2 * Written by Doug Lea and Josh Bloch with assistance from members of
3 * JCP JSR-166 Expert Group and released to the public domain, as explained
4 * at http://creativecommons.org/licenses/publicdomain
10 * A linear collection that supports element insertion and removal at
11 * both ends. The name <i>deque</i> is short for "double ended queue"
12 * and is usually pronounced "deck". Most <tt>Deque</tt>
13 * implementations place no fixed limits on the number of elements
14 * they may contain, but this interface supports capacity-restricted
15 * deques as well as those with no fixed size limit.
17 * <p>This interface defines methods to access the elements at both
18 * ends of the deque. Methods are provided to insert, remove, and
19 * examine the element. Each of these methods exists in two forms:
20 * one throws an exception if the operation fails, the other returns a
21 * special value (either <tt>null</tt> or <tt>false</tt>, depending on
22 * the operation). The latter form of the insert operation is
23 * designed specifically for use with capacity-restricted
24 * <tt>Deque</tt> implementations; in most implementations, insert
25 * operations cannot fail.
27 * <p>The twelve methods described above are summarized in the
31 * <table BORDER CELLPADDING=3 CELLSPACING=1>
34 * <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>
35 * <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>
39 * <td ALIGN=CENTER><em>Throws exception</em></td>
40 * <td ALIGN=CENTER><em>Special value</em></td>
41 * <td ALIGN=CENTER><em>Throws exception</em></td>
42 * <td ALIGN=CENTER><em>Special value</em></td>
45 * <td><b>Insert</b></td>
46 * <td>{@link #addFirst addFirst(e)}</td>
47 * <td>{@link #offerFirst offerFirst(e)}</td>
48 * <td>{@link #addLast addLast(e)}</td>
49 * <td>{@link #offerLast offerLast(e)}</td>
52 * <td><b>Remove</b></td>
53 * <td>{@link #removeFirst removeFirst()}</td>
54 * <td>{@link #pollFirst pollFirst()}</td>
55 * <td>{@link #removeLast removeLast()}</td>
56 * <td>{@link #pollLast pollLast()}</td>
59 * <td><b>Examine</b></td>
60 * <td>{@link #getFirst getFirst()}</td>
61 * <td>{@link #peekFirst peekFirst()}</td>
62 * <td>{@link #getLast getLast()}</td>
63 * <td>{@link #peekLast peekLast()}</td>
67 * <p>This interface extends the {@link Queue} interface. When a deque is
68 * used as a queue, FIFO (First-In-First-Out) behavior results. Elements are
69 * added at the end of the deque and removed from the beginning. The methods
70 * inherited from the <tt>Queue</tt> interface are precisely equivalent to
71 * <tt>Deque</tt> methods as indicated in the following table:
74 * <table BORDER CELLPADDING=3 CELLSPACING=1>
76 * <td ALIGN=CENTER> <b><tt>Queue</tt> Method</b></td>
77 * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
80 * <td>{@link java.util.Queue#add add(e)}</td>
81 * <td>{@link #addLast addLast(e)}</td>
84 * <td>{@link java.util.Queue#offer offer(e)}</td>
85 * <td>{@link #offerLast offerLast(e)}</td>
88 * <td>{@link java.util.Queue#remove remove()}</td>
89 * <td>{@link #removeFirst removeFirst()}</td>
92 * <td>{@link java.util.Queue#poll poll()}</td>
93 * <td>{@link #pollFirst pollFirst()}</td>
96 * <td>{@link java.util.Queue#element element()}</td>
97 * <td>{@link #getFirst getFirst()}</td>
100 * <td>{@link java.util.Queue#peek peek()}</td>
101 * <td>{@link #peek peekFirst()}</td>
105 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This
106 * interface should be used in preference to the legacy {@link Stack} class.
107 * When a deque is used as a stack, elements are pushed and popped from the
108 * beginning of the deque. Stack methods are precisely equivalent to
109 * <tt>Deque</tt> methods as indicated in the table below:
112 * <table BORDER CELLPADDING=3 CELLSPACING=1>
114 * <td ALIGN=CENTER> <b>Stack Method</b></td>
115 * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
118 * <td>{@link #push push(e)}</td>
119 * <td>{@link #addFirst addFirst(e)}</td>
122 * <td>{@link #pop pop()}</td>
123 * <td>{@link #removeFirst removeFirst()}</td>
126 * <td>{@link #peek peek()}</td>
127 * <td>{@link #peekFirst peekFirst()}</td>
131 * <p>Note that the {@link #peek peek} method works equally well when
132 * a deque is used as a queue or a stack; in either case, elements are
133 * drawn from the beginning of the deque.
