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1 : tim 1.2 package java.util;
2 : tim 1.1
3 :     /**
4 : tim 1.2 * An unbounded priority queue based on a priority heap. This queue orders
5 : brian 1.6 * elements according to an order specified at construction time, which is
6 :     * specified in the same manner as {@link TreeSet} and {@link TreeMap}: elements are ordered
7 : tim 1.2 * either according to their <i>natural order</i> (see {@link Comparable}), or
8 :     * according to a {@link Comparator}, depending on which constructor is used.
9 :     * The {@link #peek}, {@link #poll}, and {@link #remove} methods return the
10 :     * minimal element with respect to the specified ordering. If multiple
11 : brian 1.6 * elements are tied for least value, no guarantees are made as to
12 :     * which of these elements is returned.
13 : tim 1.2 *
14 : brian 1.6 * <p>A priority queue has a <i>capacity</i>. The capacity is the size of
15 :     * the array used internally to store the elements on the queue. It is always at least
16 :     * as large as the queue size. As elements are added to a priority queue,
17 : tim 1.2 * its capacity grows automatically. The details of the growth policy are not
18 :     * specified.
19 :     *
20 :     *<p>Implementation note: this implementation provides O(log(n)) time for
21 : brian 1.6 * the insertion methods (<tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>)
22 : tim 1.2 * methods; linear time for the <tt>remove(Object)</tt> and
23 : brian 1.6 * <tt>contains(Object)</tt> methods; and constant time for the retrieval methods (<tt>peek</tt>,
24 :     * <tt>element</tt>, and <tt>size</tt>).
25 : tim 1.2 *
26 :     * <p>This class is a member of the
27 :     * <a href="{@docRoot}/../guide/collections/index.html">
28 :     * Java Collections Framework</a>.
29 :     */
30 :     public class PriorityQueue<E> extends AbstractQueue<E>
31 :     implements Queue<E>
32 :     {
33 :     private static final int DEFAULT_INITIAL_CAPACITY = 11;
34 : tim 1.1
35 : tim 1.2 /**
36 :     * Priority queue represented as a balanced binary heap: the two children
37 :     * of queue[n] are queue[2*n] and queue[2*n + 1]. The priority queue is
38 :     * ordered by comparator, or by the elements' natural ordering, if
39 : brian 1.6 * comparator is null: For each node n in the heap and each descendant d
40 :     * of n, n <= d.
41 : tim 1.2 *
42 : brian 1.6 * The element with the lowest value is in queue[1], assuming the queue is
43 :     * nonempty. (A one-based array is used in preference to the traditional
44 :     * zero-based array to simplify parent and child calculations.)
45 : tim 1.2 *
46 :     * queue.length must be >= 2, even if size == 0.
47 :     */
48 : dl 1.5 private transient E[] queue;
49 : tim 1.1
50 : tim 1.2 /**
51 :     * The number of elements in the priority queue.
52 :     */
53 :     private int size = 0;
54 : tim 1.1
55 : tim 1.2 /**
56 :     * The comparator, or null if priority queue uses elements'
57 :     * natural ordering.
58 :     */
59 :     private final Comparator<E> comparator;
60 :    
61 :     /**
62 :     * The number of times this priority queue has been
63 :     * <i>structurally modified</i>. See AbstractList for gory details.
64 :     */
65 : dl 1.5 private transient int modCount = 0;
66 : tim 1.2
67 :     /**
68 :     * Create a new priority queue with the default initial capacity (11)
69 : brian 1.6 * that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)
70 : tim 1.2 */
71 :     public PriorityQueue() {
72 :     this(DEFAULT_INITIAL_CAPACITY);
73 : tim 1.1 }
74 : tim 1.2
75 :     /**
76 :     * Create a new priority queue with the specified initial capacity
77 : brian 1.6 * that orders its elements according to their natural ordering (using <tt>Comparable</tt>.)
78 : tim 1.2 *
79 :     * @param initialCapacity the initial capacity for this priority queue.
80 :     */
81 :     public PriorityQueue(int initialCapacity) {
82 :     this(initialCapacity, null);
83 : tim 1.1 }
84 : tim 1.2
85 :     /**
86 :     * Create a new priority queue with the specified initial capacity (11)
87 :     * that orders its elements according to the specified comparator.
88 :     *
89 :     * @param initialCapacity the initial capacity for this priority queue.
90 :     * @param comparator the comparator used to order this priority queue.
