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1 : dl 1.1 /*
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
5 :     */
6 :    
7 :     package java.util;
8 :    
9 :     /**
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.
16 :     *
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.
26 :     *
27 : dl 1.3 * <p>The twelve methods described above are summarized in the
28 : jsr166 1.5 * following table:
29 : dl 1.3 *
30 : jsr166 1.5 * <p><table BORDER CELLPADDING=3 CELLSPACING=1>
31 : dl 1.1 * <tr>
32 :     * <td></td>
33 :     * <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>
34 :     * <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>
35 :     * </tr>
36 :     * <tr>
37 :     * <td></td>
38 :     * <td ALIGN=CENTER><em>Throws exception</em></td>
39 :     * <td ALIGN=CENTER><em>Returns special value</em></td>
40 :     * <td ALIGN=CENTER><em>Throws exception</em></td>
41 :     * <td ALIGN=CENTER><em>Returns special value</em></td>
42 :     * </tr>
43 :     * <tr>
44 :     * <td><b>Insert</b></td>
45 :     * <td>{@link #addFirst addFirst(e)}</td>
46 :     * <td>{@link #offerFirst offerFirst(e)}</td>
47 :     * <td>{@link #addLast addLast(e)}</td>
48 :     * <td>{@link #offerLast offerLast(e)}</td>
49 :     * </tr>
50 :     * <tr>
51 :     * <td><b>Remove</b></td>
52 :     * <td>{@link #removeFirst removeFirst()}</td>
53 :     * <td>{@link #pollFirst pollFirst()}</td>
54 :     * <td>{@link #removeLast removeLast()}</td>
55 :     * <td>{@link #pollLast pollLast()}</td>
56 :     * </tr>
57 :     * <tr>
58 :     * <td><b>Examine</b></td>
59 :     * <td>{@link #getFirst getFirst()}</td>
60 :     * <td>{@link #peekFirst peekFirst()}</td>
61 :     * <td>{@link #getLast getLast()}</td>
62 :     * <td>{@link #peekLast peekLast()}</td>
63 :     * </tr>
64 :     * </table>
65 :     *
66 :     * <p>This interface extends the {@link Queue} interface. When a deque is
67 :     * used as a queue, FIFO (First-In-First-Out) behavior results. Elements are
68 : dl 1.4 * added at the end of the deque and removed from the beginning. The methods
69 : dl 1.1 * inherited from the <tt>Queue</tt> interface are precisely equivalent to
70 : jsr166 1.5 * <tt>Deque</tt> methods as indicated in the following table:
71 : dl 1.1 *
72 : jsr166 1.5 * <p><table BORDER CELLPADDING=3 CELLSPACING=1>
73 : dl 1.1 * <tr>
74 :     * <td ALIGN=CENTER> <b><tt>Queue</tt> Method</b></td>
75 :     * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
76 :     * </tr>
77 :     * <tr>
78 :     * <td>{@link java.util.Queue#offer offer(e)}</td>
79 :     * <td>{@link #offerLast offerLast(e)}</td>
80 : jsr166 1.5 * </tr>
81 :     * <tr>
82 : dl 1.1 * <td>{@link java.util.Queue#add add(e)}</td>
83 :     * <td>{@link #addLast addLast(e)}</td>
84 : jsr166 1.5 * </tr>
85 :     * <tr>
86 : dl 1.1 * <td>{@link java.util.Queue#poll poll()}</td>
87 :     * <td>{@link #pollFirst pollFirst()}</td>
88 : jsr166 1.5 * </tr>
89 :     * <tr>
90 : dl 1.1 * <td>{@link java.util.Queue#remove remove()}</td>
91 :     * <td>{@link #removeFirst removeFirst()}</td>
92 : jsr166 1.5 * </tr>
93 :     * <tr>
94 : dl 1.1 * <td>{@link java.util.Queue#peek peek()}</td>
95 :     * <td>{@link #peek peekFirst()}</td>
96 : jsr166 1.5 * </tr>
97 :     * <tr>
98 : dl 1.1 * <td>{@link java.util.Queue#element element()}</td>
99 :     * <td>{@link #getFirst getFirst()}</td>
100 : jsr166 1.5 * </tr>
101 : dl 1.1 * </table>
102 :     *
103 :     * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This
104 :     * interface should be used in preference to the legacy {@link Stack} class.
