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package java.util; |
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|
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/** |
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* An unbounded priority {@linkplain Queue queue} based on a priority heap. |
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* This queue orders elements according to an order specified at construction |
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* time, which is specified in the same manner as {@link java.util.TreeSet} |
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* and {@link java.util.TreeMap}: elements are ordered either according to |
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* their <i>natural order</i> (see {@link Comparable}), or according to a |
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* {@link java.util.Comparator}, depending on which constructor is used. |
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* <p>The <em>head</em> of this queue is the <em>least</em> element with |
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* respect to the specified ordering. If multiple elements are tied for least |
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* value, the head is one of those elements. A priority queue does not permit |
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* <tt>null</tt> elements. |
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* |
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* <p>The {@link #remove()} and {@link #poll()} methods remove and |
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* return the head of the queue. |
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* |
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* <p>The {@link #element()} and {@link #peek()} methods return, but do |
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* not delete, the head of the queue. |
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* |
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* <p>A priority queue has a <i>capacity</i>. The capacity is the |
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* size of the array used internally to store the elements on the |
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* queue. |
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* It is always at least as large as the queue size. As |
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* elements are added to a priority queue, its capacity grows |
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* automatically. The details of the growth policy are not specified. |
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* |
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* <p>The Iterator provided in method {@link #iterator()} is <em>not</em> |
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* guaranteed to traverse the elements of the PriorityQueue in any |
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* particular order. If you need ordered traversal, consider using |
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* <tt>Arrays.sort(pq.toArray())</tt>. |
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* |
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* <p> <strong>Note that this implementation is not synchronized.</strong> |
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* Multiple threads should not access a <tt>PriorityQueue</tt> |
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* instance concurrently if any of the threads modifies the list |
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* structurally. Instead, use the thread-safe {@link |
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* java.util.concurrent.PriorityBlockingQueue} class. |
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* |
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* |
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* <p>Implementation note: this implementation provides O(log(n)) time |
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* for the insertion methods (<tt>offer</tt>, <tt>poll</tt>, |
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* <tt>remove()</tt> and <tt>add</tt>) methods; linear time for the |
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* <tt>remove(Object)</tt> and <tt>contains(Object)</tt> methods; and |
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* constant time for the retrieval methods (<tt>peek</tt>, |
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* <tt>element</tt>, and <tt>size</tt>). |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../guide/collections/index.html"> |
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* Java Collections Framework</a>. |
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* @since 1.5 |
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* @author Josh Bloch |
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*/ |
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public class PriorityQueue<E> extends AbstractQueue<E> |
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implements Queue<E>, java.io.Serializable { |
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|
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private static final long serialVersionUID = -7720805057305804111L; |
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|
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private static final int DEFAULT_INITIAL_CAPACITY = 11; |
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|
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/** |
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* Priority queue represented as a balanced binary heap: the two children |
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* of queue[n] are queue[2*n] and queue[2*n + 1]. The priority queue is |
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* ordered by comparator, or by the elements' natural ordering, if |
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* comparator is null: For each node n in the heap and each descendant d |
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* of n, n <= d. |
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* |
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* The element with the lowest value is in queue[1], assuming the queue is |
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* nonempty. (A one-based array is used in preference to the traditional |
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* zero-based array to simplify parent and child calculations.) |
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* |
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* queue.length must be >= 2, even if size == 0. |
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*/ |
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private transient Object[] queue; |
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|
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/** |
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* The number of elements in the priority queue. |
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*/ |