91 :     */
92 :     public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
93 :     if (initialCapacity < 1)
94 :     initialCapacity = 1;
95 :     queue = new E[initialCapacity + 1];
96 :     this.comparator = comparator;
97 : tim 1.1 }
98 :    
99 : tim 1.2 /**
100 :     * Create a new priority queue containing the elements in the specified
101 :     * collection. The priority queue has an initial capacity of 110% of the
102 :     * size of the specified collection. If the specified collection
103 :     * implements the {@link Sorted} interface, the priority queue will be
104 :     * sorted according to the same comparator, or according to its elements'
105 :     * natural order if the collection is sorted according to its elements'
106 : brian 1.6 * natural order. If the specified collection does not implement
107 :     * <tt>Sorted</tt>, the priority queue is ordered according to
108 : tim 1.2 * its elements' natural order.
109 :     *
110 :     * @param initialElements the collection whose elements are to be placed
111 :     * into this priority queue.
112 :     * @throws ClassCastException if elements of the specified collection
113 :     * cannot be compared to one another according to the priority
114 :     * queue's ordering.
115 :     * @throws NullPointerException if the specified collection or an
116 :     * element of the specified collection is <tt>null</tt>.
117 :     */
118 :     public PriorityQueue(Collection<E> initialElements) {
119 :     int sz = initialElements.size();
120 :     int initialCapacity = (int)Math.min((sz * 110L) / 100,
121 :     Integer.MAX_VALUE - 1);
122 :     if (initialCapacity < 1)
123 :     initialCapacity = 1;
124 :     queue = new E[initialCapacity + 1];
125 :    
126 : dl 1.5 /* Commented out to compile with generics compiler
127 :    
128 : tim 1.2 if (initialElements instanceof Sorted) {
129 :     comparator = ((Sorted)initialElements).comparator();
130 :     for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
131 :     queue[++size] = i.next();
132 :     } else {
133 : dl 1.5 */
134 :     {
135 : tim 1.2 comparator = null;
136 :     for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
137 :     add(i.next());
138 :     }
139 : tim 1.1 }
140 :    
141 : tim 1.2 // Queue Methods
142 :    
143 :     /**
144 :     * Remove and return the minimal element from this priority queue if
145 : brian 1.6 * it contains one or more elements, otherwise return <tt>null</tt>. The term
146 : tim 1.2 * <i>minimal</i> is defined according to this priority queue's order.
147 :     *
148 :     * @return the minimal element from this priority queue if it contains
149 :     * one or more elements, otherwise <tt>null</tt>.
150 :     */
151 : tim 1.1 public E poll() {
152 : tim 1.2 if (size == 0)
153 :     return null;
154 :     return remove(1);
155 : tim 1.1 }
156 : tim 1.2
157 :     /**
158 :     * Return, but do not remove, the minimal element from the priority queue,
159 : brian 1.6 * or return <tt>null</tt> if the queue is empty. The term <i>minimal</i> is
160 : tim 1.2 * defined according to this priority queue's order. This method returns
161 :     * the same object reference that would be returned by by the
162 : dl 1.5 * <tt>poll</tt> method. The two methods differ in that this method
163 : tim 1.2 * does not remove the element from the priority queue.
164 :     *
165 :     * @return the minimal element from this priority queue if it contains
166 :     * one or more elements, otherwise <tt>null</tt>.
167 :     */
168 : tim 1.1 public E peek() {
169 : tim 1.2 return queue[1];
170 : tim 1.1 }
171 :    
172 : tim 1.2 // Collection Methods
173 :    
174 :     /**
175 :     * Removes a single instance of the specified element from this priority
176 :     * queue, if it is present. Returns true if this collection contained the
177 :     * specified element (or equivalently, if this collection changed as a
178 :     * result of the call).
179 :     *
180 : brian 1.6 * @param element the element to be removed from this collection, if present.
181 : tim 1.2 * @return <tt>true</tt> if this collection changed as a result of the
182 :     * call
183 :     * @throws ClassCastException if the specified element cannot be compared
184 : dl 1.5 * with elements currently in the priority queue according
185 : tim 1.2 * to the priority queue's ordering.
186 :     * @throws NullPointerException if the specified element is null.
