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 class="plain">
60 * <caption>Summary of Deque methods</caption>
61 * <tr>
62 * <td></td>
63 * <td style="text-align:center" COLSPAN = 2> <b>First Element (Head)</b></td>
64 * <td style="text-align:center" COLSPAN = 2> <b>Last Element (Tail)</b></td>
65 * </tr>
66 * <tr>
67 * <td></td>
68 * <td style="text-align:center"><em>Throws exception</em></td>
69 * <td style="text-align:center"><em>Special value</em></td>
70 * <td style="text-align:center"><em>Throws exception</em></td>
71 * <td style="text-align:center"><em>Special value</em></td>
72 * </tr>
73 * <tr>
74 * <td><b>Insert</b></td>
75 * <td>{@link #addFirst(Object) addFirst(e)}</td>
76 * <td>{@link #offerFirst(Object) offerFirst(e)}</td>
77 * <td>{@link #addLast(Object) addLast(e)}</td>
78 * <td>{@link #offerLast(Object) 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 {@code Queue} interface are precisely equivalent to
100 * {@code Deque} methods as indicated in the following table:
101 *
102 * <table class="plain">
103 * <caption>Comparison of Queue and Deque methods</caption>
104 * <tr>
105 * <td style="text-align:center"> <b>{@code Queue} Method</b></td>
106 * <td style="text-align:center"> <b>Equivalent {@code Deque} Method</b></td>
107 * </tr>
108 * <tr>
109 * <td>{@link #add(Object) add(e)}</td>
110 * <td>{@link #addLast(Object) addLast(e)}</td>
111 * </tr>
112 * <tr>
113 * <td>{@link #offer(Object) offer(e)}</td>
114 * <td>{@link #offerLast(Object) offerLast(e)}</td>
115 * </tr>
116 * <tr>
117 * <td>{@link #remove() remove()}</td>
118 * <td>{@link #removeFirst() removeFirst()}</td>
119 * </tr>
120 * <tr>
121 * <td>{@link #poll() poll()}</td>
122 * <td>{@link #pollFirst() pollFirst()}</td>
123 * </tr>
124 * <tr>
125 * <td>{@link #element() element()}</td>
126 * <td>{@link #getFirst() getFirst()}</td>
127 * </tr>
128 * <tr>
129 * <td>{@link #peek() peek()}</td>
130 * <td>{@link #peekFirst() 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 class="plain">
141 * <caption>Comparison of Stack and Deque methods</caption>
142 * <tr>
143 * <td style="text-align:center"> <b>Stack Method</b></td>
144 * <td style="text-align:center"> <b>Equivalent {@code Deque} Method</b></td>
145 * </tr>
146 * <tr>
147 * <td>{@link #push(Object) push(e)}</td>
148 * <td>{@link #addFirst(Object) 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 indicate 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
185 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
186 * Java Collections 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 deque
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 * {@code Objects.equals(o, e)} (if such an element exists).
348 * Returns {@code true} 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 {@code true} 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
355 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
356 * @throws NullPointerException if the specified element is null and this
357 * deque does not permit null elements
358 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
359 */
360 boolean removeFirstOccurrence(Object o);
361
362 /**
363 * Removes the last occurrence of the specified element from this deque.
364 * If the deque does not contain the element, it is unchanged.
365 * More formally, removes the last element {@code e} such that
366 * {@code Objects.equals(o, e)} (if such an element exists).
367 * Returns {@code true} if this deque contained the specified element
368 * (or equivalently, if this deque changed as a result of the call).
369 *
370 * @param o element to be removed from this deque, if present
371 * @return {@code true} if an element was removed as a result of this call
372 * @throws ClassCastException if the class of the specified element
373 * is incompatible with this deque
374 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
375 * @throws NullPointerException if the specified element is null and this
376 * deque does not permit null elements
377 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
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
434 * throws an 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 * Adds all of the elements in the specified collection at the end
482 * of this deque, as if by calling {@link #addLast} on each one,
483 * in the order that they are returned by the collection's iterator.
