| Deque.java |
1 /*
2 * %W% %E%
3 *
4 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
5 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6 */
7
8 package java.util;
9
10 /**
11 * A linear collection that supports element insertion and removal at
12 * both ends. The name <i>deque</i> is short for "double ended queue"
13 * and is usually pronounced "deck". Most <tt>Deque</tt>
14 * implementations place no fixed limits on the number of elements
15 * they may contain, but this interface supports capacity-restricted
16 * deques as well as those with no fixed size limit.
17 *
18 * <p>This interface defines methods to access the elements at both
19 * ends of the deque. Methods are provided to insert, remove, and
20 * examine the element. Each of these methods exists in two forms:
21 * one throws an exception if the operation fails, the other returns a
22 * special value (either <tt>null</tt> or <tt>false</tt>, depending on
23 * the operation). The latter form of the insert operation is
24 * designed specifically for use with capacity-restricted
25 * <tt>Deque</tt> implementations; in most implementations, insert
26 * operations cannot fail.
27 *
28 * <p>The twelve methods described above are summarized in the
29 * following table:
30 *
31 * <p>
32 * <table BORDER CELLPADDING=3 CELLSPACING=1>
33 * <tr>
34 * <td></td>
35 * <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>
36 * <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>
37 * </tr>
38 * <tr>
39 * <td></td>
40 * <td ALIGN=CENTER><em>Throws exception</em></td>
41 * <td ALIGN=CENTER><em>Special value</em></td>
42 * <td ALIGN=CENTER><em>Throws exception</em></td>
43 * <td ALIGN=CENTER><em>Special value</em></td>
44 * </tr>
45 * <tr>
46 * <td><b>Insert</b></td>
47 * <td>{@link #addFirst addFirst(e)}</td>
48 * <td>{@link #offerFirst offerFirst(e)}</td>
49 * <td>{@link #addLast addLast(e)}</td>
50 * <td>{@link #offerLast offerLast(e)}</td>
51 * </tr>
52 * <tr>
53 * <td><b>Remove</b></td>
54 * <td>{@link #removeFirst removeFirst()}</td>
55 * <td>{@link #pollFirst pollFirst()}</td>
56 * <td>{@link #removeLast removeLast()}</td>
57 * <td>{@link #pollLast pollLast()}</td>
58 * </tr>
59 * <tr>
60 * <td><b>Examine</b></td>
61 * <td>{@link #getFirst getFirst()}</td>
62 * <td>{@link #peekFirst peekFirst()}</td>
63 * <td>{@link #getLast getLast()}</td>
64 * <td>{@link #peekLast peekLast()}</td>
65 * </tr>
66 * </table>
67 *
68 * <p>This interface extends the {@link Queue} interface. When a deque is
69 * used as a queue, FIFO (First-In-First-Out) behavior results. Elements are
70 * added at the end of the deque and removed from the beginning. The methods
71 * inherited from the <tt>Queue</tt> interface are precisely equivalent to
72 * <tt>Deque</tt> methods as indicated in the following table:
73 *
74 * <p>
75 * <table BORDER CELLPADDING=3 CELLSPACING=1>
76 * <tr>
77 * <td ALIGN=CENTER> <b><tt>Queue</tt> Method</b></td>
78 * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
79 * </tr>
80 * <tr>
81 * <td>{@link java.util.Queue#add add(e)}</td>
82 * <td>{@link #addLast addLast(e)}</td>
83 * </tr>
84 * <tr>
85 * <td>{@link java.util.Queue#offer offer(e)}</td>
86 * <td>{@link #offerLast offerLast(e)}</td>
87 * </tr>
88 * <tr>
89 * <td>{@link java.util.Queue#remove remove()}</td>
90 * <td>{@link #removeFirst removeFirst()}</td>
91 * </tr>
92 * <tr>
93 * <td>{@link java.util.Queue#poll poll()}</td>
94 * <td>{@link #pollFirst pollFirst()}</td>
95 * </tr>
96 * <tr>
97 * <td>{@link java.util.Queue#element element()}</td>
98 * <td>{@link #getFirst getFirst()}</td>
99 * </tr>
100 * <tr>
101 * <td>{@link java.util.Queue#peek peek()}</td>
102 * <td>{@link #peek peekFirst()}</td>
103 * </tr>
104 * </table>
105 *
106 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This
107 * interface should be used in preference to the legacy {@link Stack} class.
