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="striped">
60 * <caption>Summary of Deque methods</caption>
61 * <thead>
62 * <tr>
63 * <td rowspan="2"></td>
64 * <th scope="col" colspan="2"> First Element (Head)</th>
65 * <th scope="col" colspan="2"> Last Element (Tail)</th>
66 * </tr>
67 * <tr>
68 * <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
69 * <th scope="col" style="font-weight:normal; font-style:italic">Special value</th>
70 * <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
71 * <th scope="col" style="font-weight:normal; font-style:italic">Special value</th>
72 * </tr>
73 * </thead>
74 * <tbody>
75 * <tr>
76 * <th scope="row">Insert</th>
77 * <td>{@link #addFirst(Object) addFirst(e)}</td>
78 * <td>{@link #offerFirst(Object) offerFirst(e)}</td>
79 * <td>{@link #addLast(Object) addLast(e)}</td>
80 * <td>{@link #offerLast(Object) offerLast(e)}</td>
81 * </tr>
82 * <tr>
83 * <th scope="row">Remove</th>
84 * <td>{@link #removeFirst() removeFirst()}</td>
85 * <td>{@link #pollFirst() pollFirst()}</td>
86 * <td>{@link #removeLast() removeLast()}</td>
87 * <td>{@link #pollLast() pollLast()}</td>
88 * </tr>
89 * <tr>
90 * <th scope="row">Examine</th>
91 * <td>{@link #getFirst() getFirst()}</td>
92 * <td>{@link #peekFirst() peekFirst()}</td>
93 * <td>{@link #getLast() getLast()}</td>
94 * <td>{@link #peekLast() peekLast()}</td>
95 * </tr>
96 * </tbody>
97 * </table>
98 *
99 * <p>This interface extends the {@link Queue} interface. When a deque is
100 * used as a queue, FIFO (First-In-First-Out) behavior results. Elements are
101 * added at the end of the deque and removed from the beginning. The methods
102 * inherited from the {@code Queue} interface are precisely equivalent to
103 * {@code Deque} methods as indicated in the following table:
104 *
105 * <table class="striped">
106 * <caption>Comparison of Queue and Deque methods</caption>
107 * <thead>
108 * <tr>
109 * <th scope="col"> {@code Queue} Method</th>
110 * <th scope="col"> Equivalent {@code Deque} Method</th>
111 * </tr>
112 * </thead>
113 * <tbody>
114 * <tr>
115 * <th scope="row">{@link #add(Object) add(e)}</th>
116 * <td>{@link #addLast(Object) addLast(e)}</td>
117 * </tr>
118 * <tr>
119 * <th scope="row">{@link #offer(Object) offer(e)}</th>
120 * <td>{@link #offerLast(Object) offerLast(e)}</td>
121 * </tr>
122 * <tr>
123 * <th scope="row">{@link #remove() remove()}</th>
124 * <td>{@link #removeFirst() removeFirst()}</td>
125 * </tr>
126 * <tr>
127 * <th scope="row">{@link #poll() poll()}</th>
128 * <td>{@link #pollFirst() pollFirst()}</td>
129 * </tr>
130 * <tr>
131 * <th scope="row">{@link #element() element()}</th>
132 * <td>{@link #getFirst() getFirst()}</td>
133 * </tr>
134 * <tr>
135 * <th scope="row">{@link #peek() peek()}</th>
136 * <td>{@link #peekFirst() peekFirst()}</td>
137 * </tr>
138 * </tbody>
139 * </table>
140 *
141 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This
142 * interface should be used in preference to the legacy {@link Stack} class.
143 * When a deque is used as a stack, elements are pushed and popped from the
144 * beginning of the deque. Stack methods are precisely equivalent to
145 * {@code Deque} methods as indicated in the table below:
146 *
147 * <table class="striped">
148 * <caption>Comparison of Stack and Deque methods</caption>
149 * <thead>
150 * <tr>
151 * <th scope="col"> Stack Method</th>
152 * <th scope="col"> Equivalent {@code Deque} Method</th>
153 * </tr>
154 * </thead>
155 * <tbody>
156 * <tr>
157 * <th scope="row">{@link #push(Object) push(e)}</th>
158 * <td>{@link #addFirst(Object) addFirst(e)}</td>
159 * </tr>
160 * <tr>
161 * <th scope="row">{@link #pop() pop()}</th>
162 * <td>{@link #removeFirst() removeFirst()}</td>
163 * </tr>
164 * <tr>
165 * <th scope="row">{@link #peek() peek()}</th>
166 * <td>{@link #peekFirst() peekFirst()}</td>
167 * </tr>
168 * </tbody>
169 * </table>
170 *
171 * <p>Note that the {@link #peek peek} method works equally well when
172 * a deque is used as a queue or a stack; in either case, elements are
173 * drawn from the beginning of the deque.
174 *
175 * <p>This interface provides two methods to remove interior
176 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
177 * {@link #removeLastOccurrence removeLastOccurrence}.
178 *
179 * <p>Unlike the {@link List} interface, this interface does not
180 * provide support for indexed access to elements.
181 *
182 * <p>While {@code Deque} implementations are not strictly required
183 * to prohibit the insertion of null elements, they are strongly
184 * encouraged to do so. Users of any {@code Deque} implementations
185 * that do allow null elements are strongly encouraged <i>not</i> to
186 * take advantage of the ability to insert nulls. This is so because
187 * {@code null} is used as a special return value by various methods
188 * to indicate that the deque is empty.
189 *
190 * <p>{@code Deque} implementations generally do not define
191 * element-based versions of the {@code equals} and {@code hashCode}
192 * methods, but instead inherit the identity-based versions from class
193 * {@code Object}.
194 *
195 * <p>This interface is a member of the
196 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
197 * Java Collections Framework</a>.
198 *
199 * @author Doug Lea
200 * @author Josh Bloch
201 * @since 1.6
202 * @param <E> the type of elements held in this deque
203 */
204 public interface Deque<E> extends Queue<E> {
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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="{@docRoot}/java.base/java/util/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="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
370 */
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="{@docRoot}/java.base/java/util/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="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
389 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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 */
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="{@docRoot}/java.base/java/util/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="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
572 */
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="{@docRoot}/java.base/java/util/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="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
588 */
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 */
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 */
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 */
614 Iterator<E> descendingIterator();
615
616 }