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 <tt>Deque</tt>
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 <tt>null</tt> or <tt>false</tt>, depending on
51 * the operation). The latter form of the insert operation is
52 * designed specifically for use with capacity-restricted
53 * <tt>Deque</tt> 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 * <p>
60 * <table BORDER CELLPADDING=3 CELLSPACING=1>
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 #addFirst addFirst(e)}</td>
76 * <td>{@link #offerFirst offerFirst(e)}</td>
77 * <td>{@link #addLast addLast(e)}</td>
78 * <td>{@link #offerLast offerLast(e)}</td>
79 * </tr>
80 * <tr>
81 * <td><b>Remove</b></td>
82 * <td>{@link #removeFirst removeFirst()}</td>
83 * <td>{@link #pollFirst pollFirst()}</td>
84 * <td>{@link #removeLast removeLast()}</td>
85 * <td>{@link #pollLast pollLast()}</td>
86 * </tr>
87 * <tr>
88 * <td><b>Examine</b></td>
89 * <td>{@link #getFirst getFirst()}</td>
90 * <td>{@link #peekFirst peekFirst()}</td>
91 * <td>{@link #getLast getLast()}</td>
92 * <td>{@link #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 <tt>Queue</tt> interface are precisely equivalent to
100 * <tt>Deque</tt> methods as indicated in the following table:
101 *
102 * <p>
103 * <table BORDER CELLPADDING=3 CELLSPACING=1>
104 * <tr>
105 * <td ALIGN=CENTER> <b><tt>Queue</tt> Method</b></td>
106 * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> 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 * <tt>Deque</tt> methods as indicated in the table below:
139 *
140 * <p>
141 * <table BORDER CELLPADDING=3 CELLSPACING=1>
142 * <tr>
143 * <td ALIGN=CENTER> <b>Stack Method</b></td>
144 * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> 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 <tt>Deque</tt> implementations are not strictly required
172 * to prohibit the insertion of null elements, they are strongly
173 * encouraged to do so. Users of any <tt>Deque</tt> 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 * <tt>null</tt> is used as a special return value by various methods
177 * to indicated that the deque is empty.
178 *
179 * <p><tt>Deque</tt> implementations generally do not define
180 * element-based versions of the <tt>equals</tt> and <tt>hashCode</tt>
181 * methods, but instead inherit the identity-based versions from class
182 * <tt>Object</tt>.
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
194 public interface Deque<E> extends Queue<E> {
195 /**
196 * Inserts the specified element at the front of this deque if it is
197 * possible to do so immediately without violating capacity restrictions.
198 * When using a capacity-restricted deque, it is generally preferable to
199 * 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 */
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 * When using a capacity-restricted deque, it is generally preferable to
217 * 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 */
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 <tt>true</tt> if the element was added to this deque, else
241 * <tt>false</tt>
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 */
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 <tt>true</tt> if the element was added to this deque, else
259 * <tt>false</tt>
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 */
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 */
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 */
287 E removeLast();
288
289 /**
290 * Retrieves and removes the first element of this deque,
291 * or returns <tt>null</tt> if this deque is empty.
292 *
293 * @return the head of this deque, or <tt>null</tt> if this deque is empty
294 */
295 E pollFirst();
296
297 /**
298 * Retrieves and removes the last element of this deque,
299 * or returns <tt>null</tt> if this deque is empty.
300 *
301 * @return the tail of this deque, or <tt>null</tt> if this deque is empty
302 */
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 */
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 */
324 E getLast();
325
326 /**
327 * Retrieves, but does not remove, the first element of this deque,
328 * or returns <tt>null</tt> if this deque is empty.
329 *
330 * @return the head of this deque, or <tt>null</tt> if this deque is empty
331 */
332 E peekFirst();
333
334 /**
335 * Retrieves, but does not remove, the last element of this deque,
336 * or returns <tt>null</tt> if this deque is empty.
337 *
338 * @return the tail of this deque, or <tt>null</tt> if this deque is empty
339 */
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 <tt>e</tt> such that
346 * <tt>(o==null ? e==null : o.equals(e))</tt>
347 * (if such an element exists).
