td>{@link #addLast addLast(e)}</td> 111 * </tr> 112 * <tr> 113 * <td>{@link java.util.Queue#offer offer(e)}</td> 114 * <td>{@link #offerLast offerLast(e)}</td> 115 * </tr> 116 * <tr> 117 * <td>{@link java.util.Queue#remove remove()}</td> 118 * <td>{@link #removeFirst removeFirst()}</td> 119 * </tr> 120 * <tr> 121 * <td>{@link java.util.Queue#poll poll()}</td> 122 * <td>{@link #pollFirst pollFirst()}</td> 123 * </tr> 124 * <tr> 125 * <td>{@link java.util.Queue#element element()}</td> 126 * <td>{@link #getFirst getFirst()}</td> 127 * </tr> 128 * <tr> 129 * <td>{@link java.util.Queue#peek peek()}</td> 130 * <td>{@link #peek peekFirst()}</td> 131 * </tr> 132 * </table> 133 * 134 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This 135 * interface should be used in preference to the legacy {@link Stack} class. 136 * When a deque is used as a stack, elements are pushed and popped from the 137 * beginning of the deque. Stack methods are precisely equivalent to 138 * {@code Deque} methods as indicated in the table below: 139 * 140 * <table BORDER CELLPADDING=3 CELLSPACING=1> 141 * <caption>Comparison of Stack and Deque methods</caption> 142 * <tr> 143 * <td ALIGN=CENTER> <b>Stack Method</b></td> 144 * <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td> 145 * </tr> 146 * <tr> 147 * <td>{@link #push push(e)}</td> 148 * <td>{@link #addFirst addFirst(e)}</td> 149 * </tr> 150 * <tr> 151 * <td>{@link #pop pop()}</td> 152 * <td>{@link #removeFirst removeFirst()}</td> 153 * </tr> 154 * <tr> 155 * <td>{@link #peek peek()}</td> 156 * <td>{@link #peekFirst peekFirst()}</td> 157 * </tr> 158 * </table> 159 * 160 * <p>Note that the {@link #peek peek} method works equally well when 161 * a deque is used as a queue or a stack; in either case, elements are 162 * drawn from the beginning of the deque. 163 * 164 * <p>This interface provides two methods to remove interior 165 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and 166 * {@link #removeLastOccurrence removeLastOccurrence}. 167 * 168 * <p>Unlike the {@link List} interface, this interface does not 169 * provide support for indexed access to elements. 170 * 171 * <p>While {@code Deque} implementations are not strictly required 172 * to prohibit the insertion of null elements, they are strongly 173 * encouraged to do so. Users of any {@code Deque} implementations 174 * that do allow null elements are strongly encouraged <i>not</i> to 175 * take advantage of the ability to insert nulls. This is so because 176 * {@code null} is used as a special return value by various methods 177 * to indicated that the deque is empty. 178 * 179 * <p>{@code Deque} implementations generally do not define 180 * element-based versions of the {@code equals} and {@code hashCode} 181 * methods, but instead inherit the identity-based versions from class 182 * {@code Object}. 183 * 184 * <p>This interface is a member of the <a 185 * href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections 186 * Framework</a>. 187 * 188 * @author Doug Lea 189 * @author Josh Bloch 190 * @since 1.6 191 * @param <E> the type of elements held in this collection 192 */ 193 public interface Deque<E> extends Queue<E> { 194 /** 195 * Inserts the specified element at the front of this deque if it is 196 * possible to do so immediately without violating capacity restrictions, 197 * throwing an {@code IllegalStateException} if no space is currently 198 * available. When using a capacity-restricted deque, it is generally 199 * preferable to use method {@link #offerFirst}. 200 * 201 * @param e the element to add 202 * @throws IllegalStateException if the element cannot be added at this 203 * time due to capacity restrictions 204 * @throws ClassCastException if the class of the specified element 205 * prevents it from being added to this deque 206 * @throws NullPointerException if the specified element is null and this 207 * deque does not permit null elements 208 * @throws IllegalArgumentException if some property of the specified 209 * element prevents it from being added to this deque 210 */ addFirst(E e)211 void addFirst(E e); 212 213 /** 214 * Inserts the specified element at the end of this deque if it is 215 * possible to do so immediately without violating capacity restrictions, 216 * throwing an {@code IllegalStateException} if no space is currently 217 * available. When using a capacity-restricted deque, it is generally 218 * preferable to use method {@link #offerLast}. 219 * 220 * <p>This method is equivalent to {@link #add}. 