484 *
485 * <p>When using a capacity-restricted deque, it is generally preferable
486 * to call {@link #offer(Object) offer} separately on each element.
487 *
488 * <p>An exception encountered while trying to add an element may result
489 * in only some of the elements having been successfully added when
490 * the associated exception is thrown.
491 *
492 * @param c the elements to be inserted into this deque
493 * @return {@code true} if this deque changed as a result of the call
494 * @throws IllegalStateException if not all the elements can be added at
495 * this time due to insertion restrictions
496 * @throws ClassCastException if the class of an element of the specified
497 * collection prevents it from being added to this deque
498 * @throws NullPointerException if the specified collection contains a
499 * null element and this deque does not permit null elements,
500 * or if the specified collection is null
501 * @throws IllegalArgumentException if some property of an element of the
502 * specified collection prevents it from being added to this deque
503 */
504 boolean addAll(Collection<? extends E> c);
505
506 // *** Stack methods ***
507
508 /**
509 * Pushes an element onto the stack represented by this deque (in other
510 * words, at the head of this deque) if it is possible to do so
511 * immediately without violating capacity restrictions, throwing an
512 * {@code IllegalStateException} if no space is currently available.
513 *
514 * <p>This method is equivalent to {@link #addFirst}.
515 *
516 * @param e the element to push
517 * @throws IllegalStateException if the element cannot be added at this
518 * time due to capacity restrictions
519 * @throws ClassCastException if the class of the specified element
520 * prevents it from being added to this deque
521 * @throws NullPointerException if the specified element is null and this
522 * deque does not permit null elements
523 * @throws IllegalArgumentException if some property of the specified
524 * element prevents it from being added to this deque
525 */
526 void push(E e);
527
528 /**
529 * Pops an element from the stack represented by this deque. In other
530 * words, removes and returns the first element of this deque.
531 *
532 * <p>This method is equivalent to {@link #removeFirst()}.
533 *
534 * @return the element at the front of this deque (which is the top
535 * of the stack represented by this deque)
536 * @throws NoSuchElementException if this deque is empty
537 */
538 E pop();
539
540
541 // *** Collection methods ***
542
543 /**
544 * Removes the first occurrence of the specified element from this deque.
545 * If the deque does not contain the element, it is unchanged.
546 * More formally, removes the first element {@code e} such that
547 * {@code Objects.equals(o, e)} (if such an element exists).
548 * Returns {@code true} if this deque contained the specified element
549 * (or equivalently, if this deque changed as a result of the call).
550 *
551 * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
552 *
553 * @param o element to be removed from this deque, if present
554 * @return {@code true} if an element was removed as a result of this call
555 * @throws ClassCastException if the class of the specified element
556 * is incompatible with this deque
557 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
558 * @throws NullPointerException if the specified element is null and this
559 * deque does not permit null elements
560 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
561 */
562 boolean remove(Object o);
563
564 /**
565 * Returns {@code true} if this deque contains the specified element.
566 * More formally, returns {@code true} if and only if this deque contains
567 * at least one element {@code e} such that {@code Objects.equals(o, e)}.
568 *
569 * @param o element whose presence in this deque is to be tested
570 * @return {@code true} if this deque contains the specified element
571 * @throws ClassCastException if the class of the specified element
572 * is incompatible with this deque
573 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
574 * @throws NullPointerException if the specified element is null and this
575 * deque does not permit null elements
576 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
577 */
578 boolean contains(Object o);
579
580 /**
581 * Returns the number of elements in this deque.
582 *
583 * @return the number of elements in this deque
584 */
585 int size();
586
587 /**
588 * Returns an iterator over the elements in this deque in proper sequence.
589 * The elements will be returned in order from first (head) to last (tail).
590 *
591 * @return an iterator over the elements in this deque in proper sequence
592 */
593 Iterator<E> iterator();
594
595 /**
596 * Returns an iterator over the elements in this deque in reverse
597 * sequential order. The elements will be returned in order from
598 * last (tail) to first (head).
599 *
600 * @return an iterator over the elements in this deque in reverse
601 * sequence
602 */
603 Iterator<E> descendingIterator();
604
605 }