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private int size = 0; |
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|
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/** |
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* The comparator, or null if priority queue uses elements' |
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* natural ordering. |
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*/ |
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private final Comparator<? super E> comparator; |
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|
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/** |
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* The number of times this priority queue has been |
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* <i>structurally modified</i>. See AbstractList for gory details. |
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*/ |
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private transient int modCount = 0; |
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|
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/** |
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* Creates a <tt>PriorityQueue</tt> with the default initial capacity |
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* (11) that orders its elements according to their natural |
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* ordering (using <tt>Comparable</tt>). |
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*/ |
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public PriorityQueue() { |
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this(DEFAULT_INITIAL_CAPACITY, null); |
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} |
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|
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/** |
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* Creates a <tt>PriorityQueue</tt> with the specified initial capacity |
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* that orders its elements according to their natural ordering |
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* (using <tt>Comparable</tt>). |
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* |
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* @param initialCapacity the initial capacity for this priority queue. |
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* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less |
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* than 1 |
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*/ |
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public PriorityQueue(int initialCapacity) { |
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this(initialCapacity, null); |
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} |
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|
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/** |
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* Creates a <tt>PriorityQueue</tt> with the specified initial capacity |
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* that orders its elements according to the specified comparator. |
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* |
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* @param initialCapacity the initial capacity for this priority queue. |
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* @param comparator the comparator used to order this priority queue. |
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* If <tt>null</tt> then the order depends on the elements' natural |
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* ordering. |
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* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less |
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* than 1 |
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*/ |
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public PriorityQueue(int initialCapacity, |
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Comparator<? super E> comparator) { |
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if (initialCapacity < 1) |
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throw new IllegalArgumentException(); |
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this.queue = new Object[initialCapacity + 1]; |
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this.comparator = comparator; |
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} |
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|
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/** |
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* Common code to initialize underlying queue array across |
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* constructors below. |
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*/ |
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private void initializeArray(Collection<? extends E> c) { |
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int sz = c.size(); |
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int initialCapacity = (int)Math.min((sz * 110L) / 100, |
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Integer.MAX_VALUE - 1); |
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if (initialCapacity < 1) |
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initialCapacity = 1; |
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|
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this.queue = new Object[initialCapacity + 1]; |
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} |
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|
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/** |
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* Initially fill elements of the queue array under the |
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* knowledge that it is sorted or is another PQ, in which |
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* case we can just place the elements in the order presented. |
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*/ |
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private void fillFromSorted(Collection<? extends E> c) { |
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for (Iterator<? extends E> i = c.iterator(); i.hasNext(); ) |
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queue[++size] = i.next(); |
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} |
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|
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/** |
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* Initially fill elements of the queue array that is not to our knowledge |
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* sorted, so we must rearrange the elements to guarantee the heap |
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* invariant. |
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*/ |
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private void fillFromUnsorted(Collection<? extends E> c) { |
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for (Iterator<? extends E> i = c.iterator(); i.hasNext(); ) |
