Java tutorial
/* * Copyright (c) 2003, 2019, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.util; import java.util.function.Consumer; import java.util.function.Predicate; import jdk.internal.access.SharedSecrets; import jdk.internal.util.ArraysSupport; /** * An unbounded priority {@linkplain Queue queue} based on a priority heap. * The elements of the priority queue are ordered according to their * {@linkplain Comparable natural ordering}, or by a {@link Comparator} * provided at queue construction time, depending on which constructor is * used. A priority queue does not permit {@code null} elements. * A priority queue relying on natural ordering also does not permit * insertion of non-comparable objects (doing so may result in * {@code ClassCastException}). * * <p>The <em>head</em> of this queue is the <em>least</em> element * with respect to the specified ordering. If multiple elements are * tied for least value, the head is one of those elements -- ties are * broken arbitrarily. The queue retrieval operations {@code poll}, * {@code remove}, {@code peek}, and {@code element} access the * element at the head of the queue. * * <p>A priority queue is unbounded, but has an internal * <i>capacity</i> governing the size of an array used to store the * elements on the queue. It is always at least as large as the queue * size. As elements are added to a priority queue, its capacity * grows automatically. The details of the growth policy are not * specified. * * <p>This class and its iterator implement all of the * <em>optional</em> methods of the {@link Collection} and {@link * Iterator} interfaces. The Iterator provided in method {@link * #iterator()} and the Spliterator provided in method {@link #spliterator()} * are <em>not</em> guaranteed to traverse the elements of * the priority queue in any particular order. If you need ordered * traversal, consider using {@code Arrays.sort(pq.toArray())}. * * <p><strong>Note that this implementation is not synchronized.</strong> * Multiple threads should not access a {@code PriorityQueue} * instance concurrently if any of the threads modifies the queue. * Instead, use the thread-safe {@link * java.util.concurrent.PriorityBlockingQueue} class. * * <p>Implementation note: this implementation provides * O(log(n)) time for the enqueuing and dequeuing methods * ({@code offer}, {@code poll}, {@code remove()} and {@code add}); * linear time for the {@code remove(Object)} and {@code contains(Object)} * methods; and constant time for the retrieval methods * ({@code peek}, {@code element}, and {@code size}). * * <p>This class is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @since 1.5 * @author Josh Bloch, Doug Lea * @param <E> the type of elements held in this queue */ @SuppressWarnings("unchecked") public class PriorityQueue<E> extends AbstractQueue<E> implements java.io.Serializable { private static final long serialVersionUID = -7720805057305804111L; private static final int DEFAULT_INITIAL_CAPACITY = 11; /** * Priority queue represented as a balanced binary heap: the two * children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The * priority queue is ordered by comparator, or by the elements' * natural ordering, if comparator is null: For each node n in the * heap and each descendant d of n, n <= d. The element with the * lowest value is in queue[0], assuming the queue is nonempty. */ transient Object[] queue; // non-private to simplify nested class access /** * The number of elements in the priority queue. */ int size; /** * The comparator, or null if priority queue uses elements' * natural ordering. */ private final Comparator<? super E> comparator; /** * The number of times this priority queue has been * <i>structurally modified</i>. See AbstractList for gory details. */ transient int modCount; // non-private to simplify nested class access /** * Creates a {@code PriorityQueue} with the default initial * capacity (11) that orders its elements according to their * {@linkplain Comparable natural ordering}. */ public PriorityQueue() { this(DEFAULT_INITIAL_CAPACITY, null); } /** * Creates a {@code PriorityQueue} with the specified initial * capacity that orders its elements according to their * {@linkplain Comparable natural ordering}. * * @param initialCapacity the initial capacity for this priority queue * @throws IllegalArgumentException