[cvs] / jsr166 / src / main / java / util / PriorityQueue.java Repository:
ViewVC logotype

Annotation of /jsr166/src/main/java/util/PriorityQueue.java

Parent Directory Parent Directory | Revision Log Revision Log


Revision 1.8 - (view) (download)

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 1.8 implements Queue<E>,
35 :     java.io.Serializable {
36 : tim 1.2 private static final int DEFAULT_INITIAL_CAPACITY = 11;
37 : tim 1.1
38 : tim 1.2 /**
39 :     * Priority queue represented as a balanced binary heap: the two children
40 :     * of queue[n] are queue[2*n] and queue[2*n + 1]. The priority queue is
41 :     * ordered by comparator, or by the elements' natural ordering, if
42 : brian 1.6 * comparator is null: For each node n in the heap and each descendant d
43 :     * of n, n <= d.
44 : tim 1.2 *
45 : brian 1.6 * The element with the lowest value is in queue[1], assuming the queue is
46 :     * nonempty. (A one-based array is used in preference to the traditional
47 :     * zero-based array to simplify parent and child calculations.)
48 : tim 1.2 *
49 :     * queue.length must be >= 2, even if size == 0.
50 :     */
51 : dl 1.5 private transient E[] queue;
52 : tim 1.1
53 : tim 1.2 /**
54 :     * The number of elements in the priority queue.
55 :     */
56 :     private int size = 0;
57 : tim 1.1
58 : tim 1.2 /**
59 :     * The comparator, or null if priority queue uses elements'
60 :     * natural ordering.
61 :     */
62 :     private final Comparator<E> comparator;
63 :    
64 :     /**
65 :     * The number of times this priority queue has been
66 :     * <i>structurally modified</i>. See AbstractList for gory details.
67 :     */
68 : dl 1.5 private transient int modCount = 0;
69 : tim 1.2
70 :     /**
71 : dl 1.7 * Create a new priority queue with the default initial capacity
72 :     * (11) that orders its elements according to their natural
73 :     * ordering (using <tt>Comparable</tt>.)
74 : tim 1.2 */
75 :     public PriorityQueue() {
76 :     this(DEFAULT_INITIAL_CAPACITY);
77 : tim 1.1 }
78 : tim 1.2
79 :     /**
80 :     * Create a new priority queue with the specified initial capacity
81 : dl 1.7 * that orders its elements according to their natural ordering
82 :     * (using <tt>Comparable</tt>.)
83 : tim 1.2 *
84 :     * @param initialCapacity the initial capacity for this priority queue.
85 :     */
86 :     public PriorityQueue(int initialCapacity) {
87 :     this(initialCapacity, null);
88 : tim 1.1 }
89 : tim 1.2
90 :     /**
91 :     * Create a new priority queue with the specified initial capacity (11)
92 :     * that orders its elements according to the specified comparator.
93 :     *
94 :     * @param initialCapacity the initial capacity for this priority queue.
95 :     * @param comparator the comparator used to order this priority queue.
96 :     */
97 :     public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
98 :     if (initialCapacity < 1)
99 :     initialCapacity = 1;
100 :     queue = new E[initialCapacity + 1];
101 :     this.comparator = comparator;
102 : tim 1.1 }
103 :    
104 : tim 1.2 /**
105 :     * Create a new priority queue containing the elements in the specified
106 :     * collection. The priority queue has an initial capacity of 110% of the
107 :     * size of the specified collection. If the specified collection
108 :     * implements the {@link Sorted} interface, the priority queue will be
109 :     * sorted according to the same comparator, or according to its elements'
110 :     * natural order if the collection is sorted according to its elements'
111 : brian 1.6 * natural order. If the specified collection does not implement
112 :     * <tt>Sorted</tt>, the priority queue is ordered according to
113 : tim 1.2 * its elements' natural order.
114 :     *
115 :     * @param initialElements the collection whose elements are to be placed
116 :     * into this priority queue.
117 :     * @throws ClassCastException if elements of the specified collection
118 :     * cannot be compared to one another according to the priority
119 :     * queue's ordering.
120 :     * @throws NullPointerException if the specified collection or an
121 :     * element of the specified collection is <tt>null</tt>.