105 : dl 1.3 * When a deque is used as a stack, elements are pushed and popped from the
106 : dl 1.1 * beginning of the deque. Stack methods are precisely equivalent to
107 : jsr166 1.5 * <tt>Deque</tt> methods as indicated in the table below:
108 : dl 1.1 *
109 : jsr166 1.5 * <p><table BORDER CELLPADDING=3 CELLSPACING=1>
110 : dl 1.1 * <tr>
111 :     * <td ALIGN=CENTER> <b>Stack Method</b></td>
112 :     * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
113 :     * </tr>
114 :     * <tr>
115 :     * <td>{@link #push push(e)}</td>
116 :     * <td>{@link #addFirst addFirst(e)}</td>
117 : jsr166 1.5 * </tr>
118 :     * <tr>
119 : dl 1.1 * <td>{@link #pop pop()}</td>
120 :     * <td>{@link #removeFirst removeFirst()}</td>
121 : jsr166 1.5 * </tr>
122 :     * <tr>
123 : dl 1.1 * <td>{@link #peek peek()}</td>
124 :     * <td>{@link #peekFirst peekFirst()}</td>
125 : jsr166 1.5 * </tr>
126 : dl 1.1 * </table>
127 :     *
128 :     * <p>Note that the {@link #peek peek} method works equally well when
129 :     * a deque is used as a queue or a stack; in either case, elements are
130 :     * drawn from the beginning of the deque.
131 :     *
132 : dl 1.2 * <p>This interface provides two methods to remove interior
133 : dl 1.1 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
134 :     * {@link #removeLastOccurrence removeLastOccurrence}. Unlike the
135 :     * {@link List} interface, this interface does not provide support for
136 :     * indexed access to elements.
137 :     *
138 :     * <p>While <tt>Deque</tt> implementations are not strictly required
139 :     * to prohibit the insertion of null elements, they are strongly
140 :     * encouraged to do so. Users of any <tt>Deque</tt> implementations
141 :     * that do allow null elements are strongly encouraged <i>not</i> to
142 :     * take advantage of the ability to insert nulls. This is so because
143 :     * <tt>null</tt> is used as a special return value by various methods
144 :     * to indicated that the deque is empty.
145 : dl 1.3 *
146 : dl 1.1 * <p><tt>Deque</tt> implementations generally do not define
147 :     * element-based versions of the <tt>equals</tt> and <tt>hashCode</tt>
148 :     * methods, but instead inherit the identity-based versions from class
149 :     * <tt>Object</tt>.
150 :     *
151 :     * <p>This interface is a member of the <a
152 :     * href="{@docRoot}/../guide/collections/index.html"> Java Collections
153 :     * Framework</a>.
154 :     *
155 :     * @author Doug Lea
156 :     * @author Josh Bloch
157 :     * @since 1.6
158 :     * @param <E> the type of elements held in this collection
159 :     */
160 :    
161 :     public interface Deque<E> extends Queue<E> {
162 :     /**
163 : dl 1.3 * Inserts the specified element at the front of this deque unless it would
164 : dl 1.1 * violate capacity restrictions. When using a capacity-restricted deque,
165 :     * this method is generally preferable to method <tt>addFirst</tt>, which
166 :     * can fail to insert an element only by throwing an exception.
167 :     *
168 :     * @param e the element to insert
169 :     * @return <tt>true</tt> if it was possible to insert the element,
170 :     * else <tt>false</tt>
171 : dl 1.3 * @throws NullPointerException if the specified element is null and this
172 : dl 1.1 * deque does not permit null elements
173 :     */
174 :     boolean offerFirst(E e);
175 :    
176 :     /**
177 : dl 1.4 * Inserts the specified element at the end of this deque unless it would
178 : dl 1.1 * violate capacity restrictions. When using a capacity-restricted deque,
179 :     * this method is generally preferable to method <tt>addLast</tt> which
180 :     * can fail to insert an element only by throwing an exception.
181 :     *
182 :     * @param e the element to insert
183 :     * @return <tt>true</tt> if it was possible to insert the element,
184 :     * else <tt>false</tt>
185 : dl 1.3 * @throws NullPointerException if the specified element is null and this
186 : dl 1.1 * deque does not permit null elements
187 :     */
188 :     boolean offerLast(E e);
189 :    
190 :     /**
191 : dl 1.3 * Inserts the specified element at the front of this deque unless it
192 : dl 1.1 * would violate capacity restrictions.
193 :     *
194 :     * @param e the element to insert
195 :     * @throws IllegalStateException if it was not possible to insert
196 :     * the element due to capacity restrictions
197 : dl 1.3 * @throws NullPointerException if the specified element is null and this
198 : dl 1.1 * deque does not permit null elements
199 :     */
200 :     void addFirst(E e);
201 :    
202 :     /**
203 : dl 1.4 * Inserts the specified element at the end of this deque unless it would
204 : dl 1.1 * violate capacity restrictions.
205 :     *
206 :     * @param e the element to insert
207 :     * @throws IllegalStateException if it was not possible to insert
208 :     * the element due to capacity restrictions
209 : dl 1.3 * @throws NullPointerException if the specified element is null and this
210 : dl 1.1 * deque does not permit null elements
211 :     */
212 :     void addLast(E e);
213 :    
214 :     /**
215 :     * Retrieves and removes the first element of this deque, or
216 :     * <tt>null</tt> if this deque is empty.