348 * Returns <tt>true</tt> if this deque contained the specified element
349 * (or equivalently, if this deque changed as a result of the call).
350 *
351 * @param o element to be removed from this deque, if present
352 * @return <tt>true</tt> if an element was removed as a result of this call
353 * @throws ClassCastException if the class of the specified element
354 * is incompatible with this deque (optional)
355 * @throws NullPointerException if the specified element is null and this
356 * deque does not permit null elements (optional)
357 */
358 boolean removeFirstOccurrence(Object o);
359
360 /**
361 * Removes the last occurrence of the specified element from this deque.
362 * If the deque does not contain the element, it is unchanged.
363 * More formally, removes the last element <tt>e</tt> such that
364 * <tt>(o==null ? e==null : o.equals(e))</tt>
365 * (if such an element exists).
366 * Returns <tt>true</tt> 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 <tt>true</tt> 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 (optional)
373 * @throws NullPointerException if the specified element is null and this
374 * deque does not permit null elements (optional)
375 */
376 boolean removeLastOccurrence(Object o);
377
378 // *** Queue methods ***
379
380 /**
381 * Inserts the specified element into the queue represented by this deque
382 * (in other words, at the tail of this deque) if it is possible to do so
383 * immediately without violating capacity restrictions, returning
384 * <tt>true</tt> upon success and throwing an
385 * <tt>IllegalStateException</tt> if no space is currently available.
386 * When using a capacity-restricted deque, it is generally preferable to
387 * use {@link #offer(Object) offer}.
388 *
389 * <p>This method is equivalent to {@link #addLast}.
390 *
391 * @param e the element to add
392 * @return <tt>true</tt> (as specified by {@link Collection#add})
393 * @throws IllegalStateException if the element cannot be added at this
394 * time due to capacity restrictions
395 * @throws ClassCastException if the class of the specified element
396 * prevents it from being added to this deque
397 * @throws NullPointerException if the specified element is null and this
398 * deque does not permit null elements
399 * @throws IllegalArgumentException if some property of the specified
400 * element prevents it from being added to this deque
401 */
402 boolean add(E e);
403
404 /**
405 * Inserts the specified element into the queue represented by this deque
406 * (in other words, at the tail of this deque) if it is possible to do so
407 * immediately without violating capacity restrictions, returning
408 * <tt>true</tt> upon success and <tt>false</tt> if no space is currently
409 * available. When using a capacity-restricted deque, this method is
410 * generally preferable to the {@link #add} method, which can fail to
411 * insert an element only by throwing an exception.
412 *
413 * <p>This method is equivalent to {@link #offerLast}.
414 *
415 * @param e the element to add
416 * @return <tt>true</tt> if the element was added to this deque, else
417 * <tt>false</tt>
418 * @throws ClassCastException if the class of the specified element
419 * prevents it from being added to this deque
420 * @throws NullPointerException if the specified element is null and this
421 * deque does not permit null elements
422 * @throws IllegalArgumentException if some property of the specified
423 * element prevents it from being added to this deque
424 */
425 boolean offer(E e);
426
427 /**
428 * Retrieves and removes the head of the queue represented by this deque
429 * (in other words, the first element of this deque).
430 * This method differs from {@link #poll poll} only in that it throws an
431 * exception if this deque is empty.
432 *
433 * <p>This method is equivalent to {@link #removeFirst()}.
434 *
435 * @return the head of the queue represented by this deque
436 * @throws NoSuchElementException if this deque is empty
437 */
438 E remove();
439
440 /**
441 * Retrieves and removes the head of the queue represented by this deque
442 * (in other words, the first element of this deque), or returns
443 * <tt>null</tt> if this deque is empty.
444 *
445 * <p>This method is equivalent to {@link #pollFirst()}.
446 *
447 * @return the first element of this deque, or <tt>null</tt> if
448 * this deque is empty
449 */
450 E poll();
451
452 /**
453 * Retrieves, but does not remove, the head of the queue represented by
454 * this deque (in other words, the first element of this deque).