135 * <p>This interface provides two methods to remove interior
136 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
137 * {@link #removeLastOccurrence removeLastOccurrence}.
139 * <p>Unlike the {@link List} interface, this interface does not
140 * provide support for indexed access to elements.
142 * <p>While <tt>Deque</tt> implementations are not strictly required
143 * to prohibit the insertion of null elements, they are strongly
144 * encouraged to do so. Users of any <tt>Deque</tt> implementations
145 * that do allow null elements are strongly encouraged <i>not</i> to
146 * take advantage of the ability to insert nulls. This is so because
147 * <tt>null</tt> is used as a special return value by various methods
148 * to indicated that the deque is empty.
150 * <p><tt>Deque</tt> implementations generally do not define
151 * element-based versions of the <tt>equals</tt> and <tt>hashCode</tt>
152 * methods, but instead inherit the identity-based versions from class
155 * <p>This interface is a member of the <a
156 * href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections
162 * @param <E> the type of elements held in this collection
165 public interface Deque<E> extends Queue<E> {
167 * Inserts the specified element at the front of this deque if it is
168 * possible to do so immediately without violating capacity restrictions.
169 * When using a capacity-restricted deque, it is generally preferable to
170 * use method {@link #offerFirst}.
172 * @param e the element to add
173 * @throws IllegalStateException if the element cannot be added at this
174 * time due to capacity restrictions
175 * @throws ClassCastException if the class of the specified element
176 * prevents it from being added to this deque
177 * @throws NullPointerException if the specified element is null and this
178 * deque does not permit null elements
179 * @throws IllegalArgumentException if some property of the specified
180 * element prevents it from being added to this deque
185 * Inserts the specified element at the end of this deque if it is
186 * possible to do so immediately without violating capacity restrictions.
187 * When using a capacity-restricted deque, it is generally preferable to
188 * use method {@link #offerLast}.
190 * <p>This method is equivalent to {@link #add}.
192 * @param e the element to add
193 * @throws IllegalStateException if the element cannot be added at this
194 * time due to capacity restrictions
195 * @throws ClassCastException if the class of the specified element
196 * prevents it from being added to this deque
197 * @throws NullPointerException if the specified element is null and this
198 * deque does not permit null elements
199 * @throws IllegalArgumentException if some property of the specified
200 * element prevents it from being added to this deque
205 * Inserts the specified element at the front of this deque unless it would
206 * violate capacity restrictions. When using a capacity-restricted deque,
207 * this method is generally preferable to the {@link #addFirst} method,
208 * which can fail to insert an element only by throwing an exception.
210 * @param e the element to add
211 * @return <tt>true</tt> if the element was added to this deque, else
213 * @throws ClassCastException if the class of the specified element
214 * prevents it from being added to this deque
215 * @throws NullPointerException if the specified element is null and this
216 * deque does not permit null elements
217 * @throws IllegalArgumentException if some property of the specified
218 * element prevents it from being added to this deque
220 boolean offerFirst(E e);
223 * Inserts the specified element at the end of this deque unless it would
224 * violate capacity restrictions. When using a capacity-restricted deque,
225 * this method is generally preferable to the {@link #addLast} method,
226 * which can fail to insert an element only by throwing an exception.
228 * @param e the element to add
229 * @return <tt>true</tt> if the element was added to this deque, else
231 * @throws ClassCastException if the class of the specified element
232 * prevents it from being added to this deque
233 * @throws NullPointerException if the specified element is null and this
234 * deque does not permit null elements
235 * @throws IllegalArgumentException if some property of the specified
236 * element prevents it from being added to this deque
238 boolean offerLast(E e);
241 * Retrieves and removes the first element of this deque. This method
242 * differs from {@link #pollFirst pollFirst} only in that it throws an
243 * exception if this deque is empty.
245 * @return the head of this deque
246 * @throws NoSuchElementException if this deque is empty
251 * Retrieves and removes the last element of this deque. This method
252 * differs from {@link #pollLast pollLast} only in that it throws an
253 * exception if this deque is empty.
255 * @return the tail of this deque
256 * @throws NoSuchElementException if this deque is empty
261 * Retrieves and removes the first element of this deque,
262 * or returns <tt>null</tt> if this deque is empty.
