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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 : dl 1.7 * <p>A priority queue has a <i>capacity</i>. The capacity is the
15 :     * size of the array used internally to store the elements on the
16 :     * queue. It is always at least as large as the queue size. As
17 :     * elements are added to a priority queue, its capacity grows
18 :     * automatically. The details of the growth policy are not specified.
19 : tim 1.2 *
20 : dl 1.7 *<p>Implementation note: this implementation provides O(log(n)) time
21 :     *for the insertion methods (<tt>offer</tt>, <tt>poll</tt>,
22 :     *<tt>remove()</tt> and <tt>add</tt>) methods; linear time for the
23 :     *<tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and
24 :     *constant time for the retrieval methods (<tt>peek</tt>,
25 :     *<tt>element</tt>, and <tt>size</tt>).
26 : tim 1.2 *
27 :     * <p>This class is a member of the
28 :     * <a href="{@docRoot}/../guide/collections/index.html">
29 :     * Java Collections Framework</a>.
30 : dl 1.7 * @since 1.5
31 :     * @author Josh Bloch
32 : tim 1.2 */
33 :     public class PriorityQueue<E> extends AbstractQueue<E>
34 : dl 1.7 implements Queue<E> {
35 : tim 1.2 private static final int DEFAULT_INITIAL_CAPACITY = 11;
36 : tim 1.1
37 : tim 1.2 /**
38 :     * Priority queue represented as a balanced binary heap: the two children
39 :     * of queue[n] are queue[2*n] and queue[2*n + 1]. The priority queue is
40 :     * ordered by comparator, or by the elements' natural ordering, if
41 : brian 1.6 * comparator is null: For each node n in the heap and each descendant d
42 :     * of n, n <= d.
43 : tim 1.2 *
44 : brian 1.6 * The element with the lowest value is in queue[1], assuming the queue is
45 :     * nonempty. (A one-based array is used in preference to the traditional
46 :     * zero-based array to simplify parent and child calculations.)
47 : tim 1.2 *
48 :     * queue.length must be >= 2, even if size == 0.
49 :     */
50 : dl 1.5 private transient E[] queue;
51 : tim 1.1
52 : tim 1.2 /**
53 :     * The number of elements in the priority queue.
54 :     */
55 :     private int size = 0;
56 : tim 1.1
57 : tim 1.2 /**
58 :     * The comparator, or null if priority queue uses elements'
59 :     * natural ordering.
60 :     */
61 :     private final Comparator<E> comparator;
62 :    
63 :     /**
64 :     * The number of times this priority queue has been
65 :     * <i>structurally modified</i>. See AbstractList for gory details.
66 :     */
67 : dl 1.5 private transient int modCount = 0;
68 : tim 1.2
69 :     /**
70 : dl 1.7 * Create a new priority queue with the default initial capacity
71 :     * (11) that orders its elements according to their natural
72 :     * ordering (using <tt>Comparable</tt>.)
73 : tim 1.2 */
74 :     public PriorityQueue() {
75 :     this(DEFAULT_INITIAL_CAPACITY);
76 : tim 1.1 }
77 : tim 1.2
78 :     /**
79 :     * Create a new priority queue with the specified initial capacity
80 : dl 1.7 * that orders its elements according to their natural ordering
81 :     * (using <tt>Comparable</tt>.)
82 : tim 1.2 *
83 :     * @param initialCapacity the initial capacity for this priority queue.
84 :     */
85 :     public PriorityQueue(int initialCapacity) {
86 :     this(initialCapacity, null);
87 : tim 1.1 }
88 : tim 1.2
89 :     /**
90 :     * Create a new priority queue with the specified initial capacity (11)
91 :     * that orders its elements according to the specified comparator.
92 :     *
93 :     * @param initialCapacity the initial capacity for this priority queue.
94 :     * @param comparator the comparator used to order this priority queue.
95 :     */
96 :     public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
97 :     if (initialCapacity < 1)
98 :     initialCapacity = 1;
99 :     queue = new E[initialCapacity + 1];
100 :     this.comparator = comparator;
101 : tim 1.1 }
102 :    
103 : tim 1.2 /**
104 :     * Create a new priority queue containing the elements in the specified
105 :     * collection. The priority queue has an initial capacity of 110% of the
106 :     * size of the specified collection. If the specified collection
107 :     * implements the {@link Sorted} interface, the priority queue will be
108 :     * sorted according to the same comparator, or according to its elements'
109 :     * natural order if the collection is sorted according to its elements'
110 : brian 1.6 * natural order. If the specified collection does not implement
111 :     * <tt>Sorted</tt>, the priority queue is ordered according to
112 : tim 1.2 * its elements' natural order.