122 :     */
123 :     public PriorityQueue(Collection<E> initialElements) {
124 :     int sz = initialElements.size();
125 :     int initialCapacity = (int)Math.min((sz * 110L) / 100,
126 :     Integer.MAX_VALUE - 1);
127 :     if (initialCapacity < 1)
128 :     initialCapacity = 1;
129 :     queue = new E[initialCapacity + 1];
130 :    
131 : dl 1.5
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 :     comparator = null;
138 :     for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
139 :     add(i.next());
140 :     }
141 : tim 1.1 }
142 :    
143 : tim 1.2 // Queue Methods
144 :    
145 :     /**
146 : dl 1.7 * Remove and return the minimal element from this priority queue
147 :     * if it contains one or more elements, otherwise return
148 :     * <tt>null</tt>. The term <i>minimal</i> is defined according to
149 :     * this priority queue's order.
150 : tim 1.2 *
151 :     * @return the minimal element from this priority queue if it contains
152 :     * one or more elements, otherwise <tt>null</tt>.
153 :     */
154 : tim 1.1 public E poll() {
155 : tim 1.2 if (size == 0)
156 :     return null;
157 :     return remove(1);
158 : tim 1.1 }
159 : tim 1.2
160 :     /**
161 : dl 1.7 * Return, but do not remove, the minimal element from the
162 :     * priority queue, or return <tt>null</tt> if the queue is empty.
163 :     * The term <i>minimal</i> is defined according to this priority
164 :     * queue's order. This method returns the same object reference
165 :     * that would be returned by by the <tt>poll</tt> method. The two
166 :     * methods differ in that this method does not remove the element
167 :     * from the priority queue.
168 : tim 1.2 *
169 :     * @return the minimal element from this priority queue if it contains
170 :     * one or more elements, otherwise <tt>null</tt>.
171 :     */
172 : tim 1.1 public E peek() {
173 : tim 1.2 return queue[1];
174 : tim 1.1 }
175 :    
176 : tim 1.2 // Collection Methods
177 :    
178 :     /**
179 :     * Removes a single instance of the specified element from this priority
180 :     * queue, if it is present. Returns true if this collection contained the
181 :     * specified element (or equivalently, if this collection changed as a
182 :     * result of the call).
183 :     *
184 : dl 1.7 * @param element the element to be removed from this collection,
185 :     * if present.
186 : tim 1.2 * @return <tt>true</tt> if this collection changed as a result of the
187 :     * call
188 :     * @throws ClassCastException if the specified element cannot be compared
189 : dl 1.7 * with elements currently in the priority queue according
190 : tim 1.2 * to the priority queue's ordering.
191 :     * @throws NullPointerException if the specified element is null.
192 :     */
193 :     public boolean remove(Object element) {
194 :     if (element == null)
195 :     throw new NullPointerException();
196 :    
197 :     if (comparator == null) {
198 :     for (int i = 1; i <= size; i++) {
199 :     if (((Comparable)queue[i]).compareTo(element) == 0) {
200 :     remove(i);
201 :     return true;
202 :     }
203 :     }
204 :     } else {
205 :     for (int i = 1; i <= size; i++) {
206 :     if (comparator.compare(queue[i], (E) element) == 0) {
207 :     remove(i);
208 :     return true;
209 :     }
210 :     }
211 :     }
212 : tim 1.1 return false;
213 :     }
214 : tim 1.2
215 :     /**
216 :     * Returns an iterator over the elements in this priority queue. The
217 : brian 1.6 * elements of the priority queue will be returned by this iterator in the
218 :     * order specified by the queue, which is to say the order they would be
219 :     * returned by repeated calls to <tt>poll</tt>.
220 : dl 1.5 *
221 : tim 1.2 * @return an <tt>Iterator</tt> over the elements in this priority queue.
222 :     */
223 :     public Iterator<E> iterator() {
224 : dl 1.7 return new Itr();
225 : tim 1.2 }
226 :    
227 :     private class Itr implements Iterator<E> {
228 : dl 1.7 /**
229 :     * Index (into queue array) of element to be returned by
230 : tim 1.2 * subsequent call to next.
