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}: |
7 |
* elements are ordered |
8 |
* either according to their <i>natural order</i> (see {@link Comparable}), or |
9 |
* according to a {@link Comparator}, depending on which constructor is used. |
10 |
* The <em>head</em> of this queue is the least element with respect to the |
11 |
* specified ordering. If multiple elements are tied for least value, the |
12 |
* head is one of those elements. A priority queue does not permit |
13 |
* <tt>null</tt> elements. |
14 |
* |
15 |
* <p>The {@link #remove()} and {@link #poll()} methods remove and |
16 |
* return the head of the queue. |
17 |
* |
18 |
* <p>The {@link #element()} and {@link #peek()} methods return, but do |
19 |
* not delete, the head of the queue. |
20 |
* |
21 |
* <p>A priority queue has a <i>capacity</i>. The capacity is the |
22 |
* size of the array used internally to store the elements on the |
23 |
* queue. It is always at least as large as the queue size. As |
24 |
* elements are added to a priority queue, its capacity grows |
25 |
* automatically. The details of the growth policy are not specified. |
26 |
* |
27 |
* <p>Implementation note: this implementation provides O(log(n)) time |
28 |
* for the insertion methods (<tt>offer</tt>, <tt>poll</tt>, |
29 |
* <tt>remove()</tt> and <tt>add</tt>) methods; linear time for the |
30 |
* <tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and |
31 |
* constant time for the retrieval methods (<tt>peek</tt>, |
32 |
* <tt>element</tt>, and <tt>size</tt>). |
33 |
* |
34 |
* <p>This class is a member of the |
35 |
* <a href="{@docRoot}/../guide/collections/index.html"> |
36 |
* Java Collections Framework</a>. |
37 |
* @since 1.5 |
38 |
* @author Josh Bloch |
39 |
*/ |
40 |
public class PriorityQueue<E> extends AbstractQueue<E> |
41 |
implements Queue<E>, java.io.Serializable { |
42 |
|
43 |
private static final int DEFAULT_INITIAL_CAPACITY = 11; |
44 |
|
45 |
/** |
46 |
* Priority queue represented as a balanced binary heap: the two children |
47 |
* of queue[n] are queue[2*n] and queue[2*n + 1]. The priority queue is |
48 |
* ordered by comparator, or by the elements' natural ordering, if |
49 |
* comparator is null: For each node n in the heap and each descendant d |
50 |
* of n, n <= d. |
51 |
* |
52 |
* The element with the lowest value is in queue[1], assuming the queue is |
53 |
* nonempty. (A one-based array is used in preference to the traditional |
54 |
* zero-based array to simplify parent and child calculations.) |
55 |
* |
56 |
* queue.length must be >= 2, even if size == 0. |
57 |
*/ |
58 |
private transient Object[] queue; |
59 |
|
60 |
/** |
61 |
* The number of elements in the priority queue. |
62 |
*/ |
63 |
private int size = 0; |
64 |
|
65 |
/** |
66 |
* The comparator, or null if priority queue uses elements' |
67 |
* natural ordering. |
68 |
*/ |
69 |
private final Comparator<? super E> comparator; |
70 |
|
71 |
/** |
72 |
* The number of times this priority queue has been |
73 |
* <i>structurally modified</i>. See AbstractList for gory details. |
74 |
*/ |
75 |
private transient int modCount = 0; |
76 |
|
77 |
/** |
78 |
* Create a <tt>PriorityQueue</tt> with the default initial capacity |
79 |
* (11) that orders its elements according to their natural |
80 |
* ordering (using <tt>Comparable</tt>.) |
81 |
*/ |
82 |
public PriorityQueue() { |
83 |
this(DEFAULT_INITIAL_CAPACITY, null); |
84 |
} |
85 |
|
86 |
/** |
87 |
* Create a <tt>PriorityQueue</tt> with the specified initial capacity |
88 |
* that orders its elements according to their natural ordering |
89 |
* (using <tt>Comparable</tt>.) |
90 |
* |
91 |
* @param initialCapacity the initial capacity for this priority queue. |
92 |
*/ |
93 |
public PriorityQueue(int initialCapacity) { |
94 |
this(initialCapacity, null); |
95 |
} |
96 |
|
97 |
/** |
98 |