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queue[++size] = i.next(); |
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heapify(); |
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} |
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|
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/** |
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* Creates a <tt>PriorityQueue</tt> containing the elements in the |
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* specified collection. The priority queue has an initial |
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* capacity of 110% of the size of the specified collection or 1 |
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* if the collection is empty. If the specified collection is an |
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* instance of a {@link java.util.SortedSet} or is another |
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* <tt>PriorityQueue</tt>, the priority queue will be sorted |
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* according to the same comparator, or according to its elements' |
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* natural order if the collection is sorted according to its |
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* elements' natural order. Otherwise, the priority queue is |
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* ordered according to its elements' natural order. |
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* |
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* @param c the collection whose elements are to be placed |
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* into this priority queue. |
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* @throws ClassCastException if elements of the specified collection |
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* cannot be compared to one another according to the priority |
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* queue's ordering. |
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* @throws NullPointerException if <tt>c</tt> or any element within it |
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* is <tt>null</tt> |
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*/ |
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public PriorityQueue(Collection<? extends E> c) { |
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initializeArray(c); |
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if (c instanceof SortedSet) { |
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// @fixme double-cast workaround for compiler |
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SortedSet<? extends E> s = (SortedSet<? extends E>) (SortedSet)c; |
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comparator = (Comparator<? super E>)s.comparator(); |
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fillFromSorted(s); |
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} else if (c instanceof PriorityQueue) { |
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PriorityQueue<? extends E> s = (PriorityQueue<? extends E>) c; |
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comparator = (Comparator<? super E>)s.comparator(); |
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fillFromSorted(s); |
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} else { |
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comparator = null; |
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fillFromUnsorted(c); |
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} |
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} |
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|
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/** |
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* Creates a <tt>PriorityQueue</tt> containing the elements in the |
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* specified collection. The priority queue has an initial |
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* capacity of 110% of the size of the specified collection or 1 |
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* if the collection is empty. This priority queue will be sorted |
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* according to the same comparator as the given collection, or |
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* according to its elements' natural order if the collection is |
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* sorted according to its elements' natural order. |
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* |
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* @param c the collection whose elements are to be placed |
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* into this priority queue. |
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* @throws ClassCastException if elements of the specified collection |
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* cannot be compared to one another according to the priority |
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* queue's ordering. |
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* @throws NullPointerException if <tt>c</tt> or any element within it |
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* is <tt>null</tt> |
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*/ |
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public PriorityQueue(PriorityQueue<? extends E> c) { |
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initializeArray(c); |
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comparator = (Comparator<? super E>)c.comparator(); |
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fillFromSorted(c); |
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} |
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|
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/** |
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* Creates a <tt>PriorityQueue</tt> containing the elements in the |
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* specified collection. The priority queue has an initial |
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* capacity of 110% of the size of the specified collection or 1 |
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* if the collection is empty. This priority queue will be sorted |
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* according to the same comparator as the given collection, or |
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* according to its elements' natural order if the collection is |
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* sorted according to its elements' natural order. |
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* |
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* @param c the collection whose elements are to be placed |
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* into this priority queue. |
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* @throws ClassCastException if elements of the specified collection |
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* cannot be compared to one another according to the priority |