231 : dl 1.7 */
232 :     private int cursor = 1;
233 : tim 1.2
234 : dl 1.7 /**
235 :     * Index of element returned by most recent call to next or
236 :     * previous. Reset to 0 if this element is deleted by a call
237 :     * to remove.
238 :     */
239 :     private int lastRet = 0;
240 :    
241 :     /**
242 :     * The modCount value that the iterator believes that the backing
243 :     * List should have. If this expectation is violated, the iterator
244 :     * has detected concurrent modification.
245 :     */
246 :     private int expectedModCount = modCount;
247 : tim 1.2
248 : dl 1.7 public boolean hasNext() {
249 :     return cursor <= size;
250 :     }
251 :    
252 :     public E next() {
253 : tim 1.2 checkForComodification();
254 :     if (cursor > size)
255 : dl 1.7 throw new NoSuchElementException();
256 : tim 1.2 E result = queue[cursor];
257 :     lastRet = cursor++;
258 :     return result;
259 : dl 1.7 }
260 : tim 1.2
261 : dl 1.7 public void remove() {
262 :     if (lastRet == 0)
263 :     throw new IllegalStateException();
264 : tim 1.2 checkForComodification();
265 :    
266 :     PriorityQueue.this.remove(lastRet);
267 :     if (lastRet < cursor)
268 :     cursor--;
269 :     lastRet = 0;
270 :     expectedModCount = modCount;
271 : dl 1.7 }
272 : tim 1.2
273 : dl 1.7 final void checkForComodification() {
274 :     if (modCount != expectedModCount)
275 :     throw new ConcurrentModificationException();
276 :     }
277 : tim 1.2 }
278 :    
279 :     /**
280 :     * Returns the number of elements in this priority queue.
281 : dl 1.5 *
282 : tim 1.2 * @return the number of elements in this priority queue.
283 :     */
284 : tim 1.1 public int size() {
285 : tim 1.2 return size;
286 : tim 1.1 }
287 : tim 1.2
288 :     /**
289 :     * Add the specified element to this priority queue.
290 :     *
291 :     * @param element the element to add.
292 :     * @return true
293 :     * @throws ClassCastException if the specified element cannot be compared
294 : dl 1.7 * with elements currently in the priority queue according
295 : tim 1.2 * to the priority queue's ordering.
296 :     * @throws NullPointerException if the specified element is null.
297 :     */
298 :     public boolean offer(E element) {
299 :     if (element == null)
300 :     throw new NullPointerException();
301 :     modCount++;
302 :    
303 :     // Grow backing store if necessary
304 :     if (++size == queue.length) {
305 :     E[] newQueue = new E[2 * queue.length];
306 :     System.arraycopy(queue, 0, newQueue, 0, size);
307 :     queue = newQueue;
308 :     }
309 :    
310 :     queue[size] = element;
311 :     fixUp(size);
312 :     return true;
313 : tim 1.1 }
314 :    
315 : tim 1.2 /**
316 :     * Remove all elements from the priority queue.
317 :     */
318 :     public void clear() {
319 :     modCount++;
320 :    
321 :     // Null out element references to prevent memory leak
322 :     for (int i=1; i<=size; i++)
323 :     queue[i] = null;
324 :    
325 :     size = 0;
326 :     }
327 :    
328 :     /**
329 :     * Removes and returns the ith element from queue. Recall
330 :     * that queue is one-based, so 1 <= i <= size.
331 :     *
332 :     * XXX: Could further special-case i==size, but is it worth it?
333 :     * XXX: Could special-case i==0, but is it worth it?
334 :     */
335 :     private E remove(int i) {
336 :     assert i <= size;
337 :     modCount++;
338 :    
339 :     E result = queue[i];
340 :     queue[i] = queue[size];
341 :     queue[size--] = null; // Drop extra ref to prevent memory leak
342 :     if (i <= size)
343 :     fixDown(i);
344 :     return result;
345 : tim 1.1 }
346 :    
347 : tim 1.2 /**
348 :     * Establishes the heap invariant (described above) assuming the heap
349 :     * satisfies the invariant except possibly for the leaf-node indexed by k
350 :     * (which may have a nextExecutionTime less than its parent's).