455 * This method differs from {@link #peek peek} only in that it throws an
456 * exception if this deque is empty.
457 *
458 * <p>This method is equivalent to {@link #getFirst()}.
459 *
460 * @return the head of the queue represented by this deque
461 * @throws NoSuchElementException if this deque is empty
462 */
463 E element();
464
465 /**
466 * Retrieves, but does not remove, the head of the queue represented by
467 * this deque (in other words, the first element of this deque), or
468 * returns <tt>null</tt> if this deque is empty.
469 *
470 * <p>This method is equivalent to {@link #peekFirst()}.
471 *
472 * @return the head of the queue represented by this deque, or
473 * <tt>null</tt> if this deque is empty
474 */
475 E peek();
476
477
478 // *** Stack methods ***
479
480 /**
481 * Pushes an element onto the stack represented by this deque (in other
482 * words, at the head of this deque) if it is possible to do so
483 * immediately without violating capacity restrictions, returning
484 * <tt>true</tt> upon success and throwing an
485 * <tt>IllegalStateException</tt> if no space is currently available.
486 *
487 * <p>This method is equivalent to {@link #addFirst}.
488 *
489 * @param e the element to push
490 * @throws IllegalStateException if the element cannot be added at this
491 * time due to capacity restrictions
492 * @throws ClassCastException if the class of the specified element
493 * prevents it from being added to this deque
494 * @throws NullPointerException if the specified element is null and this
495 * deque does not permit null elements
496 * @throws IllegalArgumentException if some property of the specified
497 * element prevents it from being added to this deque
498 */
499 void push(E e);
500
501 /**
502 * Pops an element from the stack represented by this deque. In other
503 * words, removes and returns the first element of this deque.
504 *
505 * <p>This method is equivalent to {@link #removeFirst()}.
506 *
507 * @return the element at the front of this deque (which is the top
508 * of the stack represented by this deque)
509 * @throws NoSuchElementException if this deque is empty
510 */
511 E pop();
512
513
514 // *** Collection methods ***
515
516 /**
517 * Removes the first occurrence of the specified element from this deque.
518 * If the deque does not contain the element, it is unchanged.
519 * More formally, removes the first element <tt>e</tt> such that
520 * <tt>(o==null ? e==null : o.equals(e))</tt>
521 * (if such an element exists).
522 * Returns <tt>true</tt> 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}.
526 *
527 * @param o element to be removed from this deque, if present
528 * @return <tt>true</tt> 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 (optional)
531 * @throws NullPointerException if the specified element is null and this
532 * deque does not permit null elements (optional)
533 */
534 boolean remove(Object o);
535
536 /**
537 * Returns <tt>true</tt> if this deque contains the specified element.
538 * More formally, returns <tt>true</tt> if and only if this deque contains
539 * at least one element <tt>e</tt> such that
540 * <tt>(o==null ? e==null : o.equals(e))</tt>.
541 *
542 * @param o element whose presence in this deque is to be tested
543 * @return <tt>true</tt> if this deque contains the specified element
544 * @throws ClassCastException if the type of the specified element
545 * is incompatible with this deque (optional)
546 * @throws NullPointerException if the specified element is null and this
547 * deque does not permit null elements (optional)
548 */
549 boolean contains(Object o);
550
551 /**
552 * Returns the number of elements in this deque.
553 *
554 * @return the number of elements in this deque
555 */
556 public int size();
557
558 /**
559 * Returns an iterator over the elements in this deque in proper sequence.
560 * The elements will be returned in order from first (head) to last (tail).
561 *
562 * @return an iterator over the elements in this deque in proper sequence
563 */
564 Iterator<E> iterator();
565
566 /**
567 * Returns an iterator over the elements in this deque in reverse
568 * sequential order. The elements will be returned in order from
569 * last (tail) to first (head).
570 *
571 * @return an iterator over the elements in this deque in reverse
572 * sequence
573 */
574 Iterator<E> descendingIterator();
575
576 }