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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 :     * elements according to the order specified at creation time. This order is
6 :     * specified as for {@link TreeSet} and {@link TreeMap}: Elements are ordered
7 :     * 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 :     * these elements are tied for least value, no guarantees are made as to
12 :     * which of elements is returned.
13 :     *
14 :     * <p>Each priority queue has a <i>capacity</i>. The capacity is the size of
15 :     * the array used 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 list,
17 :     * 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 :     * the <tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>
22 :     * methods; linear time for the <tt>remove(Object)</tt> and
23 :     * <tt>contains</tt> methods; and constant time for the <tt>peek</tt>,
24 :     * <tt>element</tt>, and <tt>size</tt> methods.
25 :     *
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 :     * comparator is null: For each node n in the heap, and each descendant
40 :     * of n, d, n <= d.
41 :     *
42 :     * 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 :     *
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 :     * that orders its elements according to their natural ordering.
70 :     */
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 :     * that orders its elements according to their natural ordering.
78 :     *
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 :     * natural order. If the specified collection does not implement the
107 :     * <tt>Sorted</tt> interface, the priority queue is ordered according to
108 :     * 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 :     * it contains one or more elements, otherwise <tt>null</tt>. The term
146 :     * <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 :     * or <tt>null</tt> if the queue is empty. The term <i>minimal</i> is
160 :     * 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 :     * @param o element to be removed from this collection, if present.
181 :     * @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 : dl 1.5 * first element returned by this iterator is the same element that
213 : tim 1.2 * would be returned by a call to <tt>peek</tt>.
214 : dl 1.5 *
215 : tim 1.2 * @return an <tt>Iterator</tt> over the elements in this priority queue.
216 :     */
217 :     public Iterator<E> iterator() {
218 : dl 1.5 return new Itr();
219 : tim 1.2 }
220 :    
221 :     private class Itr implements Iterator<E> {
222 : dl 1.5 /**
223 :     * Index (into queue array) of element to be returned by
224 : tim 1.2 * subsequent call to next.
225 : dl 1.5 */
226 :     int cursor = 1;
227 : tim 1.2
228 : dl 1.5 /**
229 :     * Index of element returned by most recent call to next or
230 :     * previous. Reset to 0 if this element is deleted by a call
231 :     * to remove.
232 :     */
233 :     int lastRet = 0;
234 :    
235 :     /**
236 :     * The modCount value that the iterator believes that the backing
237 :     * List should have. If this expectation is violated, the iterator
238 :     * has detected concurrent modification.
239 :     */
240 :     int expectedModCount = modCount;
241 :    
242 :     public boolean hasNext() {
243 :     return cursor <= size;
244 :     }
245 : tim 1.2
246 : dl 1.5 public E next() {
247 : tim 1.2 checkForComodification();
248 :     if (cursor > size)
249 : dl 1.5 throw new NoSuchElementException();
250 : tim 1.2 E result = queue[cursor];
251 :     lastRet = cursor++;
252 :     return result;
253 : dl 1.5 }
254 : tim 1.2
255 : dl 1.5 public void remove() {
256 :     if (lastRet == 0)
257 :     throw new IllegalStateException();
258 : tim 1.2 checkForComodification();
259 :    
260 :     PriorityQueue.this.remove(lastRet);
261 :     if (lastRet < cursor)
262 :     cursor--;
263 :     lastRet = 0;
264 :     expectedModCount = modCount;
265 : dl 1.5 }
266 : tim 1.2
267 : dl 1.5 final void checkForComodification() {
268 :     if (modCount != expectedModCount)
269 :     throw new ConcurrentModificationException();
270 :     }
271 : tim 1.2 }
272 :    
273 :     /**
274 :     * Returns the number of elements in this priority queue.
275 : dl 1.5 *
276 : tim 1.2 * @return the number of elements in this priority queue.
277 :     */
278 : tim 1.1 public int size() {
279 : tim 1.2 return size;
280 : tim 1.1 }
281 : tim 1.2
282 :     /**
283 :     * Add the specified element to this priority queue.
284 :     *
285 :     * @param element the element to add.
286 :     * @return true
287 :     * @throws ClassCastException if the specified element cannot be compared
288 : dl 1.5 * with elements currently in the priority queue according
289 : tim 1.2 * to the priority queue's ordering.
290 :     * @throws NullPointerException if the specified element is null.
291 :     */
292 :     public boolean offer(E element) {
293 :     if (element == null)
294 :     throw new NullPointerException();
295 :     modCount++;
296 :    
297 :     // Grow backing store if necessary
298 :     if (++size == queue.length) {
299 :     E[] newQueue = new E[2 * queue.length];
300 :     System.arraycopy(queue, 0, newQueue, 0, size);
301 :     queue = newQueue;
302 :     }
303 :    
304 :     queue[size] = element;
305 :     fixUp(size);
306 :     return true;
307 : tim 1.1 }
308 :    
309 : tim 1.2 /**
310 :     * Remove all elements from the priority queue.