* Create a <tt>PriorityQueue</tt> with the specified initial capacity |
99 |
* that orders its elements according to the specified comparator. |
100 |
* |
101 |
* @param initialCapacity the initial capacity for this priority queue. |
102 |
* @param comparator the comparator used to order this priority queue. |
103 |
* If <tt>null</tt> then the order depends on the elements' natural |
104 |
* ordering. |
105 |
* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less |
106 |
* than 1 |
107 |
*/ |
108 |
public PriorityQueue(int initialCapacity, Comparator<? super E> comparator) { |
109 |
if (initialCapacity < 1) |
110 |
throw new IllegalArgumentException(); |
111 |
this.queue = new Object[initialCapacity + 1]; |
112 |
this.comparator = comparator; |
113 |
} |
114 |
|
115 |
/** |
116 |
* Create a <tt>PriorityQueue</tt> containing the elements in the specified |
117 |
* collection. The priority queue has an initial capacity of 110% of the |
118 |
* size of the specified collection; or 1 if the collection is empty. |
119 |
* If the specified collection |
120 |
* implements the {@link Sorted} interface, the priority queue will be |
121 |
* sorted according to the same comparator, or according to its elements' |
122 |
* natural order if the collection is sorted according to its elements' |
123 |
* natural order. If the specified collection does not implement |
124 |
* <tt>Sorted</tt>, the priority queue is ordered according to |
125 |
* its elements' natural order. |
126 |
* |
127 |
* @param c the collection whose elements are to be placed |
128 |
* into this priority queue. |
129 |
* @throws ClassCastException if elements of the specified collection |
130 |
* cannot be compared to one another according to the priority |
131 |
* queue's ordering. |
132 |
* @throws NullPointerException if <tt>c</tt> or any element within it |
133 |
* is <tt>null</tt> |
134 |
*/ |
135 |
public PriorityQueue(Collection<? extends E> c) { |
136 |
int sz = c.size(); |
137 |
int initialCapacity = (int)Math.min((sz * 110L) / 100, |
138 |
Integer.MAX_VALUE - 1); |
139 |
if (initialCapacity < 1) |
140 |
initialCapacity = 1; |
141 |
|
142 |
this.queue = new Object[initialCapacity + 1]; |
143 |
|
144 |
if (c instanceof Sorted) { |
145 |
// FIXME: this code assumes too much |
146 |
this.comparator = (Comparator<? super E>) ((Sorted)c).comparator(); |
147 |
for (Iterator<? extends E> i = c.iterator(); i.hasNext(); ) |
148 |
queue[++size] = i.next(); |
149 |
} else { |
150 |
comparator = null; |
151 |
for (Iterator<? extends E> i = c.iterator(); i.hasNext(); ) |
152 |
add(i.next()); |
153 |
} |
154 |
} |
155 |
|
156 |
// Queue Methods |
157 |
|
158 |
/** |
159 |
* Add the specified element to this priority queue. |
160 |
* |
161 |
* @param element the element to add. |
162 |
* @return <tt>true</tt> |
163 |
* @throws ClassCastException if the specified element cannot be compared |
164 |
* with elements currently in the priority queue according |
165 |
* to the priority queue's ordering. |
166 |
* @throws NullPointerException if the specified element is null. |
167 |
*/ |
168 |
public boolean offer(E element) { |
169 |
if (element == null) |
170 |
throw new NullPointerException(); |
171 |
modCount++; |
172 |
++size; |
173 |
|
174 |
// Grow backing store if necessary |
175 |
while (size >= queue.length) { |
176 |
Object[] newQueue = new Object[2 * queue.length]; |
177 |
System.arraycopy(queue, 0, newQueue, 0, queue.length); |
178 |
queue = newQueue; |
179 |
} |
180 |
|
181 |
queue[size] = element; |
182 |
fixUp(size); |
183 |
return true; |
184 |
} |
185 |
|
186 |
public E poll() { |
187 |
if (size == 0) |
188 |
return null; |
189 |
return (E) remove(1); |
190 |
} |
191 |
|
192 |
public E peek() { |
193 |
return (E) queue[1]; |
194 |
} |
195 |
|
196 |
// Collection Methods |
197 |
|
198 |
// these first two override just to get the throws docs |
199 |
|
200 |