if {@code initialCapacity} is less * than 1 */ public PriorityQueue(int initialCapacity) { this(initialCapacity, null); } /** * Creates a {@code PriorityQueue} with the default initial capacity and * whose elements are ordered according to the specified comparator. * * @param comparator the comparator that will be used to order this * priority queue. If {@code null}, the {@linkplain Comparable * natural ordering} of the elements will be used. * @since 1.8 */ public PriorityQueue(Comparator<? super E> comparator) { this(DEFAULT_INITIAL_CAPACITY, comparator); } /** * Creates a {@code PriorityQueue} with the specified initial capacity * that orders its elements according to the specified comparator. * * @param initialCapacity the initial capacity for this priority queue * @param comparator the comparator that will be used to order this * priority queue. If {@code null}, the {@linkplain Comparable * natural ordering} of the elements will be used. * @throws IllegalArgumentException if {@code initialCapacity} is * less than 1 */ public PriorityQueue(int initialCapacity, Comparator<? super E> comparator) { // Note: This restriction of at least one is not actually needed, // but continues for 1.5 compatibility if (initialCapacity < 1) throw new IllegalArgumentException(); this.queue = new Object[initialCapacity]; this.comparator = comparator; } /** * Creates a {@code PriorityQueue} containing the elements in the * specified collection. If the specified collection is an instance of * a {@link SortedSet} or is another {@code PriorityQueue}, this * priority queue will be ordered according to the same ordering. * Otherwise, this priority queue will be ordered according to the * {@linkplain Comparable natural ordering} of its elements. * * @param c the collection whose elements are to be placed * into this priority queue * @throws ClassCastException if elements of the specified collection * cannot be compared to one another according to the priority * queue's ordering * @throws NullPointerException if the specified collection or any * of its elements are null */ public PriorityQueue(Collection<? extends E> c) { if (c instanceof SortedSet<?>) { SortedSet<? extends E> ss = (SortedSet<? extends E>) c; this.comparator = (Comparator<? super E>) ss.comparator(); initElementsFromCollection(ss); } else if (c instanceof PriorityQueue<?>) { PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c; this.comparator = (Comparator<? super E>) pq.comparator(); initFromPriorityQueue(pq); } else { this.comparator = null; initFromCollection(c); } } /** * Creates a {@code PriorityQueue} containing the elements in the * specified priority queue. This priority queue will be * ordered according to the same ordering as the given priority * queue. * * @param c the priority queue whose elements are to be placed * into this priority queue * @throws ClassCastException if elements of {@code c} cannot be * compared to one another according to {@code c}'s * ordering * @throws NullPointerException if the specified priority queue or any * of its elements are null */ public PriorityQueue(PriorityQueue<? extends E> c) { this.comparator = (Comparator<? super E>) c.comparator(); initFromPriorityQueue(c); } /** * Creates a {@code PriorityQueue} containing the elements in the * specified sorted set. This priority queue will be ordered * according to the same ordering as the given sorted set. * * @param c the sorted set whose elements are to be placed * into this priority queue * @throws ClassCastException if elements of the specified sorted * set cannot be compared to one another according to the * sorted set's ordering * @throws NullPointerException if the specified sorted set or any * of its elements are null */ public PriorityQueue(SortedSet<? extends E> c) { this.comparator = (Comparator<? super E>) c.comparator(); initElementsFromCollection(c); } /** Ensures that queue[0] exists, helping peek() and poll(). */ private static Object[] ensureNonEmpty(Object[] es) { return (es.length > 0) ? es : new Object[1]; } private void initFromPriorityQueue(PriorityQueue<? extends E> c) { if (c.getClass() == PriorityQueue.class) { this.queue = ensureNonEmpty(c.toArray()); this.size = c.size(); } else { initFromCollection(c); } } private void initElementsFromCollection(Collection<? extends E> c) { Object[] es = c.toArray(); int len = es.length; // If