113 :     *
114 :     * @param initialElements the collection whose elements are to be placed
115 :     * into this priority queue.
116 :     * @throws ClassCastException if elements of the specified collection
117 :     * cannot be compared to one another according to the priority
118 :     * queue's ordering.
119 :     * @throws NullPointerException if the specified collection or an
120 :     * element of the specified collection is <tt>null</tt>.
121 :     */
122 :     public PriorityQueue(Collection<E> initialElements) {
123 :     int sz = initialElements.size();
124 :     int initialCapacity = (int)Math.min((sz * 110L) / 100,
125 :     Integer.MAX_VALUE - 1);
126 :     if (initialCapacity < 1)
127 :     initialCapacity = 1;
128 :     queue = new E[initialCapacity + 1];
129 :    
130 : dl 1.5 /* Commented out to compile with generics compiler
131 :    
132 : tim 1.2 if (initialElements instanceof Sorted) {
133 :     comparator = ((Sorted)initialElements).comparator();
134 :     for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
135 :     queue[++size] = i.next();
136 :     } else {
137 : dl 1.5 */
138 :     {
139 : tim 1.2 comparator = null;
140 :     for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
141 :     add(i.next());
142 :     }
143 : tim 1.1 }
144 :    
145 : tim 1.2 // Queue Methods
146 :    
147 :     /**
148 : dl 1.7 * Remove and return the minimal element from this priority queue
149 :     * if it contains one or more elements, otherwise return
150 :     * <tt>null</tt>. The term <i>minimal</i> is defined according to
151 :     * this priority queue's order.
152 : tim 1.2 *
153 :     * @return the minimal element from this priority queue if it contains
154 :     * one or more elements, otherwise <tt>null</tt>.
155 :     */
156 : tim 1.1 public E poll() {
157 : tim 1.2 if (size == 0)
158 :     return null;
159 :     return remove(1);
160 : tim 1.1 }
161 : tim 1.2
162 :     /**
163 : dl 1.7 * Return, but do not remove, the minimal element from the
164 :     * priority queue, or return <tt>null</tt> if the queue is empty.
165 :     * The term <i>minimal</i> is defined according to this priority
166 :     * queue's order. This method returns the same object reference
167 :     * that would be returned by by the <tt>poll</tt> method. The two
168 :     * methods differ in that this method does not remove the element
169 :     * from the priority queue.
170 : tim 1.2 *
171 :     * @return the minimal element from this priority queue if it contains
172 :     * one or more elements, otherwise <tt>null</tt>.
173 :     */
174 : tim 1.1 public E peek() {
175 : tim 1.2 return queue[1];
176 : tim 1.1 }
177 :    
178 : tim 1.2 // Collection Methods
179 :    
180 :     /**
181 :     * Removes a single instance of the specified element from this priority
182 :     * queue, if it is present. Returns true if this collection contained the
183 :     * specified element (or equivalently, if this collection changed as a
184 :     * result of the call).
185 :     *
186 : dl 1.7 * @param element the element to be removed from this collection,
187 :     * if present.
188 : tim 1.2 * @return <tt>true</tt> if this collection changed as a result of the
189 :     * call
190 :     * @throws ClassCastException if the specified element cannot be compared
191 : dl 1.7 * with elements currently in the priority queue according
192 : tim 1.2 * to the priority queue's ordering.
193 :     * @throws NullPointerException if the specified element is null.
194 :     */
195 :     public boolean remove(Object element) {
196 :     if (element == null)
197 :     throw new NullPointerException();
198 :    
199 :     if (comparator == null) {
200 :     for (int i = 1; i <= size; i++) {
201 :     if (((Comparable)queue[i]).compareTo(element) == 0) {
202 :     remove(i);
203 :     return true;
204 :     }
205 :     }
206 :     } else {
207 :     for (int i = 1; i <= size; i++) {
208 :     if (comparator.compare(queue[i], (E) element) == 0) {
209 :     remove(i);
210 :     return true;
211 :     }
212 :     }
213 :     }
214 : tim 1.1 return false;
215 :     }
216 : tim 1.2
217 :     /**
218 :     * Returns an iterator over the elements in this priority queue. The
219 : brian 1.6 * elements of the priority queue will be returned by this iterator in the
220 :     * order specified by the queue, which is to say the order they would be
221 :     * returned by repeated calls to <tt>poll</tt>.