108 * When a deque is used as a stack, elements are pushed and popped from the
109 * beginning of the deque. Stack methods are precisely equivalent to
110 * <tt>Deque</tt> methods as indicated in the table below:
111 *
112 * <p>
113 * <table BORDER CELLPADDING=3 CELLSPACING=1>
114 * <tr>
115 * <td ALIGN=CENTER> <b>Stack Method</b></td>
116 * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
117 * </tr>
118 * <tr>
119 * <td>{@link #push push(e)}</td>
120 * <td>{@link #addFirst addFirst(e)}</td>
121 * </tr>
122 * <tr>
123 * <td>{@link #pop pop()}</td>
124 * <td>{@link #removeFirst removeFirst()}</td>
125 * </tr>
126 * <tr>
127 * <td>{@link #peek peek()}</td>
128 * <td>{@link #peekFirst peekFirst()}</td>
129 * </tr>
130 * </table>
131 *
132 * <p>Note that the {@link #peek peek} method works equally well when
133 * a deque is used as a queue or a stack; in either case, elements are
134 * drawn from the beginning of the deque.
135 *
136 * <p>This interface provides two methods to remove interior
137 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
138 * {@link #removeLastOccurrence removeLastOccurrence}.
139 *
140 * <p>Unlike the {@link List} interface, this interface does not
141 * provide support for indexed access to elements.
142 *
143 * <p>While <tt>Deque</tt> implementations are not strictly required
144 * to prohibit the insertion of null elements, they are strongly
145 * encouraged to do so. Users of any <tt>Deque</tt> implementations
146 * that do allow null elements are strongly encouraged <i>not</i> to
147 * take advantage of the ability to insert nulls. This is so because
148 * <tt>null</tt> is used as a special return value by various methods
149 * to indicated that the deque is empty.
150 *
151 * <p><tt>Deque</tt> implementations generally do not define
152 * element-based versions of the <tt>equals</tt> and <tt>hashCode</tt>
153 * methods, but instead inherit the identity-based versions from class
154 * <tt>Object</tt>.
155 *
156 * <p>This interface is a member of the <a
157 * href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections
158 * Framework</a>.
159 *
160 * @author Doug Lea
161 * @author Josh Bloch
162 * @since 1.6
163 * @param <E> the type of elements held in this collection
164 */
165
166 public interface Deque<E> extends Queue<E> {
167 /**
168 * Inserts the specified element at the front of this deque if it is
169 * possible to do so immediately without violating capacity restrictions.
170 * When using a capacity-restricted deque, it is generally preferable to
171 * use method {@link #offerFirst}.
172 *
173 * @param e the element to add
174 * @throws IllegalStateException if the element cannot be added at this
175 * time due to capacity restrictions
176 * @throws ClassCastException if the class of the specified element
177 * prevents it from being added to this deque
178 * @throws NullPointerException if the specified element is null and this
179 * deque does not permit null elements
180 * @throws IllegalArgumentException if some property of the specified
181 * element prevents it from being added to this deque
182 */
183 void addFirst(E e);
184
185 /**
186 * Inserts the specified element at the end of this deque if it is
187 * possible to do so immediately without violating capacity restrictions.
188 * When using a capacity-restricted deque, it is generally preferable to
189 * use method {@link #offerLast}.
190 *
191 * <p>This method is equivalent to {@link #add}.
192 *
193 * @param e the element to add
194 * @throws IllegalStateException if the element cannot be added at this
195 * time due to capacity restrictions
196 * @throws ClassCastException if the class of the specified element
197 * prevents it from being added to this deque
198 * @throws NullPointerException if the specified element is null and this
199 * deque does not permit null elements
200 * @throws IllegalArgumentException if some property of the specified
201 * element prevents it from being added to this deque
202 */
203 void addLast(E e);
204
205 /**
206 * Inserts the specified element at the front of this deque unless it would
207 * violate capacity restrictions. When using a capacity-restricted deque,
208 * this method is generally preferable to the {@link #addFirst} method,
209 * which can fail to insert an element only by throwing an exception.