c.toArray incorrectly doesn't return Object[], copy it. if (es.getClass() != Object[].class) es = Arrays.copyOf(es, len, Object[].class); if (len == 1 || this.comparator != null) for (Object e : es) if (e == null) throw new NullPointerException(); this.queue = ensureNonEmpty(es); this.size = len; } /** * Initializes queue array with elements from the given Collection. * * @param c the collection */ private void initFromCollection(Collection<? extends E> c) { initElementsFromCollection(c); heapify(); } /** * Increases the capacity of the array. * * @param minCapacity the desired minimum capacity */ private void grow(int minCapacity) { int oldCapacity = queue.length; // Double size if small; else grow by 50% int newCapacity = ArraysSupport.newLength(oldCapacity, minCapacity - oldCapacity, /* minimum growth */ oldCapacity < 64 ? oldCapacity + 2 : oldCapacity >> 1 /* preferred growth */); queue = Arrays.copyOf(queue, newCapacity); } /** * Inserts the specified element into this priority queue. * * @return {@code true} (as specified by {@link Collection#add}) * @throws ClassCastException if the specified element cannot be * compared with elements currently in this priority queue * according to the priority queue's ordering * @throws NullPointerException if the specified element is null */ public boolean add(E e) { return offer(e); } /** * Inserts the specified element into this priority queue. * * @return {@code true} (as specified by {@link Queue#offer}) * @throws ClassCastException if the specified element cannot be * compared with elements currently in this priority queue * according to the priority queue's ordering * @throws NullPointerException if the specified element is null */ public boolean offer(E e) { if (e == null) throw new NullPointerException(); modCount++; int i = size; if (i >= queue.length) grow(i + 1); siftUp(i, e); size = i + 1; return true; } public E peek() { return (E) queue[0]; } private int indexOf(Object o) { if (o != null) { final Object[] es = queue; for (int i = 0, n = size; i < n; i++) if (o.equals(es[i])) return i; } return -1; } /** * Removes a single instance of the specified element from this queue, * if it is present. More formally, removes an element {@code e} such * that {@code o.equals(e)}, if this queue contains one or more such * elements. Returns {@code true} if and only if this queue contained * the specified element (or equivalently, if this queue changed as a * result of the call). * * @param o element to be removed from this queue, if present * @return {@code true} if this queue changed as a result of the call */ public boolean remove(Object o) { int i = indexOf(o); if (i == -1) return false; else { removeAt(i); return true; } } /** * Identity-based version for use in Itr.remove. * * @param o element to be removed from this queue, if present */ void removeEq(Object o) { final Object[] es = queue; for (int i = 0, n = size; i < n; i++) { if (o == es[i]) { removeAt(i); break; } } } /** * Returns {@code true} if this queue contains the specified element. * More formally, returns {@code true} if and only if this queue contains * at least one element {@code e} such that {@code o.equals(e)}. * * @param o object to be checked for containment in this queue * @return {@code true} if this queue contains the specified element */ public boolean contains(Object o) { return indexOf(o) >= 0; } /** * Returns an array containing all of the elements in this queue. * The elements are in no particular order. * * <p>The returned array will be "safe" in that no references to it are * maintained by this queue. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * * <p>This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all of the elements in this queue */ public Object[] toArray() { return Arrays.copyOf(queue, size); } /** * Returns an array containing all of the elements in this queue; the * runtime type of the returned array is that of the specified array. * The returned array elements are in no particular order. * If the queue fits in the specified array, it is returned therein. * Otherwise, a new array is allocated with the runtime type of the * specified array and the size of this queue. * * <p>If the queue fits in the specified array with room to spare * (i.e., the array has more elements than the queue), the element in * the array immediately following