222 : dl 1.5 *
223 : tim 1.2 * @return an <tt>Iterator</tt> over the elements in this priority queue.
224 :     */
225 :     public Iterator<E> iterator() {
226 : dl 1.7 return new Itr();
227 : tim 1.2 }
228 :    
229 :     private class Itr implements Iterator<E> {
230 : dl 1.7 /**
231 :     * Index (into queue array) of element to be returned by
232 : tim 1.2 * subsequent call to next.
233 : dl 1.7 */
234 :     private int cursor = 1;
235 : tim 1.2
236 : dl 1.7 /**
237 :     * Index of element returned by most recent call to next or
238 :     * previous. Reset to 0 if this element is deleted by a call
239 :     * to remove.
240 :     */
241 :     private int lastRet = 0;
242 :    
243 :     /**
244 :     * The modCount value that the iterator believes that the backing
245 :     * List should have. If this expectation is violated, the iterator
246 :     * has detected concurrent modification.
247 :     */
248 :     private int expectedModCount = modCount;
249 : tim 1.2
250 : dl 1.7 public boolean hasNext() {
251 :     return cursor <= size;
252 :     }
253 :    
254 :     public E next() {
255 : tim 1.2 checkForComodification();
256 :     if (cursor > size)
257 : dl 1.7 throw new NoSuchElementException();
258 : tim 1.2 E result = queue[cursor];
259 :     lastRet = cursor++;
260 :     return result;
261 : dl 1.7 }
262 : tim 1.2
263 : dl 1.7 public void remove() {
264 :     if (lastRet == 0)
265 :     throw new IllegalStateException();
266 : tim 1.2 checkForComodification();
267 :    
268 :     PriorityQueue.this.remove(lastRet);
269 :     if (lastRet < cursor)
270 :     cursor--;
271 :     lastRet = 0;
272 :     expectedModCount = modCount;
273 : dl 1.7 }
274 : tim 1.2
275 : dl 1.7 final void checkForComodification() {
276 :     if (modCount != expectedModCount)
277 :     throw new ConcurrentModificationException();
278 :     }
279 : tim 1.2 }
280 :    
281 :     /**
282 :     * Returns the number of elements in this priority queue.
283 : dl 1.5 *
284 : tim 1.2 * @return the number of elements in this priority queue.
285 :     */
286 : tim 1.1 public int size() {
287 : tim 1.2 return size;
288 : tim 1.1 }
289 : tim 1.2
290 :     /**
291 :     * Add the specified element to this priority queue.
292 :     *
293 :     * @param element the element to add.
294 :     * @return true
295 :     * @throws ClassCastException if the specified element cannot be compared
296 : dl 1.7 * with elements currently in the priority queue according
297 : tim 1.2 * to the priority queue's ordering.
298 :     * @throws NullPointerException if the specified element is null.
299 :     */
300 :     public boolean offer(E element) {
301 :     if (element == null)
302 :     throw new NullPointerException();
303 :     modCount++;
304 :    
305 :     // Grow backing store if necessary
306 :     if (++size == queue.length) {
307 :     E[] newQueue = new E[2 * queue.length];
308 :     System.arraycopy(queue, 0, newQueue, 0, size);
309 :     queue = newQueue;
310 :     }
311 :    
312 :     queue[size] = element;
313 :     fixUp(size);
314 :     return true;
315 : tim 1.1 }
316 :    
317 : tim 1.2 /**
318 :     * Remove all elements from the priority queue.
319 :     */
320 :     public void clear() {
321 :     modCount++;
322 :    
323 :     // Null out element references to prevent memory leak
324 :     for (int i=1; i<=size; i++)
325 :     queue[i] = null;
326 :    
327 :     size = 0;
328 :     }
329 :    
330 :     /**
331 :     * Removes and returns the ith element from queue. Recall
332 :     * that queue is one-based, so 1 <= i <= size.
333 :     *
334 :     * XXX: Could further special-case i==size, but is it worth it?
335 :     * XXX: Could special-case i==0, but is it worth it?