/** |
201 |
* @throws NullPointerException if the specified element is <tt>null</tt>. |
202 |
* @throws ClassCastException if the specified element cannot be compared |
203 |
* with elements currently in the priority queue according |
204 |
* to the priority queue's ordering. |
205 |
*/ |
206 |
public boolean add(E element) { |
207 |
return super.add(element); |
208 |
} |
209 |
|
210 |
/** |
211 |
* @throws NullPointerException if any element is <tt>null</tt>. |
212 |
* @throws ClassCastException if any element cannot be compared |
213 |
* with elements currently in the priority queue according |
214 |
* to the priority queue's ordering. |
215 |
*/ |
216 |
public boolean addAll(Collection<? extends E> c) { |
217 |
return super.addAll(c); |
218 |
} |
219 |
|
220 |
public boolean remove(Object o) { |
221 |
if (o == null) |
222 |
return false; |
223 |
|
224 |
if (comparator == null) { |
225 |
for (int i = 1; i <= size; i++) { |
226 |
if (((Comparable<E>)queue[i]).compareTo((E)o) == 0) { |
227 |
remove(i); |
228 |
return true; |
229 |
} |
230 |
} |
231 |
} else { |
232 |
for (int i = 1; i <= size; i++) { |
233 |
if (comparator.compare((E)queue[i], (E)o) == 0) { |
234 |
remove(i); |
235 |
return true; |
236 |
} |
237 |
} |
238 |
} |
239 |
return false; |
240 |
} |
241 |
|
242 |
public Iterator<E> iterator() { |
243 |
return new Itr(); |
244 |
} |
245 |
|
246 |
private class Itr implements Iterator<E> { |
247 |
/** |
248 |
* Index (into queue array) of element to be returned by |
249 |
* subsequent call to next. |
250 |
*/ |
251 |
private int cursor = 1; |
252 |
|
253 |
/** |
254 |
* Index of element returned by most recent call to next or |
255 |
* previous. Reset to 0 if this element is deleted by a call |
256 |
* to remove. |
257 |
*/ |
258 |
private int lastRet = 0; |
259 |
|
260 |
/** |
261 |
* The modCount value that the iterator believes that the backing |
262 |
* List should have. If this expectation is violated, the iterator |
263 |
* has detected concurrent modification. |
264 |
*/ |
265 |
private int expectedModCount = modCount; |
266 |
|
267 |
public boolean hasNext() { |
268 |
return cursor <= size; |
269 |
} |
270 |
|
271 |
public E next() { |
272 |
checkForComodification(); |
273 |
if (cursor > size) |
274 |
throw new NoSuchElementException(); |
275 |
E result = (E) queue[cursor]; |
276 |
lastRet = cursor++; |
277 |
return result; |
278 |
} |
279 |
|
280 |
public void remove() { |
281 |
if (lastRet == 0) |
282 |
throw new IllegalStateException(); |
283 |
checkForComodification(); |
284 |
|
285 |
PriorityQueue.this.remove(lastRet); |
286 |
if (lastRet < cursor) |
287 |
cursor--; |
288 |
lastRet = 0; |
289 |
expectedModCount = modCount; |
290 |
} |
291 |
|
292 |
final void checkForComodification() { |
293 |
if (modCount != expectedModCount) |
294 |
throw new ConcurrentModificationException(); |
295 |
} |
296 |
} |
297 |
|
298 |
/** |
299 |
* Returns the number of elements in this priority queue. |
300 |
* |
301 |
* @return the number of elements in this priority queue. |
302 |
*/ |
303 |
public int size() { |
304 |
return size; |
305 |
} |
306 |
|
307 |
/** |
308 |
* Remove all elements from the priority queue. |
309 |
*/ |
310 |
public void clear() { |
311 |
modCount++; |
312 |
|
313 |
// Null out element references to prevent memory leak |
314 |
for (int i=1; i<=size; i++) |
315 |
queue[i] = null; |
316 |
|
317 |
size = 0; |
318 |
} |
319 |
|
320 |
/** |
321 |
* Removes and returns the ith element from queue. Recall |
322 |
* that queue is one-based, so 1 <= i <= size. |
323 |
* |
324 |
* XXX: Could further special-case i==size, but is it worth it? |
325 |
* XXX: Could special-case i==0, but is it worth it? |
326 |
*/ |
327 |
private E remove(int i) { |
328 |
assert i <= size; |
329 |
modCount++; |
330 |
|
331 |
E result = (E) queue[i]; |
332 |
queue[i] = queue[size]; |
333 |
