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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 1.2 * <p>The twelve methods described above are summarized in the
28 :     * following table:<p>
29 : dl 1.1 *
30 :     * <table BORDER CELLPADDING=3 CELLSPACING=1>
31 :     * <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 :     * added to the end of the deque and removed from the beginning. The methods
69 :     * inherited from the <tt>Queue</tt> interface are precisely equivalent to
70 :     * <tt>Deque</tt> methods as indicated in the following table:<p>
71 :     *
72 :     * <table BORDER CELLPADDING=3 CELLSPACING=1>
73 :     * <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 :     * <tr>
79 :     * <td>{@link java.util.Queue#offer offer(e)}</td>
80 :     * <td>{@link #offerLast offerLast(e)}</td>
81 :     * </tr>
82 :     * <tr>
83 :     * <td>{@link java.util.Queue#add add(e)}</td>
84 :     * <td>{@link #addLast addLast(e)}</td>
85 :     * </tr>
86 :     * <tr>
87 :     * <td>{@link java.util.Queue#poll poll()}</td>
88 :     * <td>{@link #pollFirst pollFirst()}</td>
89 :     * </tr>
90 :     * <tr>
91 :     * <td>{@link java.util.Queue#remove remove()}</td>
92 :     * <td>{@link #removeFirst removeFirst()}</td>
93 :     * </tr>
94 :     * <tr>
95 :     * <td>{@link java.util.Queue#peek peek()}</td>
96 :     * <td>{@link #peek peekFirst()}</td>
97 :     * </tr>
98 :     * <tr>
99 :     * <td>{@link java.util.Queue#element element()}</td>
100 :     * <td>{@link #getFirst getFirst()}</td>
101 :     * </tr>
102 :     * </table>
103 :     *
104 :     * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This
105 :     * interface should be used in preference to the legacy {@link Stack} class.
106 :     * When a dequeue is used as a stack, elements are pushed and popped from the
107 :     * beginning of the deque. Stack methods are precisely equivalent to
108 :     * <tt>Deque</tt> methods as indicated in the table below:<p>
109 :     *
110 :     * <table BORDER CELLPADDING=3 CELLSPACING=1>
111 :     * <tr>
112 :     * <td ALIGN=CENTER> <b>Stack Method</b></td>
113 :     * <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
114 :     * </tr>
115 :     * <tr>
116 :     * <tr>
117 :     * <td>{@link #push push(e)}</td>
118 :     * <td>{@link #addFirst addFirst(e)}</td>
119 :     * </tr>
120 :     * <tr>
121 :     * <td>{@link #pop pop()}</td>
122 :     * <td>{@link #removeFirst removeFirst()}</td>
123 :     * </tr>
124 :     * <tr>
125 :     * <td>{@link #peek peek()}</td>
126 :     * <td>{@link #peekFirst peekFirst()}</td>
127 :     * </tr>
128 :     * </table>
129 :     *
130 :     * <p>Note that the {@link #peek peek} method works equally well when
131 :     * a deque is used as a queue or a stack; in either case, elements are
132 :     * drawn from the beginning of the deque.
133 :     *
134 : dl 1.2 * <p>This interface provides two methods to remove interior
135 : dl 1.1 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
136 :     * {@link #removeLastOccurrence removeLastOccurrence}. Unlike the
137 :     * {@link List} interface, this interface does not provide support for
138 :     * indexed access to elements.
139 :     *
140 :     * <p>While <tt>Deque</tt> implementations are not strictly required
141 :     * to prohibit the insertion of null elements, they are strongly
142 :     * encouraged to do so. Users of any <tt>Deque</tt> implementations
143 :     * that do allow null elements are strongly encouraged <i>not</i> to
144 :     * take advantage of the ability to insert nulls. This is so because
145 :     * <tt>null</tt> is used as a special return value by various methods
146 :     * to indicated that the deque is empty.
147 :     *
148 :     * <p><tt>Deque</tt> implementations generally do not define
149 :     * element-based versions of the <tt>equals</tt> and <tt>hashCode</tt>
150 :     * methods, but instead inherit the identity-based versions from class
151 :     * <tt>Object</tt>.