221 * 222 * @param e the element to add 223 * @throws IllegalStateException if the element cannot be added at this 224 * time due to capacity restrictions 225 * @throws ClassCastException if the class of the specified element 226 * prevents it from being added to this deque 227 * @throws NullPointerException if the specified element is null and this 228 * deque does not permit null elements 229 * @throws IllegalArgumentException if some property of the specified 230 * element prevents it from being added to this deque 231 */ addLast(E e)232 void addLast(E e); 233 234 /** 235 * Inserts the specified element at the front of this deque unless it would 236 * violate capacity restrictions. When using a capacity-restricted deque, 237 * this method is generally preferable to the {@link #addFirst} method, 238 * which can fail to insert an element only by throwing an exception. 239 * 240 * @param e the element to add 241 * @return {@code true} if the element was added to this deque, else 242 * {@code false} 243 * @throws ClassCastException if the class of the specified element 244 * prevents it from being added to this deque 245 * @throws NullPointerException if the specified element is null and this 246 * deque does not permit null elements 247 * @throws IllegalArgumentException if some property of the specified 248 * element prevents it from being added to this deque 249 */ offerFirst(E e)250 boolean offerFirst(E e); 251 252 /** 253 * Inserts the specified element at the end of this deque unless it would 254 * violate capacity restrictions. When using a capacity-restricted deque, 255 * this method is generally preferable to the {@link #addLast} method, 256 * which can fail to insert an element only by throwing an exception. 257 * 258 * @param e the element to add 259 * @return {@code true} if the element was added to this deque, else 260 * {@code false} 261 * @throws ClassCastException if the class of the specified element 262 * prevents it from being added to this deque 263 * @throws NullPointerException if the specified element is null and this 264 * deque does not permit null elements 265 * @throws IllegalArgumentException if some property of the specified 266 * element prevents it from being added to this deque 267 */ offerLast(E e)268 boolean offerLast(E e); 269 270 /** 271 * Retrieves and removes the first element of this deque. This method 272 * differs from {@link #pollFirst pollFirst} only in that it throws an 273 * exception if this deque is empty. 274 * 275 * @return the head of this deque 276 * @throws NoSuchElementException if this deque is empty 277 */ removeFirst()278 E removeFirst(); 279 280 /** 281 * Retrieves and removes the last element of this deque. This method 282 * differs from {@link #pollLast pollLast} only in that it throws an 283 * exception if this deque is empty. 284 * 285 * @return the tail of this deque 286 * @throws NoSuchElementException if this deque is empty 287 */ removeLast()288 E removeLast(); 289 290 /** 291 * Retrieves and removes the first element of this deque, 292 * or returns {@code null} if this deque is empty. 293 * 294 * @return the head of this deque, or {@code null} if this deque is empty 295 */ pollFirst()296 E pollFirst(); 297 298 /** 299 * Retrieves and removes the last element of this deque, 300 * or returns {@code null} if this deque is empty. 301 * 302 * @return the tail of this deque, or {@code null} if this deque is empty 303 */ pollLast()304 E pollLast(); 305 306 /** 307 * Retrieves, but does not remove, the first element of this deque. 308 * 309 * This method differs from {@link #peekFirst peekFirst} only in that it 310 * throws an exception if this deque is empty. 311 * 312 * @return the head of this deque 313 * @throws NoSuchElementException if this deque is empty 314 */ getFirst()315 E getFirst(); 316 317 /** 318 * Retrieves, but does not remove, the last element of this deque. 319 * This method differs from {@link #peekLast peekLast} only in that it 320 * throws an exception if this deque is empty. 321 * 322 * @return the tail of this deque 323 * @throws NoSuchElementException if this deque is empty 324 */ getLast()325 E getLast(); 326 327 /** 328 * Retrieves, but does not remove, the first element of this deque, 329 * or returns {@code null} if this deque is empty. 330 * 331 * @return the head of this deque, or {@code null} if this deque is empty 332 */ peekFirst()333 E peekFirst(); 334 335 /** 336 * Retrieves, but does not remove, the last element of this deque, 337 * or returns {@code null} if this deque is empty. 338 * 339 * @return the tail of this deque, or {@code null} if this deque is empty 340 */ peekLast()341 E peekLast(); 342 343 /** 344 * Removes the first occurrence of the specified element from this deque. 