/[cvs]/jsr166/src/main/java/util/PriorityQueue.java
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Contents of /jsr166/src/main/java/util/PriorityQueue.java

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Revision 1.7 - (show annotations)
Tue Jun 24 14:34:30 2003 UTC (16 years, 2 months ago) by dl
Branch: MAIN
Changes since 1.6: +78 -69 lines
Added missing javadoc tags; minor reformatting

1 package java.util;
2
3 /**
4 * An unbounded priority queue based on a priority heap. This queue orders
5 * 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 * 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 * elements are tied for least value, no guarantees are made as to
12 * which of these elements is returned.
13 *
14 * <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 *
20 *<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 *
27 * <p>This class is a member of the
28 * <a href="{@docRoot}/../guide/collections/index.html">
29 * Java Collections Framework</a>.
30 * @since 1.5
31 * @author Josh Bloch
32 */
33 public class PriorityQueue<E> extends AbstractQueue<E>
34 implements Queue<E> {
35 private static final int DEFAULT_INITIAL_CAPACITY = 11;
36
37 /**
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 * comparator is null: For each node n in the heap and each descendant d
42 * of n, n <= d.
43 *
44 * 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 *
48 * queue.length must be >= 2, even if size == 0.
49 */
50 private transient E[] queue;
51
52 /**
53 * The number of elements in the priority queue.
54 */
55 private int size = 0;
56
57 /**
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 private transient int modCount = 0;
68
69 /**
70 * 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 */
74 public PriorityQueue() {
75 this(DEFAULT_INITIAL_CAPACITY);
76 }
77
78 /**
79 * Create a new priority queue with the specified initial capacity
80 * that orders its elements according to their natural ordering
81 * (using <tt>Comparable</tt>.)
82 *
83 * @param initialCapacity the initial capacity for this priority queue.
84 */
85 public PriorityQueue(int initialCapacity) {
86 this(initialCapacity, null);
87 }
88
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 }
102
103 /**
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 * natural order. If the specified collection does not implement
111 * <tt>Sorted</tt>, the priority queue is ordered according to
112 * 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 /* Commented out to compile with generics compiler
131
132 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 */
138 {
139 comparator = null;
140 for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
141 add(i.next());
142 }
143 }
144
145 // Queue Methods
146
147 /**
148 * 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 *
153 * @return the minimal element from this priority queue if it contains
154 * one or more elements, otherwise <tt>null</tt>.
155 */
156 public E poll() {
157 if (size == 0)
158 return null;
159 return remove(1);
160 }
161
162 /**
163 * 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 *
171 * @return the minimal element from this priority queue if it contains
172 * one or more elements, otherwise <tt>null</tt>.
173 */
174 public E peek() {
175 return queue[1];
176 }
177
178 // 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 * @param element the element to be removed from this collection,
187 * if present.
188 * @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 * with elements currently in the priority queue according
192 * 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 return false;
215 }
216
217 /**
218 * Returns an iterator over the elements in this priority queue. The
219 * 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 *
223 * @return an <tt>Iterator</tt> over the elements in this priority queue.
224 */
225 public Iterator<E> iterator() {
226 return new Itr();
227 }
228
229 private class Itr implements Iterator<E> {
230 /**
231 * Index (into queue array) of element to be returned by
232 * subsequent call to next.
233 */
234 private int cursor = 1;
235
236 /**
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
250 public boolean hasNext() {
251 return cursor <= size;
252 }
253
254 public E next() {
255 checkForComodification();
256 if (cursor > size)
257 throw new NoSuchElementException();
258 E result = queue[cursor];
259 lastRet = cursor++;
260 return result;
261 }
262
263 public void remove() {
264 if (lastRet == 0)
265 throw new IllegalStateException();
266 checkForComodification();
267
268 PriorityQueue.this.remove(lastRet);
269 if (lastRet < cursor)
270 cursor--;
271 lastRet = 0;
272 expectedModCount = modCount;
273 }
274
275 final void checkForComodification() {
276 if (modCount != expectedModCount)
277 throw new ConcurrentModificationException();
278 }
279 }
280
281 /**
282 * Returns the number of elements in this priority queue.
283 *
284 * @return the number of elements in this priority queue.
285 */
286 public int size() {
287 return size;
288 }
289
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 * with elements currently in the priority queue according
297 * 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 }
316
317 /**
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 }
348
349 /**
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 * <tt>null</tt> if it uses its elements' natural ordering.
416 */
417 Comparator comparator() {
418 return comparator;
419 }
420
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 * @param s the stream
429 */
430 private synchronized void writeObject(java.io.ObjectOutputStream s)
431 throws java.io.IOException{
432 // Write out element count, and any hidden stuff
433 s.defaultWriteObject();
434
435 // Write out array length
436 s.writeInt(queue.length);
437
438 // Write out all elements in the proper order.
439 for (int i=0; i<size; i++)
440 s.writeObject(queue[i]);
441 }
442
443 /**
444 * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
445 * deserialize it).
446 * @param s the stream
447 */
448 private synchronized void readObject(java.io.ObjectInputStream s)
449 throws java.io.IOException, ClassNotFoundException {
450 // Read in size, and any hidden stuff
451 s.defaultReadObject();
452
453 // Read in array length and allocate array
454 int arrayLength = s.readInt();
455 queue = new E[arrayLength];
456
457 // Read in all elements in the proper order.
458 for (int i=0; i<size; i++)
459 queue[i] = (E)s.readObject();
460 }
461
462 }

dl@cs.oswego.edu
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