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* queue's ordering. |
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* @throws NullPointerException if <tt>c</tt> or any element within it |
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* is <tt>null</tt> |
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*/ |
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public PriorityQueue(SortedSet<? extends E> c) { |
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initializeArray(c); |
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comparator = (Comparator<? super E>)c.comparator(); |
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fillFromSorted(c); |
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} |
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|
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/** |
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* Resize array, if necessary, to be able to hold given index |
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*/ |
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private void grow(int index) { |
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int newlen = queue.length; |
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if (index < newlen) // don't need to grow |
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return; |
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if (index == Integer.MAX_VALUE) |
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throw new OutOfMemoryError(); |
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while (newlen <= index) { |
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if (newlen >= Integer.MAX_VALUE / 2) // avoid overflow |
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newlen = Integer.MAX_VALUE; |
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else |
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newlen <<= 2; |
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} |
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Object[] newQueue = new Object[newlen]; |
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System.arraycopy(queue, 0, newQueue, 0, queue.length); |
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queue = newQueue; |
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} |
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|
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|
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// Queue Methods |
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|
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/** |
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* Add the specified element to this priority queue. |
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* |
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* @return <tt>true</tt> |
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* @throws ClassCastException if the specified element cannot be compared |
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* with elements currently in the priority queue according |
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* to the priority queue's ordering. |
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* @throws NullPointerException if the specified element is <tt>null</tt>. |
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*/ |
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public boolean offer(E o) { |
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if (o == null) |
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throw new NullPointerException(); |
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modCount++; |
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++size; |
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|
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// Grow backing store if necessary |
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if (size >= queue.length) |
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grow(size); |
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|
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queue[size] = o; |
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fixUp(size); |
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return true; |
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} |
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|
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public E poll() { |
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if (size == 0) |
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return null; |
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return remove(); |
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} |
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|
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public E peek() { |
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return (E) queue[1]; |
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} |
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|
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// Collection Methods - the first two override to update docs |
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|
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/** |
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* Adds the specified element to this queue. |
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* @return <tt>true</tt> (as per the general contract of |
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* <tt>Collection.add</tt>). |
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* |
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* @throws NullPointerException {@inheritDoc} |
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* @throws ClassCastException if the specified element cannot be compared |
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* with elements currently in the priority queue according |
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* to the priority queue's ordering. |
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*/ |
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public boolean add(E o) { |
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return super.add(o); |
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} |
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|
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|
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/** |
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* Adds all of the elements in the specified collection to this queue. |
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* The behavior of this operation is undefined if |
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* the specified collection is modified while the operation is in |
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* progress. (This implies that the behavior of this call is undefined if |
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* the specified collection is this queue, and this queue is nonempty.) |
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* <p> |