264 * @return the head of this deque, or <tt>null</tt> if this deque is empty
269 * Retrieves and removes the last element of this deque,
270 * or returns <tt>null</tt> if this deque is empty.
272 * @return the tail of this deque, or <tt>null</tt> if this deque is empty
277 * Retrieves, but does not remove, the first element of this deque.
279 * This method differs from {@link #peekFirst peekFirst} only in that it
280 * throws an exception if this deque is empty.
282 * @return the head of this deque
283 * @throws NoSuchElementException if this deque is empty
288 * Retrieves, but does not remove, the last element of this deque.
289 * This method differs from {@link #peekLast peekLast} only in that it
290 * throws an exception if this deque is empty.
292 * @return the tail of this deque
293 * @throws NoSuchElementException if this deque is empty
298 * Retrieves, but does not remove, the first element of this deque,
299 * or returns <tt>null</tt> if this deque is empty.
301 * @return the head of this deque, or <tt>null</tt> if this deque is empty
306 * Retrieves, but does not remove, the last element of this deque,
307 * or returns <tt>null</tt> if this deque is empty.
309 * @return the tail of this deque, or <tt>null</tt> if this deque is empty
314 * Removes the first occurrence of the specified element from this deque.
315 * If the deque does not contain the element, it is unchanged.
316 * More formally, removes the first element <tt>e</tt> such that
317 * <tt>(o==null ? e==null : o.equals(e))</tt>
318 * (if such an element exists).
319 * Returns <tt>true</tt> if this deque contained the specified element
320 * (or equivalently, if this deque changed as a result of the call).
322 * @param o element to be removed from this deque, if present
323 * @return <tt>true</tt> if an element was removed as a result of this call
324 * @throws ClassCastException if the class of the specified element
325 * is incompatible with this deque (optional)
326 * @throws NullPointerException if the specified element is null and this
327 * deque does not permit null elements (optional)
329 boolean removeFirstOccurrence(Object o);
332 * Removes the last occurrence of the specified element from this deque.
333 * If the deque does not contain the element, it is unchanged.
334 * More formally, removes the last element <tt>e</tt> such that
335 * <tt>(o==null ? e==null : o.equals(e))</tt>
336 * (if such an element exists).
337 * Returns <tt>true</tt> if this deque contained the specified element
338 * (or equivalently, if this deque changed as a result of the call).
340 * @param o element to be removed from this deque, if present
341 * @return <tt>true</tt> if an element was removed as a result of this call
342 * @throws ClassCastException if the class of the specified element
343 * is incompatible with this deque (optional)
344 * @throws NullPointerException if the specified element is null and this
345 * deque does not permit null elements (optional)
347 boolean removeLastOccurrence(Object o);
349 // *** Queue methods ***
352 * Inserts the specified element into the queue represented by this deque
353 * (in other words, at the tail of this deque) if it is possible to do so
354 * immediately without violating capacity restrictions, returning
355 * <tt>true</tt> upon success and throwing an
356 * <tt>IllegalStateException</tt> if no space is currently available.
357 * When using a capacity-restricted deque, it is generally preferable to
358 * use {@link #offer(Object) offer}.
360 * <p>This method is equivalent to {@link #addLast}.
362 * @param e the element to add
363 * @return <tt>true</tt> (as specified by {@link Collection#add})
364 * @throws IllegalStateException if the element cannot be added at this
365 * time due to capacity restrictions
366 * @throws ClassCastException if the class of the specified element
367 * prevents it from being added to this deque
368 * @throws NullPointerException if the specified element is null and this
369 * deque does not permit null elements
370 * @throws IllegalArgumentException if some property of the specified
371 * element prevents it from being added to this deque
376 * Inserts the specified element into the queue represented by this deque
377 * (in other words, at the tail of this deque) if it is possible to do so
378 * immediately without violating capacity restrictions, returning
379 * <tt>true</tt> upon success and <tt>false</tt> if no space is currently
380 * available. When using a capacity-restricted deque, this method is
381 * generally preferable to the {@link #add} method, which can fail to
382 * insert an element only by throwing an exception.
384 * <p>This method is equivalent to {@link #offerLast}.
386 * @param e the element to add
387 * @return <tt>true</tt> if the element was added to this deque, else
389 * @throws ClassCastException if the class of the specified element
390 * prevents it from being added to this deque
391 * @throws NullPointerException if the specified element is null and this
392 * deque does not permit null elements
393 * @throws IllegalArgumentException if some property of the specified
394 * element prevents it from being added to this deque
399 * Retrieves and removes the head of the queue represented by this deque
400 * (in other words, the first element of this deque).