210 *
211 * @param e the element to add
212 * @return <tt>true</tt> if the element was added to this deque, else
213 * <tt>false</tt>
214 * @throws ClassCastException if the class of the specified element
215 * prevents it from being added to this deque
216 * @throws NullPointerException if the specified element is null and this
217 * deque does not permit null elements
218 * @throws IllegalArgumentException if some property of the specified
219 * element prevents it from being added to this deque
220 */
221 boolean offerFirst(E e);
222
223 /**
224 * Inserts the specified element at the end of this deque unless it would
225 * violate capacity restrictions. When using a capacity-restricted deque,
226 * this method is generally preferable to the {@link #addLast} method,
227 * which can fail to insert an element only by throwing an exception.
228 *
229 * @param e the element to add
230 * @return <tt>true</tt> if the element was added to this deque, else
231 * <tt>false</tt>
232 * @throws ClassCastException if the class of the specified element
233 * prevents it from being added to this deque
234 * @throws NullPointerException if the specified element is null and this
235 * deque does not permit null elements
236 * @throws IllegalArgumentException if some property of the specified
237 * element prevents it from being added to this deque
238 */
239 boolean offerLast(E e);
240
241 /**
242 * Retrieves and removes the first element of this deque. This method
243 * differs from {@link #pollFirst pollFirst} only in that it throws an
244 * exception if this deque is empty.
245 *
246 * @return the head of this deque
247 * @throws NoSuchElementException if this deque is empty
248 */
249 E removeFirst();
250
251 /**
252 * Retrieves and removes the last element of this deque. This method
253 * differs from {@link #pollLast pollLast} only in that it throws an
254 * exception if this deque is empty.
255 *
256 * @return the tail of this deque
257 * @throws NoSuchElementException if this deque is empty
258 */
259 E removeLast();
260
261 /**
262 * Retrieves and removes the first element of this deque,
263 * or returns <tt>null</tt> if this deque is empty.
264 *
265 * @return the head of this deque, or <tt>null</tt> if this deque is empty
266 */
267 E pollFirst();
268
269 /**
270 * Retrieves and removes the last element of this deque,
271 * or returns <tt>null</tt> if this deque is empty.
272 *
273 * @return the tail of this deque, or <tt>null</tt> if this deque is empty
274 */
275 E pollLast();
276
277 /**
278 * Retrieves, but does not remove, the first element of this deque.
279 *
280 * This method differs from {@link #peekFirst peekFirst} only in that it
281 * throws an exception if this deque is empty.
282 *
283 * @return the head of this deque
284 * @throws NoSuchElementException if this deque is empty
285 */
286 E getFirst();
287
288 /**
289 * Retrieves, but does not remove, the last element of this deque.
290 * This method differs from {@link #peekLast peekLast} only in that it
291 * throws an exception if this deque is empty.
292 *
293 * @return the tail of this deque
294 * @throws NoSuchElementException if this deque is empty
295 */
296 E getLast();
297
298 /**
299 * Retrieves, but does not remove, the first element of this deque,
300 * or returns <tt>null</tt> if this deque is empty.
301 *
302 * @return the head of this deque, or <tt>null</tt> if this deque is empty
303 */
304 E peekFirst();
305
306 /**
307 * Retrieves, but does not remove, the last element of this deque,
308 * or returns <tt>null</tt> if this deque is empty.
309 *
310 * @return the tail of this deque, or <tt>null</tt> if this deque is empty
311 */
312 E peekLast();
313
314 /**
315 * Removes the first occurrence of the specified element from this deque.
316 * If the deque does not contain the element, it is unchanged.
317 * More formally, removes the first element <tt>e</tt> such that
318 * <tt>(o==null ? e==null : o.equals(e))</tt>
319 * (if such an element exists).
320 * Returns <tt>true</tt> if this deque contained the specified element
321 * (or equivalently, if this deque changed as a result of the call).