351 :     *
352 :     * This method functions by "promoting" queue[k] up the hierarchy
353 :     * (by swapping it with its parent) repeatedly until queue[k]
354 :     * is greater than or equal to its parent.
355 :     */
356 :     private void fixUp(int k) {
357 :     if (comparator == null) {
358 :     while (k > 1) {
359 :     int j = k >> 1;
360 :     if (((Comparable)queue[j]).compareTo(queue[k]) <= 0)
361 :     break;
362 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
363 :     k = j;
364 :     }
365 :     } else {
366 :     while (k > 1) {
367 :     int j = k >> 1;
368 :     if (comparator.compare(queue[j], queue[k]) <= 0)
369 :     break;
370 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
371 :     k = j;
372 :     }
373 :     }
374 :     }
375 :    
376 :     /**
377 :     * Establishes the heap invariant (described above) in the subtree
378 :     * rooted at k, which is assumed to satisfy the heap invariant except
379 :     * possibly for node k itself (which may be greater than its children).
380 :     *
381 :     * This method functions by "demoting" queue[k] down the hierarchy
382 :     * (by swapping it with its smaller child) repeatedly until queue[k]
383 :     * is less than or equal to its children.
384 :     */
385 :     private void fixDown(int k) {
386 :     int j;
387 :     if (comparator == null) {
388 :     while ((j = k << 1) <= size) {
389 :     if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0)
390 :     j++; // j indexes smallest kid
391 :     if (((Comparable)queue[k]).compareTo(queue[j]) <= 0)
392 :     break;
393 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
394 :     k = j;
395 :     }
396 :     } else {
397 :     while ((j = k << 1) <= size) {
398 :     if (j < size && comparator.compare(queue[j], queue[j+1]) > 0)
399 :     j++; // j indexes smallest kid
400 :     if (comparator.compare(queue[k], queue[j]) <= 0)
401 :     break;
402 :     E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp;
403 :     k = j;
404 :     }
405 :     }
406 :     }
407 :    
408 :     /**
409 :     * Returns the comparator associated with this priority queue, or
410 :     * <tt>null</tt> if it uses its elements' natural ordering.
411 :     *
412 :     * @return the comparator associated with this priority queue, or
413 : dl 1.7 * <tt>null</tt> if it uses its elements' natural ordering.
414 : tim 1.2 */
415 : dl 1.8 public Comparator comparator() {
416 : tim 1.2 return comparator;
417 :     }
418 : dl 1.5
419 :     /**
420 :     * Save the state of the instance to a stream (that
421 :     * is, serialize it).
422 :     *
423 :     * @serialData The length of the array backing the instance is
424 :     * emitted (int), followed by all of its elements (each an
425 :     * <tt>Object</tt>) in the proper order.
426 : dl 1.7 * @param s the stream
427 : dl 1.5 */
428 :     private synchronized void writeObject(java.io.ObjectOutputStream s)
429 :     throws java.io.IOException{
430 : dl 1.7 // Write out element count, and any hidden stuff
431 :     s.defaultWriteObject();
432 : dl 1.5
433 :     // Write out array length
434 :     s.writeInt(queue.length);
435 :    
436 : dl 1.7 // Write out all elements in the proper order.
437 :     for (int i=0; i<size; i++)
438 : dl 1.5 s.writeObject(queue[i]);
439 :     }
440 :    
441 :     /**
442 :     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
443 :     * deserialize it).
444 : dl 1.7 * @param s the stream
445 : dl 1.5 */
446 :     private synchronized void readObject(java.io.ObjectInputStream s)
447 :     throws java.io.IOException, ClassNotFoundException {
448 : dl 1.7 // Read in size, and any hidden stuff
449 :     s.defaultReadObject();
450 : dl 1.5
451 :     // Read in array length and allocate array
452 :     int arrayLength = s.readInt();
453 :     queue = new E[arrayLength];
454 :    
455 : dl 1.7 // Read in all elements in the proper order.
456 :     for (int i=0; i<size; i++)
457 : dl 1.5 queue[i] = (E)s.readObject();
458 :     }
459 :    
460 : tim 1.1 }

Doug Lea
ViewVC Help
Powered by ViewVC 1.0.8