187 :     */
188 :     public boolean remove(Object element) {
189 :     if (element == null)
190 :     throw new NullPointerException();
191 :    
192 :     if (comparator == null) {
193 :     for (int i = 1; i <= size; i++) {
194 :     if (((Comparable)queue[i]).compareTo(element) == 0) {
195 :     remove(i);
196 :     return true;
197 :     }
198 :     }
199 :     } else {
200 :     for (int i = 1; i <= size; i++) {
201 :     if (comparator.compare(queue[i], (E) element) == 0) {
202 :     remove(i);
203 :     return true;
204 :     }
205 :     }
206 :     }
207 : tim 1.1 return false;
208 :     }
209 : tim 1.2
210 :     /**
211 :     * Returns an iterator over the elements in this priority queue. The
212 : brian 1.6 * elements of the priority queue will be returned by this iterator in the
213 :     * order specified by the queue, which is to say the order they would be
214 :     * returned by repeated calls to <tt>poll</tt>.
215 : dl 1.5 *
216 : tim 1.2 * @return an <tt>Iterator</tt> over the elements in this priority queue.
217 :     */
218 :     public Iterator<E> iterator() {
219 : dl 1.5 return new Itr();
220 : tim 1.2 }
221 :    
222 :     private class Itr implements Iterator<E> {
223 : dl 1.5 /**
224 :     * Index (into queue array) of element to be returned by
225 : tim 1.2 * subsequent call to next.
226 : dl 1.5 */
227 :     int cursor = 1;
228 : tim 1.2
229 : dl 1.5 /**
230 :     * Index of element returned by most recent call to next or
231 :     * previous. Reset to 0 if this element is deleted by a call
232 :     * to remove.
233 :     */
234 :     int lastRet = 0;
235 :    
236 :     /**
237 :     * The modCount value that the iterator believes that the backing
238 :     * List should have. If this expectation is violated, the iterator
239 :     * has detected concurrent modification.
240 :     */
241 :     int expectedModCount = modCount;
242 :    
243 :     public boolean hasNext() {
244 :     return cursor <= size;
245 :     }
246 : tim 1.2
247 : dl 1.5 public E next() {
248 : tim 1.2 checkForComodification();
249 :     if (cursor > size)
250 : dl 1.5 throw new NoSuchElementException();
251 : tim 1.2 E result = queue[cursor];
252 :     lastRet = cursor++;
253 :     return result;
254 : dl 1.5 }
255 : tim 1.2
256 : dl 1.5 public void remove() {
257 :     if (lastRet == 0)
258 :     throw new IllegalStateException();
259 : tim 1.2 checkForComodification();
260 :    
261 :     PriorityQueue.this.remove(lastRet);
262 :     if (lastRet < cursor)
263 :     cursor--;
264 :     lastRet = 0;
265 :     expectedModCount = modCount;
266 : dl 1.5 }
267 : tim 1.2
268 : dl 1.5 final void checkForComodification() {
269 :     if (modCount != expectedModCount)
270 :     throw new ConcurrentModificationException();
271 :     }
272 : tim 1.2 }
273 :    
274 :     /**
275 :     * Returns the number of elements in this priority queue.
276 : dl 1.5 *
277 : tim 1.2 * @return the number of elements in this priority queue.
278 :     */
279 : tim 1.1 public int size() {
280 : tim 1.2 return size;
281 : tim 1.1 }
282 : tim 1.2
283 :     /**
284 :     * Add the specified element to this priority queue.
285 :     *
286 :     * @param element the element to add.
287 :     * @return true
288 :     * @throws ClassCastException if the specified element cannot be compared
289 : dl 1.5 * with elements currently in the priority queue according
290 : tim 1.2 * to the priority queue's ordering.
291 :     * @throws NullPointerException if the specified element is null.
292 :     */
293 :     public boolean offer(E element) {
294 :     if (element == null)
295 :     throw new NullPointerException();
296 :     modCount++;
297 :    
298 :     // Grow backing store if necessary
299 :     if (++size == queue.length) {
300 :     E[] newQueue = new E[2 * queue.length];
301 :     System.arraycopy(queue, 0, newQueue, 0, size);
302 :     queue = newQueue;
303 :     }
304 :    
305 :     queue[size] = element;
306 :     fixUp(size);
307 :     return true;
308 : tim 1.1 }
309 :    
310 : tim 1.2 /**
311 :     * Remove all elements from the priority queue.
312 :     */
313 :     public void clear() {
314 :     modCount++;
315 :    
316 :     // Null out element references to prevent memory leak
317 :     for (int i=1; i<=size; i++)
318 :     queue[i] = null;
319 :    
320 :     size = 0;
321 :     }
322 :    
323 :     /**
324 :     * Removes and returns the ith element from queue. Recall
325 :     * that queue is one-based, so 1 <= i <= size.