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* This implementation iterates over the specified collection, and adds |
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* each object returned by the iterator to this collection, in turn. |
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* @throws NullPointerException {@inheritDoc} |
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* @throws ClassCastException if any element cannot be compared |
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* with elements currently in the priority queue according |
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* to the priority queue's ordering. |
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*/ |
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public boolean addAll(Collection<? extends E> c) { |
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return super.addAll(c); |
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} |
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|
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|
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/** |
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* Removes a single instance of the specified element from this |
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* queue, if it is present. More formally, |
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* removes an element <tt>e</tt> such that <tt>(o==null ? e==null : |
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* o.equals(e))</tt>, if the queue contains one or more such |
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* elements. Returns <tt>true</tt> if the queue contained the |
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* specified element (or equivalently, if the queue changed as a |
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* result of the call). |
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* |
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* <p>This implementation iterates over the queue looking for the |
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* specified element. If it finds the element, it removes the element |
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* from the queue using the iterator's remove method.<p> |
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* |
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*/ |
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public boolean remove(Object o) { |
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if (o == null) |
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return false; |
361 |
|
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if (comparator == null) { |
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for (int i = 1; i <= size; i++) { |
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if (((Comparable<E>)queue[i]).compareTo((E)o) == 0) { |
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removeAt(i); |
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return true; |
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} |
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} |
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} else { |
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for (int i = 1; i <= size; i++) { |
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if (comparator.compare((E)queue[i], (E)o) == 0) { |
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removeAt(i); |
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return true; |
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} |
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} |
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} |
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return false; |
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} |
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|
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/** |
381 |
* Returns an iterator over the elements in this queue. The iterator |
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* does not return the elements in any particular order. |
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* |
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* @return an iterator over the elements in this queue. |
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*/ |
386 |
public Iterator<E> iterator() { |
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return new Itr(); |
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} |
389 |
|
390 |
private class Itr implements Iterator<E> { |
391 |
|
392 |
/** |
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* Index (into queue array) of element to be returned by |
394 |
* subsequent call to next. |
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*/ |
396 |
private int cursor = 1; |
397 |
|
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/** |
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* Index of element returned by most recent call to next, |
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* unless that element came from the forgetMeNot list. |
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* Reset to 0 if element is deleted by a call to remove. |
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*/ |
403 |
private int lastRet = 0; |
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|
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/** |
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* The modCount value that the iterator believes that the backing |
407 |
* List should have. If this expectation is violated, the iterator |
408 |
* has detected concurrent modification. |
409 |
*/ |
410 |
private int expectedModCount = modCount; |
411 |
|
412 |
/** |
413 |
* A list of elements that were moved from the unvisited portion of |
414 |
* the heap into the visited portion as a result of "unlucky" element |
415 |
* removals during the iteration. (Unlucky element removals are those |
416 |
* that require a fixup instead of a fixdown.) We must visit all of |
417 |
* the elements in this list to complete the iteration. We do this |
418 |
* after we've completed the "normal" iteration. |
419 |
* |
420 |
* We expect that most iterations, even those involving removals, |
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* will not use need to store elements in this field. |
422 |
*/ |
423 |
private ArrayList<E> forgetMeNot = null; |
424 |
|
425 |
/** |
426 |
* Element returned by the most recent call to next iff that |
427 |
* element was drawn from the forgetMeNot list. |
428 |
*/ |
429 |
private Object lastRetElt = null; |
430 |
|
431 |
public boolean hasNext() { |
432 |
return cursor <= size || forgetMeNot != null; |
433 |
} |
434 |
|
435 |
public E next() { |
436 |
checkForComodification(); |
437 |
E result; |
438 |
if (cursor <= size) { |
439 |
result = (E) queue[cursor]; |
440 |
lastRet = cursor++; |
441 |
} |
442 |
else if (forgetMeNot == null) |
443 |
throw new NoSuchElementException(); |
444 |
else { |
445 |
int remaining = forgetMeNot.size(); |
446 |
result = forgetMeNot.remove(remaining - 1); |
447 |
if (remaining == 1) |
448 |
forgetMeNot = null; |
449 |
lastRet = 0; |
450 |