queue[size--] = null; // Drop extra ref to prevent memory leak |
334 |
if (i <= size) |
335 |
fixDown(i); |
336 |
return result; |
337 |
} |
338 |
|
339 |
/** |
340 |
* Establishes the heap invariant (described above) assuming the heap |
341 |
* satisfies the invariant except possibly for the leaf-node indexed by k |
342 |
* (which may have a nextExecutionTime less than its parent's). |
343 |
* |
344 |
* This method functions by "promoting" queue[k] up the hierarchy |
345 |
* (by swapping it with its parent) repeatedly until queue[k] |
346 |
* is greater than or equal to its parent. |
347 |
*/ |
348 |
private void fixUp(int k) { |
349 |
if (comparator == null) { |
350 |
while (k > 1) { |
351 |
int j = k >> 1; |
352 |
if (((Comparable<E>)queue[j]).compareTo((E)queue[k]) <= 0) |
353 |
break; |
354 |
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
355 |
k = j; |
356 |
} |
357 |
} else { |
358 |
while (k > 1) { |
359 |
int j = k >> 1; |
360 |
if (comparator.compare((E)queue[j], (E)queue[k]) <= 0) |
361 |
break; |
362 |
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
363 |
k = j; |
364 |
} |
365 |
} |
366 |
} |
367 |
|
368 |
/** |
369 |
* Establishes the heap invariant (described above) in the subtree |
370 |
* rooted at k, which is assumed to satisfy the heap invariant except |
371 |
* possibly for node k itself (which may be greater than its children). |
372 |
* |
373 |
* This method functions by "demoting" queue[k] down the hierarchy |
374 |
* (by swapping it with its smaller child) repeatedly until queue[k] |
375 |
* is less than or equal to its children. |
376 |
*/ |
377 |
private void fixDown(int k) { |
378 |
int j; |
379 |
if (comparator == null) { |
380 |
while ((j = k << 1) <= size) { |
381 |
if (j<size && ((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0) |
382 |
j++; // j indexes smallest kid |
383 |
if (((Comparable<E>)queue[k]).compareTo((E)queue[j]) <= 0) |
384 |
break; |
385 |
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
386 |
k = j; |
387 |
} |
388 |
} else { |
389 |
while ((j = k << 1) <= size) { |
390 |
if (j < size && comparator.compare((E)queue[j], (E)queue[j+1]) > 0) |
391 |
j++; // j indexes smallest kid |
392 |
if (comparator.compare((E)queue[k], (E)queue[j]) <= 0) |
393 |
break; |
394 |
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
395 |
k = j; |
396 |
} |
397 |
} |
398 |
} |
399 |
|
400 |
public Comparator<? super E> comparator() { |
401 |
return comparator; |
402 |
} |
403 |
|
404 |
/** |
405 |
* Save the state of the instance to a stream (that |
406 |
* is, serialize it). |
407 |
* |
408 |
* @serialData The length of the array backing the instance is |
409 |
* emitted (int), followed by all of its elements (each an |
410 |
* <tt>Object</tt>) in the proper order. |
411 |
* @param s the stream |
412 |
*/ |
413 |
private synchronized void writeObject(java.io.ObjectOutputStream s) |
414 |
throws java.io.IOException{ |
415 |
// Write out element count, and any hidden stuff |
416 |
s.defaultWriteObject(); |
417 |
|
418 |
// Write out array length |
419 |
s.writeInt(queue.length); |
420 |
|
421 |
// Write out all elements in the proper order. |
422 |
for (int i=0; i<size; i++) |
423 |
s.writeObject(queue[i]); |
424 |
} |
425 |
|
426 |
/** |
427 |
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, |
428 |
* deserialize it). |
429 |
* @param s the stream |
430 |
*/ |
431 |
private synchronized void readObject(java.io.ObjectInputStream s) |
432 |
throws java.io.IOException, ClassNotFoundException { |
433 |
// Read in size, and any hidden stuff |
434 |
s.defaultReadObject(); |
435 |
|
436 |
// Read in array length and allocate array |
437 |
int arrayLength = s.readInt(); |
438 |
queue = new Object[arrayLength]; |
439 |
|
440 |
// Read in all elements in the proper order. |
441 |
for (int i=0; i<size; i++) |
442 |
queue[i] = s.readObject(); |
443 |
} |
444 |
|
445 |
} |
446 |
|