336 :     */
337 :     private E remove(int i) {
338 :     assert i <= size;
339 :     modCount++;
340 :    
341 :     E result = queue[i];
342 :     queue[i] = queue[size];
343 :     queue[size--] = null; // Drop extra ref to prevent memory leak
344 :     if (i <= size)
345 :     fixDown(i);
346 :     return result;
347 : tim 1.1 }
348 :    
349 : tim 1.2 /**
350 :     * Establishes the heap invariant (described above) assuming the heap
351 :     * satisfies the invariant except possibly for the leaf-node indexed by k
352 :     * (which may have a nextExecutionTime less than its parent's).
353 :     *
354 :     * This method functions by "promoting" queue[k] up the hierarchy
355 :     * (by swapping it with its parent) repeatedly until queue[k]
356 :     * is greater than or equal to its parent.
357 :     */
358 :     private void fixUp(int k) {
359 :     if (comparator == null) {
360 :     while (k > 1) {
361 :     int j = k >> 1;
362 :     if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
363 :     break;
364 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
365 :     k = j;
366 :     }
367 :     } else {
368 :     while (k > 1) {
369 :     int j = k >> 1;
370 :     if (comparator.compare(queue[j], queue[k]) <= 0)
371 :     break;
372 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
373 :     k = j;
374 :     }
375 :     }
376 :     }
377 :    
378 :     /**
379 :     * Establishes the heap invariant (described above) in the subtree
380 :     * rooted at k, which is assumed to satisfy the heap invariant except
381 :     * possibly for node k itself (which may be greater than its children).
382 :     *
383 :     * This method functions by "demoting" queue[k] down the hierarchy
384 :     * (by swapping it with its smaller child) repeatedly until queue[k]
385 :     * is less than or equal to its children.
386 :     */
387 :     private void fixDown(int k) {
388 :     int j;
389 :     if (comparator == null) {
390 :     while ((j = k << 1) <= size) {
391 :     if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
392 :     j++; // j indexes smallest kid
393 :     if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
394 :     break;
395 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
396 :     k = j;
397 :     }
398 :     } else {
399 :     while ((j = k << 1) <= size) {
400 :     if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
401 :     j++; // j indexes smallest kid
402 :     if (comparator.compare(queue[k], queue[j]) <= 0)
403 :     break;
404 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
405 :     k = j;
406 :     }
407 :     }
408 :     }
409 :    
410 :     /**
411 :     * Returns the comparator associated with this priority queue, or
412 :     * <tt>null</tt> if it uses its elements' natural ordering.
413 :     *
414 :     * @return the comparator associated with this priority queue, or
415 : dl 1.7 * <tt>null</tt> if it uses its elements' natural ordering.
416 : tim 1.2 */
417 : dl 1.5 Comparator comparator() {
418 : tim 1.2 return comparator;
419 :     }
420 : dl 1.5
421 :     /**
422 :     * Save the state of the instance to a stream (that
423 :     * is, serialize it).
424 :     *
425 :     * @serialData The length of the array backing the instance is
426 :     * emitted (int), followed by all of its elements (each an
427 :     * <tt>Object</tt>) in the proper order.
428 : dl 1.7 * @param s the stream
429 : dl 1.5 */
430 :     private synchronized void writeObject(java.io.ObjectOutputStream s)
431 :     throws java.io.IOException{
432 : dl 1.7 // Write out element count, and any hidden stuff
433 :     s.defaultWriteObject();
434 : dl 1.5
435 :     // Write out array length
436 :     s.writeInt(queue.length);
437 :    
438 : dl 1.7 // Write out all elements in the proper order.
439 :     for (int i=0; i<size; i++)
440 : dl 1.5 s.writeObject(queue[i]);
441 :     }
442 :    
443 :     /**
444 :     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
445 :     * deserialize it).
446 : dl 1.7 * @param s the stream
447 : dl 1.5 */
448 :     private synchronized void readObject(java.io.ObjectInputStream s)
449 :     throws java.io.IOException, ClassNotFoundException {
450 : dl 1.7 // Read in size, and any hidden stuff
451 :     s.defaultReadObject();
452 : dl 1.5
453 :     // Read in array length and allocate array
454 :     int arrayLength = s.readInt();
455 :     queue = new E[arrayLength];
456 :    
457 : dl 1.7 // Read in all elements in the proper order.
458 :     for (int i=0; i<size; i++)
459 : dl 1.5 queue[i] = (E)s.readObject();
460 :     }
461 :    
462 : tim 1.1 }

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