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revision 1.1, Wed May 14 21:30:45 2003 UTC revision 1.2, Sun May 18 18:10:02 2003 UTC
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1  package java.util;  package java.util;
2    
3  import java.util.*;  /*
4     * Todo
5     *
6     *   1) Make it serializable.
7     */
8    
9  /**  /**
10   * An unbounded (resizable) priority queue based on a priority   * An unbounded priority queue based on a priority heap.  This queue orders
11   * heap.The take operation returns the least element with respect to   * elements according to the order specified at creation time.  This order is
12   * the given ordering. (If more than one element is tied for least   * specified as for {@link TreeSet} and {@link TreeMap}: Elements are ordered
13   * value, one of them is arbitrarily chosen to be returned -- no   * either according to their <i>natural order</i> (see {@link Comparable}), or
14   * guarantees are made for ordering across ties.) Ordering follows the   * according to a {@link Comparator}, depending on which constructor is used.
15   * java.util.Collection conventions: Either the elements must be   * The {@link #peek}, {@link #poll}, and {@link #remove} methods return the
16   * Comparable, or a Comparator must be supplied. Comparison failures   * minimal element with respect to the specified ordering.  If multiple
17   * throw ClassCastExceptions during insertions and extractions.   * these elements are tied for least value, no guarantees are made as to
18   **/   * which of elements is returned.
19  public class PriorityQueue<E> extends AbstractCollection<E> implements Queue<E> {   *
20      public PriorityQueue(int initialCapacity) {}   * <p>Each priority queue has a <i>capacity</i>.  The capacity is the size of
21      public PriorityQueue(int initialCapacity, Comparator comparator) {}   * the array used to store the elements on the queue.  It is always at least
22     * as large as the queue size.  As elements are added to a priority list,
23     * its capacity grows automatically.  The details of the growth policy are not
24     * specified.
25     *
26     *<p>Implementation note: this implementation provides O(log(n)) time for
27     * the <tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt>
28     * methods; linear time for the <tt>remove(Object)</tt> and
29     * <tt>contains</tt> methods; and constant time for the <tt>peek</tt>,
30     * <tt>element</tt>, and <tt>size</tt> methods.
31     *
32     * <p>This class is a member of the
33     * <a href="{@docRoot}/../guide/collections/index.html">
34     * Java Collections Framework</a>.
35     */
36    public class PriorityQueue<E> extends AbstractQueue<E>
37                                  implements Queue<E>
38    {
39        private static final int DEFAULT_INITIAL_CAPACITY = 11;
40    
41      public PriorityQueue(int initialCapacity, Collection initialElements) {}      /**
42         * Priority queue represented as a balanced binary heap: the two children
43         * of queue[n] are queue[2*n] and queue[2*n + 1].  The priority queue is
44         * ordered by comparator, or by the elements' natural ordering, if
45         * comparator is null:  For each node n in the heap, and each descendant
46         * of n, d, n <= d.
47         *
48         * The element with the lowest value is in queue[1] (assuming the queue is
49         * nonempty). A one-based array is used in preference to the traditional
50         * zero-based array to simplify parent and child calculations.
51         *
52         * queue.length must be >= 2, even if size == 0.
53         */
54        private E[] queue;
55    
56      public PriorityQueue(int initialCapacity, Comparator comparator, Collection initialElements) {}      /**
57         * The number of elements in the priority queue.
58         */
59        private int size = 0;
60    
61      public boolean add(E x) {      /**
62          return false;       * The comparator, or null if priority queue uses elements'
63         * natural ordering.
64         */
65        private final Comparator<E> comparator;
66    
67        /**
68         * The number of times this priority queue has been
69         * <i>structurally modified</i>.  See AbstractList for gory details.
70         */
71        private int modCount = 0;
72    
73        /**
74         * Create a new priority queue with the default initial capacity (11)
75         * that orders its elements according to their natural ordering.
76         */
77        public PriorityQueue() {
78            this(DEFAULT_INITIAL_CAPACITY);
79      }      }
80      public boolean offer(E x) {  
81          return false;      /**
82         * Create a new priority queue with the specified initial capacity
83         * that orders its elements according to their natural ordering.
84         *
85         * @param initialCapacity the initial capacity for this priority queue.
86         */
87        public PriorityQueue(int initialCapacity) {
88            this(initialCapacity, null);
89      }      }
90      public boolean remove(Object x) {  
91          return false;      /**
92         * Create a new priority queue with the specified initial capacity (11)
93         * that orders its elements according to the specified comparator.
94         *
95         * @param initialCapacity the initial capacity for this priority queue.
96         * @param comparator the comparator used to order this priority queue.
97         */
98        public PriorityQueue(int initialCapacity, Comparator<E> comparator) {
99            if (initialCapacity < 1)
100                initialCapacity = 1;
101            queue = new E[initialCapacity + 1];
102            this.comparator = comparator;
103      }      }
104    
105      public E remove() {      /**
106          return null;       * Create a new priority queue containing the elements in the specified
107         * collection.  The priority queue has an initial capacity of 110% of the
108         * size of the specified collection. If the specified collection
109         * implements the {@link Sorted} interface, the priority queue will be
110         * sorted according to the same comparator, or according to its elements'
111         * natural order if the collection is sorted according to its elements'
112         * natural order.  If the specified collection does not implement the
113         * <tt>Sorted</tt> interface, the priority queue is ordered according to
114         * its elements' natural order.
