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 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 |
* 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 insertion methods (<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(Object)</tt> methods; and constant time for the retrieval methods (<tt>peek</tt>, |
24 |
* <tt>element</tt>, and <tt>size</tt>). |
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 |
|
35 |
/** |
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 d |
40 |
* of n, 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 |
private transient E[] queue; |
49 |
|
50 |
/** |
51 |
* The number of elements in the priority queue. |
52 |
*/ |
53 |
private int size = 0; |
54 |
|
55 |
/** |
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 |
private transient int modCount = 0; |
66 |
|
67 |
/** |
68 |
* Create a new priority queue with the default initial capacity (11) |
69 |
* that orders its elements according to their natural ordering (using <tt>Comparable</tt>.) |
70 |
*/ |
71 |
public PriorityQueue() { |
72 |
this(DEFAULT_INITIAL_CAPACITY); |
73 |
} |
74 |
|
75 |
/** |
76 |
* Create a new priority queue with the specified initial capacity |
77 |
* that orders its elements according to their natural ordering (using <tt>Comparable</tt>.) |
78 |
* |
79 |
* @param initialCapacity the initial capacity for this priority queue. |
80 |
*/ |
81 |
public PriorityQueue(int initialCapacity) { |
82 |
this(initialCapacity, null); |
83 |
} |
84 |
|
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 |
} |
98 |
|
99 |
/** |
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 |
107 |
* <tt>Sorted</tt>, 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 |
/* Commented out to compile with generics compiler |
127 |
|
128 |
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 |
*/ |
134 |
{ |
135 |
comparator = null; |
136 |
for (Iterator<E> i = initialElements.iterator(); i.hasNext(); ) |
137 |
add(i.next()); |
138 |
} |
139 |
} |
140 |
|
141 |
// Queue Methods |
142 |
|
143 |
/** |
144 |
* Remove and return the minimal element from this priority queue if |
145 |
* it contains one or more elements, otherwise return <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 |
public E poll() { |
152 |
if (size == 0) |
153 |
return null; |
154 |
return remove(1); |
155 |
} |
156 |
|
157 |
/** |
158 |
* Return, but do not remove, the minimal element from the priority queue, |
159 |
* or return <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 |
* <tt>poll</tt> method. The two methods differ in that this method |
163 |
* 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 |
public E peek() { |
169 |
return queue[1]; |
170 |
} |
171 |
|
172 |
// 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 element the 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 |
* with elements currently in the priority queue according |
185 |
* 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 |
return false; |
208 |
} |
209 |
|
210 |
/** |
211 |
* Returns an iterator over the elements in this priority queue. The |
212 |
* 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 |
* |
216 |
* @return an <tt>Iterator</tt> over the elements in this priority queue. |
217 |
*/ |
218 |
public Iterator<E> iterator() { |
219 |
return new Itr(); |
220 |
} |
221 |
|
222 |
private class Itr implements Iterator<E> { |
223 |
/** |
224 |
* Index (into queue array) of element to be returned by |
225 |
* subsequent call to next. |
226 |
*/ |
227 |
int cursor = 1; |
228 |
|
229 |
/** |
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 |
|
247 |
public E next() { |
248 |
checkForComodification(); |
249 |
if (cursor > size) |
250 |
throw new NoSuchElementException(); |
251 |
E result = queue[cursor]; |
252 |
lastRet = cursor++; |
253 |
return result; |
254 |
} |
255 |
|
256 |
public void remove() { |
257 |
if (lastRet == 0) |
258 |
throw new IllegalStateException(); |
259 |
checkForComodification(); |
260 |
|
261 |
PriorityQueue.this.remove(lastRet); |
262 |
if (lastRet < cursor) |
263 |
cursor--; |
264 |
lastRet = 0; |
265 |
expectedModCount = modCount; |
266 |
} |
267 |
|
268 |
final void checkForComodification() { |
269 |
if (modCount != expectedModCount) |
270 |
throw new ConcurrentModificationException(); |
271 |
} |
272 |
} |
273 |
|
274 |
/** |
275 |
* Returns the number of elements in this priority queue. |
276 |
* |
277 |
* @return the number of elements in this priority queue. |
278 |
*/ |
279 |
public int size() { |
280 |
return size; |
281 |
} |
282 |
|
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 |
* with elements currently in the priority queue according |
290 |
* 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 |
} |
309 |
|
310 |
/** |
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 |
} |
341 |
|
342 |
/** |
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 |
* <tt>null</tt> if it uses its elements' natural ordering. |
409 |
*/ |
410 |
Comparator comparator() { |
411 |
return comparator; |
412 |
} |
413 |
|
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 |
} |