311 :     */
312 :     public void clear() {
313 :     modCount++;
314 :    
315 :     // Null out element references to prevent memory leak
316 :     for (int i=1; i<=size; i++)
317 :     queue[i] = null;
318 :    
319 :     size = 0;
320 :     }
321 :    
322 :     /**
323 :     * Removes and returns the ith element from queue. Recall
324 :     * that queue is one-based, so 1 <= i <= size.
325 :     *
326 :     * XXX: Could further special-case i==size, but is it worth it?
327 :     * XXX: Could special-case i==0, but is it worth it?
328 :     */
329 :     private E remove(int i) {
330 :     assert i <= size;
331 :     modCount++;
332 :    
333 :     E result = queue[i];
334 :     queue[i] = queue[size];
335 :     queue[size--] = null; // Drop extra ref to prevent memory leak
336 :     if (i <= size)
337 :     fixDown(i);
338 :     return result;
339 : tim 1.1 }
340 :    
341 : tim 1.2 /**
342 :     * Establishes the heap invariant (described above) assuming the heap
343 :     * satisfies the invariant except possibly for the leaf-node indexed by k
344 :     * (which may have a nextExecutionTime less than its parent's).
345 :     *
346 :     * This method functions by "promoting" queue[k] up the hierarchy
347 :     * (by swapping it with its parent) repeatedly until queue[k]
348 :     * is greater than or equal to its parent.
349 :     */
350 :     private void fixUp(int k) {
351 :     if (comparator == null) {
352 :     while (k > 1) {
353 :     int j = k >> 1;
354 :     if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
355 :     break;
356 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
357 :     k = j;
358 :     }
359 :     } else {
360 :     while (k > 1) {
361 :     int j = k >> 1;
362 :     if (comparator.compare(queue[j], queue[k]) <= 0)
363 :     break;
364 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
365 :     k = j;
366 :     }
367 :     }
368 :     }
369 :    
370 :     /**
371 :     * Establishes the heap invariant (described above) in the subtree
372 :     * rooted at k, which is assumed to satisfy the heap invariant except
373 :     * possibly for node k itself (which may be greater than its children).
374 :     *
375 :     * This method functions by "demoting" queue[k] down the hierarchy
376 :     * (by swapping it with its smaller child) repeatedly until queue[k]
377 :     * is less than or equal to its children.
378 :     */
379 :     private void fixDown(int k) {
380 :     int j;
381 :     if (comparator == null) {
382 :     while ((j = k << 1) <= size) {
383 :     if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
384 :     j++; // j indexes smallest kid
385 :     if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
386 :     break;
387 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
388 :     k = j;
389 :     }
390 :     } else {
391 :     while ((j = k << 1) <= size) {
392 :     if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
393 :     j++; // j indexes smallest kid
394 :     if (comparator.compare(queue[k], queue[j]) <= 0)
395 :     break;
396 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
397 :     k = j;
398 :     }
399 :     }
400 :     }
401 :    
402 :     /**
403 :     * Returns the comparator associated with this priority queue, or
404 :     * <tt>null</tt> if it uses its elements' natural ordering.
405 :     *
406 :     * @return the comparator associated with this priority queue, or
407 : dl 1.5 * <tt>null</tt> if it uses its elements' natural ordering.
408 : tim 1.2 */
409 : dl 1.5 Comparator comparator() {
410 : tim 1.2 return comparator;
411 :     }
412 : dl 1.5
413 :     /**
414 :     * Save the state of the instance to a stream (that
415 :     * is, serialize it).
416 :     *
417 :     * @serialData The length of the array backing the instance is
418 :     * emitted (int), followed by all of its elements (each an
419 :     * <tt>Object</tt>) in the proper order.
420 :     */
421 :     private synchronized void writeObject(java.io.ObjectOutputStream s)
422 :     throws java.io.IOException{
423 :     // Write out element count, and any hidden stuff
424 :     s.defaultWriteObject();
425 :    
426 :     // Write out array length
427 :     s.writeInt(queue.length);
428 :    
429 :     // Write out all elements in the proper order.
430 :     for (int i=0; i<size; i++)
431 :     s.writeObject(queue[i]);
432 :     }
433 :    
434 :     /**
435 :     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
436 :     * deserialize it).
437 :     */
438 :     private synchronized void readObject(java.io.ObjectInputStream s)
439 :     throws java.io.IOException, ClassNotFoundException {
440 :     // Read in size, and any hidden stuff
441 :     s.defaultReadObject();
442 :    
443 :     // Read in array length and allocate array
444 :     int arrayLength = s.readInt();
445 :     queue = new E[arrayLength];
446 :    
447 :     // Read in all elements in the proper order.
448 :     for (int i=0; i<size; i++)
449 :     queue[i] = (E)s.readObject();
450 :     }
451 :    
452 : tim 1.1 }

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