326 :     *
327 :     * XXX: Could further special-case i==size, but is it worth it?
328 :     * XXX: Could special-case i==0, but is it worth it?
329 :     */
330 :     private E remove(int i) {
331 :     assert i <= size;
332 :     modCount++;
333 :    
334 :     E result = queue[i];
335 :     queue[i] = queue[size];
336 :     queue[size--] = null; // Drop extra ref to prevent memory leak
337 :     if (i <= size)
338 :     fixDown(i);
339 :     return result;
340 : tim 1.1 }
341 :    
342 : tim 1.2 /**
343 :     * Establishes the heap invariant (described above) assuming the heap
344 :     * satisfies the invariant except possibly for the leaf-node indexed by k
345 :     * (which may have a nextExecutionTime less than its parent's).
346 :     *
347 :     * This method functions by "promoting" queue[k] up the hierarchy
348 :     * (by swapping it with its parent) repeatedly until queue[k]
349 :     * is greater than or equal to its parent.
350 :     */
351 :     private void fixUp(int k) {
352 :     if (comparator == null) {
353 :     while (k > 1) {
354 :     int j = k >> 1;
355 :     if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
356 :     break;
357 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
358 :     k = j;
359 :     }
360 :     } else {
361 :     while (k > 1) {
362 :     int j = k >> 1;
363 :     if (comparator.compare(queue[j], queue[k]) <= 0)
364 :     break;
365 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
366 :     k = j;
367 :     }
368 :     }
369 :     }
370 :    
371 :     /**
372 :     * Establishes the heap invariant (described above) in the subtree
373 :     * rooted at k, which is assumed to satisfy the heap invariant except
374 :     * possibly for node k itself (which may be greater than its children).
375 :     *
376 :     * This method functions by "demoting" queue[k] down the hierarchy
377 :     * (by swapping it with its smaller child) repeatedly until queue[k]
378 :     * is less than or equal to its children.
379 :     */
380 :     private void fixDown(int k) {
381 :     int j;
382 :     if (comparator == null) {
383 :     while ((j = k << 1) <= size) {
384 :     if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
385 :     j++; // j indexes smallest kid
386 :     if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
387 :     break;
388 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
389 :     k = j;
390 :     }
391 :     } else {
392 :     while ((j = k << 1) <= size) {
393 :     if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
394 :     j++; // j indexes smallest kid
395 :     if (comparator.compare(queue[k], queue[j]) <= 0)
396 :     break;
397 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
398 :     k = j;
399 :     }
400 :     }
401 :     }
402 :    
403 :     /**
404 :     * Returns the comparator associated with this priority queue, or
405 :     * <tt>null</tt> if it uses its elements' natural ordering.
406 :     *
407 :     * @return the comparator associated with this priority queue, or
408 : dl 1.5 * <tt>null</tt> if it uses its elements' natural ordering.
409 : tim 1.2 */
410 : dl 1.5 Comparator comparator() {
411 : tim 1.2 return comparator;
412 :     }
413 : dl 1.5
414 :     /**
415 :     * Save the state of the instance to a stream (that
416 :     * is, serialize it).
417 :     *
418 :     * @serialData The length of the array backing the instance is
419 :     * emitted (int), followed by all of its elements (each an
420 :     * <tt>Object</tt>) in the proper order.
421 :     */
422 :     private synchronized void writeObject(java.io.ObjectOutputStream s)
423 :     throws java.io.IOException{
424 :     // Write out element count, and any hidden stuff
425 :     s.defaultWriteObject();
426 :    
427 :     // Write out array length
428 :     s.writeInt(queue.length);
429 :    
430 :     // Write out all elements in the proper order.
431 :     for (int i=0; i<size; i++)
432 :     s.writeObject(queue[i]);
433 :     }
434 :    
435 :     /**
436 :     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
437 :     * deserialize it).
438 :     */
439 :     private synchronized void readObject(java.io.ObjectInputStream s)
440 :     throws java.io.IOException, ClassNotFoundException {
441 :     // Read in size, and any hidden stuff
442 :     s.defaultReadObject();
443 :    
444 :     // Read in array length and allocate array
445 :     int arrayLength = s.readInt();
446 :     queue = new E[arrayLength];
447 :    
448 :     // Read in all elements in the proper order.
449 :     for (int i=0; i<size; i++)
450 :     queue[i] = (E)s.readObject();
451 :     }
452 :    
453 : tim 1.1 }

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