152 :     *
153 :     * <p>This interface is a member of the <a
154 :     * href="{@docRoot}/../guide/collections/index.html"> Java Collections
155 :     * Framework</a>.
156 :     *
157 :     * @author Doug Lea
158 :     * @author Josh Bloch
159 :     * @since 1.6
160 :     * @param <E> the type of elements held in this collection
161 :     */
162 :    
163 :     public interface Deque<E> extends Queue<E> {
164 :     /**
165 :     * Inserts the specified element to the front this deque unless it would
166 :     * violate capacity restrictions. When using a capacity-restricted deque,
167 :     * this method is generally preferable to method <tt>addFirst</tt>, which
168 :     * can fail to insert an element only by throwing an exception.
169 :     *
170 :     * @param e the element to insert
171 :     * @return <tt>true</tt> if it was possible to insert the element,
172 :     * else <tt>false</tt>
173 :     * @throws NullPointerException if <tt>e</tt> is null and this
174 :     * deque does not permit null elements
175 :     */
176 :     boolean offerFirst(E e);
177 :    
178 :     /**
179 :     * Inserts the specified element to the end of this deque unless it would
180 :     * violate capacity restrictions. When using a capacity-restricted deque,
181 :     * this method is generally preferable to method <tt>addLast</tt> which
182 :     * can fail to insert an element only by throwing an exception.
183 :     *
184 :     * @param e the element to insert
185 :     * @return <tt>true</tt> if it was possible to insert the element,
186 :     * else <tt>false</tt>
187 :     * @throws NullPointerException if <tt>e</tt> is null and this
188 :     * deque does not permit null elements
189 :     */
190 :     boolean offerLast(E e);
191 :    
192 :     /**
193 :     * Inserts the specified element to the front of this deque unless it
194 :     * would violate capacity restrictions.
195 :     *
196 :     * @param e the element to insert
197 :     * @throws IllegalStateException if it was not possible to insert
198 :     * the element due to capacity restrictions
199 :     * @throws NullPointerException if <tt>e</tt> is null and this
200 :     * deque does not permit null elements
201 :     */
202 :     void addFirst(E e);
203 :    
204 :     /**
205 :     * Inserts the specified element to the end of this deque unless it would
206 :     * violate capacity restrictions.
207 :     *
208 :     * @param e the element to insert
209 :     * @throws IllegalStateException if it was not possible to insert
210 :     * the element due to capacity restrictions
211 :     * @throws NullPointerException if <tt>e</tt> is null and this
212 :     * deque does not permit null elements
213 :     */
214 :     void addLast(E e);
215 :    
216 :     /**
217 :     * Retrieves and removes the first element of this deque, or
218 :     * <tt>null</tt> if this deque is empty.
219 :     *
220 :     * @return the first element of this deque, or <tt>null</tt> if
221 :     * this deque is empty
222 :     */
223 :     E pollFirst();
224 :    
225 :     /**
226 :     * Retrieves and removes the last element of this deque, or
227 :     * <tt>null</tt> if this deque is empty.
228 :     *
229 :     * @return the last element of this deque, or <tt>null</tt> if
230 :     * this deque is empty
231 :     */
232 :     E pollLast();
233 :    
234 :     /**
235 :     * Removes and returns the first element of this deque. This method
236 :     * differs from the <tt>pollFirst</tt> method only in that it throws an
237 :     * exception if this deque is empty.
238 :     *
239 :     * @return the first element of this deque
240 :     * @throws NoSuchElementException if this deque is empty
241 :     */
242 :     E removeFirst();
243 :    
244 :     /**
245 :     * Retrieves and removes the last element of this deque. This method
246 :     * differs from the <tt>pollLast</tt> method only in that it throws an
247 :     * exception if this deque is empty.
248 :     *
249 :     * @return the last element of this deque
250 :     * @throws NoSuchElementException if this deque is empty
251 :     */
252 :     E removeLast();
253 :    
254 :     /**
255 :     * Retrieves, but does not remove, the first element of this deque,
256 :     * returning <tt>null</tt> if this deque is empty.
257 :     *
258 :     * @return the first element of this deque, or <tt>null</tt> if
259 :     * this deque is empty
260 :     */
261 :     E peekFirst();
262 :    
263 :     /**
264 :     * Retrieves, but does not remove, the last element of this deque,
265 :     * returning <tt>null</tt> if this deque is empty.