217 :     *
218 :     * @return the first element of this deque, or <tt>null</tt> if
219 :     * this deque is empty
220 :     */
221 :     E pollFirst();
222 :    
223 :     /**
224 :     * Retrieves and removes the last element of this deque, or
225 :     * <tt>null</tt> if this deque is empty.
226 :     *
227 :     * @return the last element of this deque, or <tt>null</tt> if
228 :     * this deque is empty
229 :     */
230 :     E pollLast();
231 :    
232 :     /**
233 : jsr166 1.5 * Retrieves and removes the first element of this deque. This method
234 : jsr166 1.6 * differs from the {@link #pollFirst} method only in that it throws an
235 : dl 1.1 * exception if this deque is empty.
236 :     *
237 :     * @return the first element of this deque
238 :     * @throws NoSuchElementException if this deque is empty
239 :     */
240 :     E removeFirst();
241 :    
242 :     /**
243 :     * Retrieves and removes the last element of this deque. This method
244 : jsr166 1.6 * differs from the {@link #pollLast} method only in that it throws an
245 : dl 1.1 * exception if this deque is empty.
246 :     *
247 :     * @return the last element of this deque
248 :     * @throws NoSuchElementException if this deque is empty
249 :     */
250 :     E removeLast();
251 :    
252 :     /**
253 :     * Retrieves, but does not remove, the first element of this deque,
254 :     * returning <tt>null</tt> if this deque is empty.
255 :     *
256 :     * @return the first element of this deque, or <tt>null</tt> if
257 :     * this deque is empty
258 :     */
259 :     E peekFirst();
260 :    
261 :     /**
262 :     * Retrieves, but does not remove, the last element of this deque,
263 :     * returning <tt>null</tt> if this deque is empty.
264 :     *
265 :     * @return the last element of this deque, or <tt>null</tt> if this deque
266 :     * is empty
267 :     */
268 :     E peekLast();
269 :    
270 :     /**
271 :     * Retrieves, but does not remove, the first element of this
272 : jsr166 1.6 * deque. This method differs from the {@link #peekFirst} method only
273 : dl 1.1 * in that it throws an exception if this deque is empty.
274 :     *
275 :     * @return the first element of this deque
276 :     * @throws NoSuchElementException if this deque is empty
277 :     */
278 :     E getFirst();
279 :    
280 :     /**
281 :     * Retrieves, but does not remove, the last element of this
282 : jsr166 1.6 * deque. This method differs from the {@link #peekLast} method only
283 : dl 1.1 * in that it throws an exception if this deque is empty.
284 :     *
285 :     * @return the last element of this deque
286 :     * @throws NoSuchElementException if this deque is empty
287 :     */
288 :     E getLast();
289 :    
290 :     /**
291 :     * Removes the first occurrence of the specified element in this
292 :     * deque. If the deque does not contain the element, it is
293 :     * unchanged. More formally, removes the first element <tt>e</tt>
294 :     * such that <tt>(o==null ? e==null : o.equals(e))</tt> (if
295 :     * such an element exists).
296 :     *
297 : dl 1.3 * @param o element to be removed from this deque, if present
298 : dl 1.1 * @return <tt>true</tt> if the deque contained the specified element
299 : dl 1.4 * @throws NullPointerException if the specified element is null and this
300 :     * deque does not permit null elements
301 : dl 1.1 */
302 : dl 1.3 boolean removeFirstOccurrence(Object o);
303 : dl 1.1
304 :     /**
305 :     * Removes the last occurrence of the specified element in this
306 :     * deque. If the deque does not contain the element, it is
307 :     * unchanged. More formally, removes the last element <tt>e</tt>
308 :     * such that <tt>(o==null ? e==null : o.equals(e))</tt> (if
309 :     * such an element exists).
310 :     *
311 : dl 1.3 * @param o element to be removed from this deque, if present
312 : dl 1.1 * @return <tt>true</tt> if the deque contained the specified element
313 : dl 1.4 * @throws NullPointerException if the specified element is null and this
314 :     * deque does not permit null elements
315 : dl 1.1 */
316 : dl 1.3 boolean removeLastOccurrence(Object o);
317 : dl 1.1
318 :    
319 :     // *** Queue methods ***
320 :    
321 :     /**
322 :     * Inserts the specified element into the queue represented by this deque
323 :     * unless it would violate capacity restrictions. In other words, inserts
324 : dl 1.4 * the specified element at the end of this deque. When using a
325 : dl 1.1 * capacity-restricted deque, this method is generally preferable to the
326 :     * {@link #add} method, which can fail to insert an element only by
327 :     * throwing an exception.