the end of the collection is set to * {@code null}. * * <p>Like the {@link #toArray()} method, this method acts as bridge between * array-based and collection-based APIs. Further, this method allows * precise control over the runtime type of the output array, and may, * under certain circumstances, be used to save allocation costs. * * <p>Suppose {@code x} is a queue known to contain only strings. * The following code can be used to dump the queue into a newly * allocated array of {@code String}: * * <pre> {@code String[] y = x.toArray(new String[0]);}</pre> * * Note that {@code toArray(new Object[0])} is identical in function to * {@code toArray()}. * * @param a the array into which the elements of the queue are to * be stored, if it is big enough; otherwise, a new array of the * same runtime type is allocated for this purpose. * @return an array containing all of the elements in this queue * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this queue * @throws NullPointerException if the specified array is null */ public <T> T[] toArray(T[] a) { final int size = this.size; if (a.length < size) // Make a new array of a's runtime type, but my contents: return (T[]) Arrays.copyOf(queue, size, a.getClass()); System.arraycopy(queue, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } /** * Returns an iterator over the elements in this queue. The iterator * does not return the elements in any particular order. * * @return an iterator over the elements in this queue */ public Iterator<E> iterator() { return new Itr(); } private final class Itr implements Iterator<E> { /** * Index (into queue array) of element to be returned by * subsequent call to next. */ private int cursor; /** * Index of element returned by most recent call to next, * unless that element came from the forgetMeNot list. * Set to -1 if element is deleted by a call to remove. */ private int lastRet = -1; /** * A queue of elements that were moved from the unvisited portion of * the heap into the visited portion as a result of "unlucky" element * removals during the iteration. (Unlucky element removals are those * that require a siftup instead of a siftdown.) We must visit all of * the elements in this list to complete the iteration. We do this * after we've completed the "normal" iteration. * * We expect that most iterations, even those involving removals, * will not need to store elements in this field. */ private ArrayDeque<E> forgetMeNot; /** * Element returned by the most recent call to next iff that * element was drawn from the forgetMeNot list. */ private E lastRetElt; /** * The modCount value that the iterator believes that the backing * Queue should have. If this expectation is violated, the iterator * has detected concurrent modification. */ private int expectedModCount = modCount; Itr() { } // prevent access constructor creation public boolean hasNext() { return cursor < size || (forgetMeNot != null && !forgetMeNot.isEmpty()); } public E next() { if (expectedModCount != modCount) throw new ConcurrentModificationException(); if (cursor < size) return (E) queue[lastRet = cursor++]; if (forgetMeNot != null) { lastRet = -1; lastRetElt = forgetMeNot.poll(); if (lastRetElt != null) return lastRetElt; } throw new NoSuchElementException(); } public void remove() { if (expectedModCount != modCount) throw new ConcurrentModificationException(); if (lastRet != -1) { E moved = PriorityQueue.this.removeAt(lastRet); lastRet = -1; if (moved == null) cursor--; else { if (forgetMeNot == null) forgetMeNot = new ArrayDeque<>(); forgetMeNot.add(moved); } } else if (lastRetElt != null) { PriorityQueue.this.removeEq(lastRetElt); lastRetElt = null; } else { throw new IllegalStateException(); } expectedModCount = modCount; } } public int size() { return size; } /** * Removes all of the elements from this priority queue. * The queue will be empty after this call returns. */ public void clear() { modCount++; final Object[] es = queue; for (int i = 0, n = size; i < n; i++) es[i] = null; size = 0; } public E poll() { final Object[] es; final E result; if ((result = (E) ((es = queue)[0])) != null) { modCount++; final int n; final E x = (E) es[(n = --size)]; es[n] = null; if (n > 0) { final Comparator<? super E> cmp; if ((cmp = comparator) == null) siftDownComparable(0, x, es, n); else siftDownUsingComparator(0, x, es, n, cmp); } } return