322 *
323 * @param o element to be removed from this deque, if present
324 * @return <tt>true</tt> if an element was removed as a result of this call
325 * @throws ClassCastException if the class of the specified element
326 * is incompatible with this deque (optional)
327 * @throws NullPointerException if the specified element is null and this
328 * deque does not permit null elements (optional)
329 */
330 boolean removeFirstOccurrence(Object o);
331
332 /**
333 * Removes the last occurrence of the specified element from this deque.
334 * If the deque does not contain the element, it is unchanged.
335 * More formally, removes the last element <tt>e</tt> such that
336 * <tt>(o==null ? e==null : o.equals(e))</tt>
337 * (if such an element exists).
338 * Returns <tt>true</tt> if this deque contained the specified element
339 * (or equivalently, if this deque changed as a result of the call).
340 *
341 * @param o element to be removed from this deque, if present
342 * @return <tt>true</tt> if an element was removed as a result of this call
343 * @throws ClassCastException if the class of the specified element
344 * is incompatible with this deque (optional)
345 * @throws NullPointerException if the specified element is null and this
346 * deque does not permit null elements (optional)
347 */
348 boolean removeLastOccurrence(Object o);
349
350 // *** Queue methods ***
351
352 /**
353 * Inserts the specified element into the queue represented by this deque
354 * (in other words, at the tail of this deque) if it is possible to do so
355 * immediately without violating capacity restrictions, returning
356 * <tt>true</tt> upon success and throwing an
357 * <tt>IllegalStateException</tt> if no space is currently available.
358 * When using a capacity-restricted deque, it is generally preferable to
359 * use {@link #offer(Object) offer}.
360 *
361 * <p>This method is equivalent to {@link #addLast}.
362 *
363 * @param e the element to add
364 * @return <tt>true</tt> (as specified by {@link Collection#add})
365 * @throws IllegalStateException if the element cannot be added at this
366 * time due to capacity restrictions
367 * @throws ClassCastException if the class of the specified element
368 * prevents it from being added to this deque
369 * @throws NullPointerException if the specified element is null and this
370 * deque does not permit null elements
371 * @throws IllegalArgumentException if some property of the specified
372 * element prevents it from being added to this deque
373 */
374 boolean add(E e);
375
376 /**
377 * Inserts the specified element into the queue represented by this deque
378 * (in other words, at the tail of this deque) if it is possible to do so
379 * immediately without violating capacity restrictions, returning
380 * <tt>true</tt> upon success and <tt>false</tt> if no space is currently
381 * available. When using a capacity-restricted deque, this method is
382 * generally preferable to the {@link #add} method, which can fail to
383 * insert an element only by throwing an exception.
384 *
385 * <p>This method is equivalent to {@link #offerLast}.
386 *
387 * @param e the element to add
388 * @return <tt>true</tt> if the element was added to this deque, else
389 * <tt>false</tt>
390 * @throws ClassCastException if the class of the specified element
391 * prevents it from being added to this deque
392 * @throws NullPointerException if the specified element is null and this
393 * deque does not permit null elements
394 * @throws IllegalArgumentException if some property of the specified
395 * element prevents it from being added to this deque
396 */
397 boolean offer(E e);
398
399 /**
400 * Retrieves and removes the head of the queue represented by this deque
401 * (in other words, the first element of this deque).
402 * This method differs from {@link #poll poll} only in that it throws an
403 * exception if this deque is empty.
404 *
405 * <p>This method is equivalent to {@link #removeFirst()}.
406 *
407 * @return the head of the queue represented by this deque
408 * @throws NoSuchElementException if this deque is empty
409 */
410 E remove();
411
412 /**
413 * Retrieves and removes the head of the queue represented by this deque
414 * (in other words, the first element of this deque), or returns
415 * <tt>null</tt> if this deque is empty.
416 *
417 * <p>This method is equivalent to {@link #pollFirst()}.
418 *
419 * @return the first element of this deque, or <tt>null</tt> if
420 * this deque is empty
421 */
422 E poll();
423
424 /**
425 * Retrieves, but does not remove, the head of the queue represented by
426 * this deque (in other words, the first element of this deque).
427 * This method differs from {@link #peek peek} only in that it throws an
428 * exception if this deque is empty.