328 :     *
329 :     * <p>This method is equivalent to {@link #offerLast}.
330 :     *
331 :     * @param e the element to insert
332 :     * @return <tt>true</tt> if it was possible to insert the element,
333 :     * else <tt>false</tt>
334 : dl 1.3 * @throws NullPointerException if the specified element is null and this
335 : dl 1.1 * deque does not permit null elements
336 :     */
337 :     boolean offer(E e);
338 :    
339 :     /**
340 :     * Inserts the specified element into the queue represented by this
341 :     * deque unless it would violate capacity restrictions. In other words,
342 : dl 1.3 * inserts the specified element as the last element of this deque.
343 : dl 1.1 *
344 :     * <p>This method is equivalent to {@link #addLast}.
345 :     *
346 :     * @param e the element to insert
347 :     * @return <tt>true</tt> (as per the spec for {@link Collection#add})
348 :     * @throws IllegalStateException if it was not possible to insert
349 :     * the element due to capacity restrictions
350 : dl 1.3 * @throws NullPointerException if the specified element is null and this
351 : dl 1.1 * deque does not permit null elements
352 :     */
353 :     boolean add(E e);
354 :    
355 :     /**
356 :     * Retrieves and removes the head of the queue represented by
357 :     * this deque, or <tt>null</tt> if this deque is empty. In other words,
358 :     * retrieves and removes the first element of this deque, or <tt>null</tt>
359 :     * if this deque is empty.
360 :     *
361 :     * <p>This method is equivalent to {@link #pollFirst()}.
362 :     *
363 :     * @return the first element of this deque, or <tt>null</tt> if
364 :     * this deque is empty
365 :     */
366 :     E poll();
367 :    
368 :     /**
369 :     * Retrieves and removes the head of the queue represented by this deque.
370 : jsr166 1.6 * This method differs from the {@link #poll} method only in that it
371 : dl 1.1 * throws an exception if this deque is empty.
372 :     *
373 :     * <p>This method is equivalent to {@link #removeFirst()}.
374 :     *
375 :     * @return the head of the queue represented by this deque
376 :     * @throws NoSuchElementException if this deque is empty
377 :     */
378 :     E remove();
379 :    
380 :     /**
381 :     * Retrieves, but does not remove, the head of the queue represented by
382 :     * this deque, returning <tt>null</tt> if this deque is empty.
383 :     *
384 : dl 1.3 * <p>This method is equivalent to {@link #peekFirst()}.
385 : dl 1.1 *
386 :     * @return the head of the queue represented by this deque, or
387 :     * <tt>null</tt> if this deque is empty
388 :     */
389 :     E peek();
390 :    
391 :     /**
392 :     * Retrieves, but does not remove, the head of the queue represented by
393 : jsr166 1.6 * this deque. This method differs from the {@link #peek} method only in
394 : dl 1.1 * that it throws an exception if this deque is empty.
395 :     *
396 : dl 1.3 * <p>This method is equivalent to {@link #getFirst()}.
397 : dl 1.1 *
398 :     * @return the head of the queue represented by this deque
399 :     * @throws NoSuchElementException if this deque is empty
400 :     */
401 :     E element();
402 :    
403 :    
404 :     // *** Stack methods ***
405 :    
406 :     /**
407 :     * Pushes an element onto the stack represented by this deque. In other
408 : dl 1.3 * words, inserts the element at the front of this deque unless it would
409 : dl 1.1 * violate capacity restrictions.
410 :     *
411 :     * <p>This method is equivalent to {@link #addFirst}.
412 :     *
413 : dl 1.3 * @param e the element to push
414 : dl 1.1 * @throws IllegalStateException if it was not possible to insert
415 :     * the element due to capacity restrictions
416 : dl 1.3 * @throws NullPointerException if the specified element is null and this
417 : dl 1.1 * deque does not permit null elements
418 :     */
419 :     void push(E e);
420 :    
421 :     /**
422 :     * Pops an element from the stack represented by this deque. In other
423 : dl 1.2 * words, removes and returns the first element of this deque.
424 : dl 1.1 *
425 :     * <p>This method is equivalent to {@link #removeFirst()}.
426 :     *
427 :     * @return the element at the front of this deque (which is the top
428 :     * of the stack represented by this deque)
429 :     * @throws NoSuchElementException if this deque is empty
430 :     */
431 :     E pop();
432 :    
433 :    
434 :     // *** Collection Method ***
435 :    
436 :     /**
437 :     * Returns an iterator over the elements in this deque. The elements
438 :     * will be ordered from first (head) to last (tail).
439 : dl 1.3 *
440 : dl 1.1 * @return an <tt>Iterator</tt> over the elements in this deque
441 :     */
442 :     Iterator<E> iterator();
443 :     }

Doug Lea
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