345 * If the deque does not contain the element, it is unchanged. 346 * More formally, removes the first element {@code e} such that 347 * <tt>(o==null ? e==null : o.equals(e))</tt> 348 * (if such an element exists). 349 * Returns {@code true} 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 {@code true} if an element was removed as a result of this call 354 * @throws ClassCastException if the class of the specified element 355 * is incompatible with this deque 356 * (<a href="Collection.html#optional-restrictions">optional</a>) 357 * @throws NullPointerException if the specified element is null and this 358 * deque does not permit null elements 359 * (<a href="Collection.html#optional-restrictions">optional</a>) 360 */ removeFirstOccurrence(Object o)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 376 * (<a href="Collection.html#optional-restrictions">optional</a>) 377 * @throws NullPointerException if the specified element is null and this 378 * deque does not permit null elements 379 * (<a href="Collection.html#optional-restrictions">optional</a>) 380 */ removeLastOccurrence(Object o)381 boolean removeLastOccurrence(Object o); 382 383 // *** Queue methods *** 384 385 /** 386 * Inserts the specified element into the queue represented by this deque 387 * (in other words, at the tail of this deque) if it is possible to do so 388 * immediately without violating capacity restrictions, returning 389 * {@code true} upon success and throwing an 390 * {@code IllegalStateException} if no space is currently available. 391 * When using a capacity-restricted deque, it is generally preferable to 392 * use {@link #offer(Object) offer}. 393 * 394 * <p>This method is equivalent to {@link #addLast}. 395 * 396 * @param e the element to add 397 * @return {@code true} (as specified by {@link Collection#add}) 398 * @throws IllegalStateException if the element cannot be added at this 399 * time due to capacity restrictions 400 * @throws ClassCastException if the class of the specified element 401 * prevents it from being added to this deque 402 * @throws NullPointerException if the specified element is null and this 403 * deque does not permit null elements 404 * @throws IllegalArgumentException if some property of the specified 405 * element prevents it from being added to this deque 406 */ add(E e)407 boolean add(E e); 408 409 /** 410 * Inserts the specified element into the queue represented by this deque 411 * (in other words, at the tail of this deque) if it is possible to do so 412 * immediately without violating capacity restrictions, returning 413 * {@code true} upon success and {@code false} if no space is currently 414 * available. When using a capacity-restricted deque, this method is 415 * generally preferable to the {@link #add} method, which can fail to 416 * insert an element only by throwing an exception. 417 * 418 * <p>This method is equivalent to {@link #offerLast}. 419 * 420 * @param e the element to add 421 * @return {@code true} if the element was added to this deque, else 422 * {@code false} 423 * @throws ClassCastException if the class of the specified element 424 * prevents it from being added to this deque 425 * @throws NullPointerException if the specified element is null and this 426 * deque does not permit null elements 427 * @throws IllegalArgumentException if some property of the specified 428 * element prevents it from being added to this deque 429 */ offer(E e)430 boolean offer(E e); 431 432 /** 433 * Retrieves and removes the head of the queue represented by this deque 434 * (in other words, the first element of this deque). 435 * This method differs from {@link #poll poll} only in that it throws an 436 * exception if this deque is empty. 437 * 438 * <p>This method is equivalent to {@link #removeFirst()}. 439 * 440 * @return the head of the queue represented by this deque 441 * @throws NoSuchElementException if this deque is empty 442 */ remove()443 E remove(); 444 445 /** 446 * Retrieves and removes the head of the queue represented by this deque 447 * (in other words, the first element of this deque), or returns 448 * {@code null} if this deque is empty. 449 * 450 * <p>This method is equivalent to {@link #pollFirst()}. 451 * 452 * @return the first element of this deque, or {@code null} if 453 * this deque is empty 454 */ poll()455 E poll(); 456 457 /** 458 * Retrieves, but does not remove, the head of the queue represented by 459 * this deque (in other words, the first element of this deque). 460 * This method differs from {@link #peek peek} only in that it throws an 461 * exception if this deque is empty. 