115         *
116         * @param initialElements the collection whose elements are to be placed
117         *        into this priority queue.
118         * @throws ClassCastException if elements of the specified collection
119         *         cannot be compared to one another according to the priority
120         *         queue's ordering.
121         * @throws NullPointerException if the specified collection or an
122         *         element of the specified collection is <tt>null</tt>.
123         */
124        public PriorityQueue(Collection<E> initialElements) {
125            int sz = initialElements.size();
126            int initialCapacity = (int)Math.min((sz * 110L) / 100,
127                                                Integer.MAX_VALUE - 1);
128            if (initialCapacity < 1)
129                initialCapacity = 1;
130            queue = new E[initialCapacity + 1];
131    
132            /* Commented out to compile with generics compiler
133    
134            if (initialElements instanceof Sorted) {
135                comparator = ((Sorted)initialElements).comparator();
136                for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
137                    queue[++size] = i.next();
138            } else {
139            */
140            {
141                comparator = null;
142                for (Iterator<E> i = initialElements.iterator(); i.hasNext(); )
143                    add(i.next());
144      }      }
     public Iterator<E> iterator() {  
       return null;  
145      }      }
146    
147      public E element() {      // Queue Methods
148          return null;  
149      }      /**
150         * Remove and return the minimal element from this priority queue if
151         * it contains one or more elements, otherwise <tt>null</tt>.  The term
152         * <i>minimal</i> is defined according to this priority queue's order.
153         *
154         * @return the minimal element from this priority queue if it contains
155         *         one or more elements, otherwise <tt>null</tt>.
156         */
157      public E poll() {      public E poll() {
158            if (size == 0)
159          return null;          return null;
160            return remove(1);
161      }      }
162    
163        /**
164         * Return, but do not remove, the minimal element from the priority queue,
165         * or <tt>null</tt> if the queue is empty.  The term <i>minimal</i> is
166         * defined according to this priority queue's order.  This method returns
167         * the same object reference that would be returned by by the
168         * <tt>poll</tt> method.  The two methods differ in that this method
169         * does not remove the element 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() {      public E peek() {
175          return null;          return queue[1];
176      }      }
177    
178      public boolean isEmpty() {      // 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 o element to be removed from this collection, if present.
187         * @return <tt>true</tt> if this collection changed as a result of the
188         *         call
189         * @throws ClassCastException if the specified element cannot be compared
190         *            with elements currently in the priority queue according
191         *            to the priority queue's ordering.
192         * @throws NullPointerException if the specified element is null.
193         */
194        public boolean remove(Object element) {
195            if (element == null)
196                throw new NullPointerException();
197    
198            if (comparator == null) {
199                for (int i = 1; i <= size; i++) {
200                    if (((Comparable)queue[i]).compareTo(element) == 0) {
201                        remove(i);
202                        return true;
203                    }
204                }
205            } else {
206                for (int i = 1; i <= size; i++) {
207                    if (comparator.compare(queue[i], (E) element) == 0) {
208                        remove(i);
209                        return true;
210                    }
211                }
212            }
213          return false;          return false;
214      }      }
215    
216        /**
217         * Returns an iterator over the elements in this priority queue.  The
218         * first element returned by this iterator is the same element that
219         * would be returned by a call to <tt>peek</tt>.
220         *
221         * @return an <tt>Iterator</tt> over the elements in this priority queue.
222         */
223        public Iterator<E> iterator() {
224            return new Itr();
225        }
226    
227        private class Itr implements Iterator<E> {
228            /**
229             * Index (into queue array) of element to be returned by
230             * subsequent call to next.
231             */
232            int cursor = 1;
233    
234            /**
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            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            int expectedModCount = modCount;
247    
248            public boolean hasNext() {
249                return cursor <= size;
250            }
251    
252            public E next() {
253                checkForComodification();
254                if (cursor > size)
255                    throw new NoSuchElementException();
256                E result = queue[cursor];
257                lastRet = cursor++;
258                return result;
259            }
260    
261            public void remove() {
262                if (lastRet == 0)
263                    throw new IllegalStateException();
264                checkForComodification();
265    
266                PriorityQueue.this.remove(lastRet);
267                if (lastRet < cursor)
268                    cursor--;
269                lastRet = 0;
270                expectedModCount = modCount;
271            }
272    
273            final void checkForComodification() {
274                if (modCount != expectedModCount)
275                    throw new ConcurrentModificationException();
276            }
277        }
278    
279        /**
280         * Returns the number of elements in this priority queue.
281         *
282         * @return the number of elements in this priority queue.
283         */
284      public int size() {      public int size() {
285          return 0;          return size;
286      }      }
287      public Object[] toArray() {  
288          return null;      /**
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         *            with elements currently in the priority queue according
295         *            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      public <T> T[] toArray(T[] array) {          queue[size] = element;
311          return null;          fixUp(size);
312            return true;
313      }      }
314    
315        /**
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        }
346    
347        /**
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         *         <tt>null</tt> if it uses its elements' natural ordering.
414         */
415        Comparator comparator() {
416            return comparator;
417        }
418  }  }

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