266 :     *
267 :     * @return the last element of this deque, or <tt>null</tt> if this deque
268 :     * is empty
269 :     */
270 :     E peekLast();
271 :    
272 :     /**
273 :     * Retrieves, but does not remove, the first element of this
274 :     * deque. This method differs from the <tt>peek</tt> method only
275 :     * in that it throws an exception if this deque is empty.
276 :     *
277 :     * @return the first element of this deque
278 :     * @throws NoSuchElementException if this deque is empty
279 :     */
280 :     E getFirst();
281 :    
282 :     /**
283 :     * Retrieves, but does not remove, the last element of this
284 :     * deque. This method differs from the <tt>peek</tt> method only
285 :     * in that it throws an exception if this deque is empty.
286 :     *
287 :     * @return the last element of this deque
288 :     * @throws NoSuchElementException if this deque is empty
289 :     */
290 :     E getLast();
291 :    
292 :     /**
293 :     * Removes the first occurrence of the specified element in this
294 :     * deque. If the deque does not contain the element, it is
295 :     * unchanged. More formally, removes the first element <tt>e</tt>
296 :     * such that <tt>(o==null ? e==null : o.equals(e))</tt> (if
297 :     * such an element exists).
298 :     *
299 :     * @param e element to be removed from this deque, if present
300 :     * @return <tt>true</tt> if the deque contained the specified element
301 :     * @throws NullPointerException if the specified element is <tt>null</tt>
302 :     */
303 :     boolean removeFirstOccurrence(Object e);
304 :    
305 :     /**
306 :     * Removes the last occurrence of the specified element in this
307 :     * deque. If the deque does not contain the element, it is
308 :     * unchanged. More formally, removes the last element <tt>e</tt>
309 :     * such that <tt>(o==null ? e==null : o.equals(e))</tt> (if
310 :     * such an element exists).
311 :     *
312 :     * @param e element to be removed from this deque, if present
313 :     * @return <tt>true</tt> if the deque contained the specified element
314 :     * @throws NullPointerException if the specified element is <tt>null</tt>
315 :     */
316 :     boolean removeLastOccurrence(Object e);
317 :    
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 :     * the specified element to the end of this deque. When using a
325 :     * 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 :     * @throws NullPointerException if <tt>e</tt> is null and this
335 :     * 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 :     * inserts the specified element as the last element of this deque.
343 :     *
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 :     * @throws NullPointerException if <tt>e</tt> is null and this
351 :     * 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 :     * This method differs from the <tt>poll</tt> method only in that it
371 :     * 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 :     * <p>This method is equivalent to {@link #peekFirst()}
385 :     *
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 :     * this deque. This method differs from the <tt>peek</tt> method only in
394 :     * that it throws an exception if this deque is empty.
395 :     *
396 :     * <p>This method is equivalent to {@link #getFirst()}
397 :     *
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 :     * words, inserts the element to the front this deque unless it would
409 :     * violate capacity restrictions.
410 :     *
411 :     * <p>This method is equivalent to {@link #addFirst}.
412 :     *
413 :     * @throws IllegalStateException if it was not possible to insert
414 :     * the element due to capacity restrictions
415 :     * @throws NullPointerException if <tt>e</tt> is null and this
416 :     * deque does not permit null elements
417 :     */
418 :     void push(E e);
419 :    
420 :     /**
421 :     * Pops an element from the stack represented by this deque. In other
422 : dl 1.2 * words, removes and returns the first element of this deque.
423 : dl 1.1 *
424 :     * <p>This method is equivalent to {@link #removeFirst()}.
425 :     *
426 :     * @return the element at the front of this deque (which is the top
427 :     * of the stack represented by this deque)
428 :     * @throws NoSuchElementException if this deque is empty
429 :     */
430 :     E pop();
431 :    
432 :    
433 :     // *** Collection Method ***
434 :    
435 :     /**
436 :     * Returns an iterator over the elements in this deque. The elements
437 :     * will be ordered from first (head) to last (tail).
438 :     *
439 :     * @return an <tt>Iterator</tt> over the elements in this deque
440 :     */
441 :     Iterator<E> iterator();
442 :     }

Doug Lea
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