462 * 463 * <p>This method is equivalent to {@link #getFirst()}. 464 * 465 * @return the head of the queue represented by this deque 466 * @throws NoSuchElementException if this deque is empty 467 */ element()468 E element(); 469 470 /** 471 * Retrieves, but does not remove, the head of the queue represented by 472 * this deque (in other words, the first element of this deque), or 473 * returns {@code null} 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 * {@code null} if this deque is empty 479 */ peek()480 E peek(); 481 482 483 // *** Stack methods *** 484 485 /** 486 * Pushes an element onto the stack represented by this deque (in other 487 * words, at the head of this deque) if it is possible to do so 488 * immediately without violating capacity restrictions, throwing an 489 * {@code IllegalStateException} if no space is currently available. 490 * 491 * <p>This method is equivalent to {@link #addFirst}. 492 * 493 * @param e the element to push 494 * @throws IllegalStateException if the element cannot be added at this 495 * time due to capacity restrictions 496 * @throws ClassCastException if the class of the specified element 497 * prevents it from being added to this deque 498 * @throws NullPointerException if the specified element is null and this 499 * deque does not permit null elements 500 * @throws IllegalArgumentException if some property of the specified 501 * element prevents it from being added to this deque 502 */ push(E e)503 void push(E e); 504 505 /** 506 * Pops an element from the stack represented by this deque. In other 507 * words, removes and returns the first element of this deque. 508 * 509 * <p>This method is equivalent to {@link #removeFirst()}. 510 * 511 * @return the element at the front of this deque (which is the top 512 * of the stack represented by this deque) 513 * @throws NoSuchElementException if this deque is empty 514 */ pop()515 E pop(); 516 517 518 // *** Collection methods *** 519 520 /** 521 * Removes the first occurrence of the specified element from this deque. 522 * If the deque does not contain the element, it is unchanged. 523 * More formally, removes the first element {@code e} such that 524 * <tt>(o==null ? e==null : o.equals(e))</tt> 525 * (if such an element exists). 526 * Returns {@code true} if this deque contained the specified element 527 * (or equivalently, if this deque changed as a result of the call). 528 * 529 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}. 530 * 531 * @param o element to be removed from this deque, if present 532 * @return {@code true} if an element was removed as a result of this call 533 * @throws ClassCastException if the class of the specified element 534 * is incompatible with this deque 535 * (<a href="Collection.html#optional-restrictions">optional</a>) 536 * @throws NullPointerException if the specified element is null and this 537 * deque does not permit null elements 538 * (<a href="Collection.html#optional-restrictions">optional</a>) 539 */ remove(Object o)540 boolean remove(Object o); 541 542 /** 543 * Returns {@code true} if this deque contains the specified element. 544 * More formally, returns {@code true} if and only if this deque contains 545 * at least one element {@code e} such that 546 * <tt>(o==null ? e==null : o.equals(e))</tt>. 547 * 548 * @param o element whose presence in this deque is to be tested 549 * @return {@code true} if this deque contains the specified element 550 * @throws ClassCastException if the type of the specified element 551 * is incompatible with this deque 552 * (<a href="Collection.html#optional-restrictions">optional</a>) 553 * @throws NullPointerException if the specified element is null and this 554 * deque does not permit null elements 555 * (<a href="Collection.html#optional-restrictions">optional</a>) 556 */ contains(Object o)557 boolean contains(Object o); 558 559 /** 560 * Returns the number of elements in this deque. 561 * 562 * @return the number of elements in this deque 563 */ size()564 public int size(); 565 566 /** 567 * Returns an iterator over the elements in this deque in proper sequence. 568 * The elements will be returned in order from first (head) to last (tail). 569 * 570 * @return an iterator over the elements in this deque in proper sequence 571 */ iterator()572 Iterator<E> iterator(); 573 574 /** 575 * Returns an iterator over the elements in this deque in reverse 576 * sequential order. The elements will be returned in order from 577 * last (tail) to first (head). 578 * 579 * @return an iterator over the elements in this deque in reverse 580 * sequence 581 */ descendingIterator()582 Iterator<E> descendingIterator(); 583 584 } 585