401 * This method differs from {@link #poll poll} only in that it throws an
402 * exception if this deque is empty.
404 * <p>This method is equivalent to {@link #removeFirst()}.
406 * @return the head of the queue represented by this deque
407 * @throws NoSuchElementException if this deque is empty
412 * Retrieves and removes the head of the queue represented by this deque
413 * (in other words, the first element of this deque), or returns
414 * <tt>null</tt> if this deque is empty.
416 * <p>This method is equivalent to {@link #pollFirst()}.
418 * @return the first element of this deque, or <tt>null</tt> if
419 * this deque is empty
424 * Retrieves, but does not remove, the head of the queue represented by
425 * this deque (in other words, the first element of this deque).
426 * This method differs from {@link #peek peek} only in that it throws an
427 * exception if this deque is empty.
429 * <p>This method is equivalent to {@link #getFirst()}.
431 * @return the head of the queue represented by this deque
432 * @throws NoSuchElementException if this deque is empty
437 * Retrieves, but does not remove, the head of the queue represented by
438 * this deque (in other words, the first element of this deque), or
439 * returns <tt>null</tt> if this deque is empty.
441 * <p>This method is equivalent to {@link #peekFirst()}.
443 * @return the head of the queue represented by this deque, or
444 * <tt>null</tt> if this deque is empty
449 // *** Stack methods ***
452 * Pushes an element onto the stack represented by this deque (in other
453 * words, at the head of this deque) if it is possible to do so
454 * immediately without violating capacity restrictions, returning
455 * <tt>true</tt> upon success and throwing an
456 * <tt>IllegalStateException</tt> if no space is currently available.
458 * <p>This method is equivalent to {@link #addFirst}.
460 * @param e the element to push
461 * @throws IllegalStateException if the element cannot be added at this
462 * time due to capacity restrictions
463 * @throws ClassCastException if the class of the specified element
464 * prevents it from being added to this deque
465 * @throws NullPointerException if the specified element is null and this
466 * deque does not permit null elements
467 * @throws IllegalArgumentException if some property of the specified
468 * element prevents it from being added to this deque
473 * Pops an element from the stack represented by this deque. In other
474 * words, removes and returns the first element of this deque.
476 * <p>This method is equivalent to {@link #removeFirst()}.
478 * @return the element at the front of this deque (which is the top
479 * of the stack represented by this deque)
480 * @throws NoSuchElementException if this deque is empty
485 // *** Collection methods ***
488 * Removes the first occurrence of the specified element from this deque.
489 * If the deque does not contain the element, it is unchanged.
490 * More formally, removes the first element <tt>e</tt> such that
491 * <tt>(o==null ? e==null : o.equals(e))</tt>
492 * (if such an element exists).
493 * Returns <tt>true</tt> if this deque contained the specified element
494 * (or equivalently, if this deque changed as a result of the call).
496 * <p>This method is equivalent to {@link #removeFirstOccurrence}.
498 * @param o element to be removed from this deque, if present
499 * @return <tt>true</tt> if an element was removed as a result of this call
500 * @throws ClassCastException if the class of the specified element
501 * is incompatible with this deque (optional)
502 * @throws NullPointerException if the specified element is null and this
503 * deque does not permit null elements (optional)
505 boolean remove(Object o);
508 * Returns <tt>true</tt> if this deque contains the specified element.
509 * More formally, returns <tt>true</tt> if and only if this deque contains
510 * at least one element <tt>e</tt> such that
511 * <tt>(o==null ? e==null : o.equals(e))</tt>.
513 * @param o element whose presence in this deque is to be tested
514 * @return <tt>true</tt> if this deque contains the specified element
515 * @throws ClassCastException if the type of the specified element
516 * is incompatible with this deque (optional)
517 * @throws NullPointerException if the specified element is null and this
518 * deque does not permit null elements (optional)
520 boolean contains(Object o);
523 * Returns the number of elements in this deque.
525 * @return the number of elements in this deque
530 * Returns an iterator over the elements in this deque in proper sequence.
531 * The elements will be returned in order from first (head) to last (tail).
533 * @return an iterator over the elements in this deque in proper sequence
535 Iterator<E> iterator();
538 * Returns an iterator over the elements in this deque in reverse
539 * sequential order. The elements will be returned in order from
540 * last (tail) to first (head).
542 * @return an iterator over the elements in this deque in reverse
545 Iterator<E> descendingIterator();