result; } /** * Removes the ith element from queue. * * Normally this method leaves the elements at up to i-1, * inclusive, untouched. Under these circumstances, it returns * null. Occasionally, in order to maintain the heap invariant, * it must swap a later element of the list with one earlier than * i. Under these circumstances, this method returns the element * that was previously at the end of the list and is now at some * position before i. This fact is used by iterator.remove so as to * avoid missing traversing elements. */ E removeAt(int i) { // assert i >= 0 && i < size; final Object[] es = queue; modCount++; int s = --size; if (s == i) // removed last element es[i] = null; else { E moved = (E) es[s]; es[s] = null; siftDown(i, moved); if (es[i] == moved) { siftUp(i, moved); if (es[i] != moved) return moved; } } return null; } /** * Inserts item x at position k, maintaining heap invariant by * promoting x up the tree until it is greater than or equal to * its parent, or is the root. * * To simplify and speed up coercions and comparisons, the * Comparable and Comparator versions are separated into different * methods that are otherwise identical. (Similarly for siftDown.) * * @param k the position to fill * @param x the item to insert */ private void siftUp(int k, E x) { if (comparator != null) siftUpUsingComparator(k, x, queue, comparator); else siftUpComparable(k, x, queue); } private static <T> void siftUpComparable(int k, T x, Object[] es) { Comparable<? super T> key = (Comparable<? super T>) x; while (k > 0) { int parent = (k - 1) >>> 1; Object e = es[parent]; if (key.compareTo((T) e) >= 0) break; es[k] = e; k = parent; } es[k] = key; } private static <T> void siftUpUsingComparator(int k, T x, Object[] es, Comparator<? super T> cmp) { while (k > 0) { int parent = (k - 1) >>> 1; Object e = es[parent]; if (cmp.compare(x, (T) e) >= 0) break; es[k] = e; k = parent; } es[k] = x; } /** * Inserts item x at position k, maintaining heap invariant by * demoting x down the tree repeatedly until it is less than or * equal to its children or is a leaf. * * @param k the position to fill * @param x the item to insert */ private void siftDown(int k, E x) { if (comparator != null) siftDownUsingComparator(k, x, queue, size, comparator); else siftDownComparable(k, x, queue, size); } private static <T> void siftDownComparable(int k, T x, Object[] es, int n) { // assert n > 0; Comparable<? super T> key = (Comparable<? super T>) x; int half = n >>> 1; // loop while a non-leaf while (k < half) { int child = (k << 1) + 1; // assume left child is least Object c = es[child]; int right = child + 1; if (right < n && ((Comparable<? super T>) c).compareTo((T) es[right]) > 0) c = es[child = right]; if (key.compareTo((T) c) <= 0) break; es[k] = c; k = child; } es[k] = key; } private static <T> void siftDownUsingComparator(int k, T x, Object[] es, int n, Comparator<? super T> cmp) { // assert n > 0; int half = n >>> 1; while (k < half) { int child = (k << 1) + 1; Object c = es[child]; int right = child + 1; if (right < n && cmp.compare((T) c, (T) es[right]) > 0) c = es[child = right]; if (cmp.compare(x, (T) c) <= 0) break; es[k] = c; k = child; } es[k] = x; } /** * Establishes the heap invariant (described above) in the entire tree, * assuming nothing about the order of the elements prior to the call. * This classic algorithm due to Floyd (1964) is known to be O(size). */ private void heapify() { final Object[] es = queue; int n = size, i = (n >>> 1) - 1; final Comparator<? super E> cmp; if ((cmp = comparator) == null) for (; i >= 0; i--) siftDownComparable(i, (E) es[i], es, n); else for (; i >= 0; i--) siftDownUsingComparator(i, (E) es[i], es, n, cmp); } /** * Returns the comparator used to order the elements in this * queue, or {@code null} if this queue is sorted according to * the {@linkplain Comparable natural ordering} of its elements. * * @return the comparator used to order this queue, or * {@code null} if this queue is sorted according to the * natural ordering of its elements */ public Comparator<? super E> comparator() { return comparator; } /** * Saves this queue to a stream (that is, serializes it). * * @param s the stream * @throws java.io.IOException if an I/O error occurs * @serialData The length of the array backing the instance is * emitted (int), followed by all of its elements * (each an {@code