lastRetElt = result; |
451 |
} |
452 |
return result; |
453 |
} |
454 |
|
455 |
public void remove() { |
456 |
checkForComodification(); |
457 |
|
458 |
if (lastRet != 0) { |
459 |
E moved = PriorityQueue.this.removeAt(lastRet); |
460 |
lastRet = 0; |
461 |
if (moved == null) { |
462 |
cursor--; |
463 |
} else { |
464 |
if (forgetMeNot == null) |
465 |
forgetMeNot = new ArrayList(); |
466 |
forgetMeNot.add(moved); |
467 |
} |
468 |
} else if (lastRetElt != null) { |
469 |
PriorityQueue.this.remove(lastRetElt); |
470 |
lastRetElt = null; |
471 |
} else { |
472 |
throw new IllegalStateException(); |
473 |
} |
474 |
|
475 |
expectedModCount = modCount; |
476 |
} |
477 |
|
478 |
final void checkForComodification() { |
479 |
if (modCount != expectedModCount) |
480 |
throw new ConcurrentModificationException(); |
481 |
} |
482 |
} |
483 |
|
484 |
public int size() { |
485 |
return size; |
486 |
} |
487 |
|
488 |
/** |
489 |
* Remove all elements from the priority queue. |
490 |
*/ |
491 |
public void clear() { |
492 |
modCount++; |
493 |
|
494 |
// Null out element references to prevent memory leak |
495 |
for (int i=1; i<=size; i++) |
496 |
queue[i] = null; |
497 |
|
498 |
size = 0; |
499 |
} |
500 |
|
501 |
/** |
502 |
* Removes and returns the first element from queue. |
503 |
*/ |
504 |
public E remove() { |
505 |
if (size == 0) |
506 |
throw new NoSuchElementException(); |
507 |
modCount++; |
508 |
|
509 |
E result = (E) queue[1]; |
510 |
queue[1] = queue[size]; |
511 |
queue[size--] = null; // Drop extra ref to prevent memory leak |
512 |
if (size > 1) |
513 |
fixDown(1); |
514 |
|
515 |
return result; |
516 |
} |
517 |
|
518 |
/** |
519 |
* Removes and returns the ith element from queue. (Recall that queue |
520 |
* is one-based, so 1 <= i <= size.) |
521 |
* |
522 |
* Normally this method leaves the elements at positions from 1 up to i-1, |
523 |
* inclusive, untouched. Under these circumstances, it returns null. |
524 |
* Occasionally, in order to maintain the heap invariant, it must move |
525 |
* the last element of the list to some index in the range [2, i-1], |
526 |
* and move the element previously at position (i/2) to position i. |
527 |
* Under these circumstances, this method returns the element that was |
528 |
* previously at the end of the list and is now at some position between |
529 |
* 2 and i-1 inclusive. |
530 |
*/ |
531 |
private E removeAt(int i) { |
532 |
assert i > 0 && i <= size; |
533 |
modCount++; |
534 |
|
535 |
E moved = (E) queue[size]; |
536 |
queue[i] = moved; |
537 |
queue[size--] = null; // Drop extra ref to prevent memory leak |
538 |
if (i <= size) { |
539 |
fixDown(i); |
540 |
if (queue[i] == moved) { |
541 |
fixUp(i); |
542 |
if (queue[i] != moved) |
543 |
return moved; |
544 |
} |
545 |
} |
546 |
return null; |
547 |
} |
548 |
|
549 |
/** |
550 |
* Establishes the heap invariant (described above) assuming the heap |
551 |
* satisfies the invariant except possibly for the leaf-node indexed by k |
552 |
* (which may have a nextExecutionTime less than its parent's). |
553 |
* |
554 |
* This method functions by "promoting" queue[k] up the hierarchy |
555 |
* (by swapping it with its parent) repeatedly until queue[k] |
556 |
* is greater than or equal to its parent. |
557 |
*/ |
558 |
private void fixUp(int k) { |
559 |
if (comparator == null) { |
560 |
while (k > 1) { |
561 |
int j = k >> 1; |
562 |
if (((Comparable<E>)queue[j]).compareTo((E)queue[k]) <= 0) |
563 |
break; |
564 |
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
565 |
k = j; |
566 |
} |
567 |
} else { |
568 |
while (k > 1) { |
569 |
int j = k >>> 1; |
570 |
if (comparator.compare((E)queue[j], (E)queue[k]) <= 0) |
571 |
break; |
572 |
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
573 |
k = j; |
574 |
} |
575 |
} |
576 |
} |
577 |
|
578 |
/** |
579 |
* Establishes the heap invariant (described above) in the subtree |
580 |
* rooted at k, which is assumed to satisfy the heap invariant except |
581 |
* possibly for node k itself (which may be greater than its children). |
582 |
* |
583 |
* This method functions by "demoting" queue[k] down the hierarchy |
584 |
* (by swapping it with its smaller child) repeatedly until queue[k] |
585 |
* is less than or equal to its children. |
586 |
*/ |
587 |
private void fixDown(int k) { |
588 |
int j; |
589 |
if (comparator == null) { |
590 |
while ((j = k << 1) <= size && (j > 0)) { |
591 |
if (j<size && |
592 |
((Comparable<E>)queue[j]).compareTo((E)queue[j+1]) > 0) |
593 |
j++; // j indexes smallest kid |
594 |
|
595 |
if (((Comparable<E>)queue[k]).compareTo((E)queue[j]) <= 0) |
596 |
break; |
597 |
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
598 |
k = j; |
599 |
} |
600 |
} else { |
601 |
while ((j = k << 1) <= size && (j > 0)) { |
602 |
if (j<size && |
603 |
comparator.compare((E)queue[j], (E)queue[j+1]) > 0) |
604 |
j++; // j indexes smallest kid |
605 |
if (comparator.compare((E)queue[k], (E)queue[j]) <= 0) |
606 |
break; |
607 |
Object tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
608 |
k = j; |
609 |
} |
610 |
} |
611 |
} |
612 |
|
613 |
/** |
614 |
* Establishes the heap invariant (described above) in the entire tree, |
615 |
* assuming nothing about the order of the elements prior to the call. |
616 |
*/ |
617 |
private void heapify() { |
618 |
for (int i = size/2; i >= 1; i--) |
619 |
fixDown(i); |
620 |
} |
621 |
|
622 |
/** |
623 |
* Returns the comparator used to order this collection, or <tt>null</tt> |
624 |
* if this collection is sorted according to its elements natural ordering |
625 |
* (using <tt>Comparable</tt>). |
626 |
* |
627 |
* @return the comparator used to order this collection, or <tt>null</tt> |
628 |
* if this collection is sorted according to its elements natural ordering. |
629 |
*/ |
630 |
public Comparator<? super E> comparator() { |
631 |
return comparator; |
632 |
} |
633 |
|
634 |
/** |
635 |
* Save the state of the instance to a stream (that |
636 |
* is, serialize it). |
637 |
* |
638 |
* @serialData The length of the array backing the instance is |
639 |
* emitted (int), followed by all of its elements (each an |
640 |
* <tt>Object</tt>) in the proper order. |
641 |
* @param s the stream |
642 |
*/ |
643 |
private void writeObject(java.io.ObjectOutputStream s) |
644 |
throws java.io.IOException{ |
645 |
// Write out element count, and any hidden stuff |
646 |
s.defaultWriteObject(); |
647 |
|
648 |
// Write out array length |
649 |
s.writeInt(queue.length); |
650 |
|
651 |
// Write out all elements in the proper order. |
652 |
for (int i=0; i<size; i++) |
653 |
s.writeObject(queue[i]); |
654 |
} |
655 |
|
656 |
/** |
657 |
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, |
658 |
* deserialize it). |
659 |
* @param s the stream |
660 |
*/ |
661 |
private void readObject(java.io.ObjectInputStream s) |
662 |
throws java.io.IOException, ClassNotFoundException { |
663 |
// Read in size, and any hidden stuff |
664 |
s.defaultReadObject(); |
665 |
|
666 |
// Read in array length and allocate array |
667 |
int arrayLength = s.readInt(); |
668 |
queue = new Object[arrayLength]; |
669 |
|
670 |
// Read in all elements in the proper order. |
671 |
for (int i=0; i<size; i++) |
672 |
queue[i] = s.readObject(); |
673 |
} |
674 |
|
675 |
} |