429 *
430 * <p>This method is equivalent to {@link #getFirst()}.
431 *
432 * @return the head of the queue represented by this deque
433 * @throws NoSuchElementException if this deque is empty
434 */
435 E element();
436
437 /**
438 * Retrieves, but does not remove, the head of the queue represented by
439 * this deque (in other words, the first element of this deque), or
440 * returns <tt>null</tt> if this deque is empty.
441 *
442 * <p>This method is equivalent to {@link #peekFirst()}.
443 *
444 * @return the head of the queue represented by this deque, or
445 * <tt>null</tt> if this deque is empty
446 */
447 E peek();
448
449
450 // *** Stack methods ***
451
452 /**
453 * Pushes an element onto the stack represented by this deque (in other
454 * words, at the head of this deque) if it is possible to do so
455 * immediately without violating capacity restrictions, returning
456 * <tt>true</tt> upon success and throwing an
457 * <tt>IllegalStateException</tt> if no space is currently available.
458 *
459 * <p>This method is equivalent to {@link #addFirst}.
460 *
461 * @param e the element to push
462 * @throws IllegalStateException if the element cannot be added at this
463 * time due to capacity restrictions
464 * @throws ClassCastException if the class of the specified element
465 * prevents it from being added to this deque
466 * @throws NullPointerException if the specified element is null and this
467 * deque does not permit null elements
468 * @throws IllegalArgumentException if some property of the specified
469 * element prevents it from being added to this deque
470 */
471 void push(E e);
472
473 /**
474 * Pops an element from the stack represented by this deque. In other
475 * words, removes and returns the first element of this deque.
476 *
477 * <p>This method is equivalent to {@link #removeFirst()}.
478 *
479 * @return the element at the front of this deque (which is the top
480 * of the stack represented by this deque)
481 * @throws NoSuchElementException if this deque is empty
482 */
483 E pop();
484
485
486 // *** Collection methods ***
487
488 /**
489 * Removes the first occurrence of the specified element from this deque.
490 * If the deque does not contain the element, it is unchanged.
491 * More formally, removes the first element <tt>e</tt> such that
492 * <tt>(o==null ? e==null : o.equals(e))</tt>
493 * (if such an element exists).
494 * Returns <tt>true</tt> if this deque contained the specified element
495 * (or equivalently, if this deque changed as a result of the call).
496 *
497 * <p>This method is equivalent to {@link #removeFirstOccurrence}.
498 *
499 * @param o element to be removed from this deque, if present
500 * @return <tt>true</tt> if an element was removed as a result of this call
501 * @throws ClassCastException if the class of the specified element
502 * is incompatible with this deque (optional)
503 * @throws NullPointerException if the specified element is null and this
504 * deque does not permit null elements (optional)
505 */
506 boolean remove(Object o);
507
508 /**
509 * Returns <tt>true</tt> if this deque contains the specified element.
510 * More formally, returns <tt>true</tt> if and only if this deque contains
511 * at least one element <tt>e</tt> such that
512 * <tt>(o==null ? e==null : o.equals(e))</tt>.
513 *
514 * @param o element whose presence in this deque is to be tested
515 * @return <tt>true</tt> if this deque contains the specified element
516 * @throws ClassCastException if the type of the specified element
517 * is incompatible with this deque (optional)
518 * @throws NullPointerException if the specified element is null and this
519 * deque does not permit null elements (optional)
520 */
521 boolean contains(Object o);
522
523 /**
524 * Returns the number of elements in this deque.
525 *
526 * @return the number of elements in this deque
527 */
528 public int size();
529
530 /**
531 * Returns an iterator over the elements in this deque in proper sequence.
532 * The elements will be returned in order from first (head) to last (tail).
533 *
534 * @return an iterator over the elements in this deque in proper sequence
535 */
536 Iterator<E> iterator();
537
538 /**
539 * Returns an iterator over the elements in this deque in reverse
540 * sequential order. The elements will be returned in order from
541 * last (tail) to first (head).
542 *
543 * @return an iterator over the elements in this deque in reverse
544 * sequence
545 */
546 Iterator<E> descendingIterator();
547
548 }
549