Object}) in the proper order. */ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { // Write out element count, and any hidden stuff s.defaultWriteObject(); // Write out array length, for compatibility with 1.5 version s.writeInt(Math.max(2, size + 1)); // Write out all elements in the "proper order". final Object[] es = queue; for (int i = 0, n = size; i < n; i++) s.writeObject(es[i]); } /** * Reconstitutes the {@code PriorityQueue} instance from a stream * (that is, deserializes it). * * @param s the stream * @throws ClassNotFoundException if the class of a serialized object * could not be found * @throws java.io.IOException if an I/O error occurs */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { // Read in size, and any hidden stuff s.defaultReadObject(); // Read in (and discard) array length s.readInt(); SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size); final Object[] es = queue = new Object[Math.max(size, 1)]; // Read in all elements. for (int i = 0, n = size; i < n; i++) es[i] = s.readObject(); // Elements are guaranteed to be in "proper order", but the // spec has never explained what that might be. heapify(); } /** * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em> * and <em>fail-fast</em> {@link Spliterator} over the elements in this * queue. The spliterator does not traverse elements in any particular order * (the {@link Spliterator#ORDERED ORDERED} characteristic is not reported). * * <p>The {@code Spliterator} reports {@link Spliterator#SIZED}, * {@link Spliterator#SUBSIZED}, and {@link Spliterator#NONNULL}. * Overriding implementations should document the reporting of additional * characteristic values. * * @return a {@code Spliterator} over the elements in this queue * @since 1.8 */ public final Spliterator<E> spliterator() { return new PriorityQueueSpliterator(0, -1, 0); } final class PriorityQueueSpliterator implements Spliterator<E> { private int index; // current index, modified on advance/split private int fence; // -1 until first use private int expectedModCount; // initialized when fence set /** Creates new spliterator covering the given range. */ PriorityQueueSpliterator(int origin, int fence, int expectedModCount) { this.index = origin; this.fence = fence; this.expectedModCount = expectedModCount; } private int getFence() { // initialize fence to size on first use int hi; if ((hi = fence) < 0) { expectedModCount = modCount; hi = fence = size; } return hi; } public PriorityQueueSpliterator trySplit() { int hi = getFence(), lo = index, mid = (lo + hi) >>> 1; return (lo >= mid) ? null : new PriorityQueueSpliterator(lo, index = mid, expectedModCount); } public void forEachRemaining(Consumer<? super E> action) { if (action == null) throw new NullPointerException(); if (fence < 0) { fence = size; expectedModCount = modCount; } final Object[] es = queue; int i, hi; E e; for (i = index, index = hi = fence; i < hi; i++) { if ((e = (E) es[i]) == null) break; // must be CME action.accept(e); } if (modCount != expectedModCount) throw new ConcurrentModificationException(); } public boolean tryAdvance(Consumer<? super E> action) { if (action == null) throw new NullPointerException(); if (fence < 0) { fence = size; expectedModCount = modCount; } int i; if ((i = index) < fence) { index = i + 1; E e; if ((e = (E) queue[i]) == null || modCount != expectedModCount) throw new ConcurrentModificationException(); action.accept(e); return true; } return false; } public long estimateSize() { return getFence() - index; } public int characteristics() { return Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.NONNULL; } } /** * @throws NullPointerException {@inheritDoc} */ public boolean removeIf(Predicate<? super E> filter) { Objects.requireNonNull(filter); return bulkRemove(filter); } /** * @throws NullPointerException {@inheritDoc} */ public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return bulkRemove(e -> c.contains(e)); } /** * @throws NullPointerException {@inheritDoc} */ public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); return bulkRemove(e -> !c.contains(e)); } // A tiny bit set implementation private static long[] nBits(int n) { return new long[((n - 1) >> 6) + 1]; } private static void setBit(long[] bits, int i) { bits[i >> 6] |= 1L << i; } private static boolean isClear(long[] bits, int i) { return (bits[i >> 6] & (1L << i)) == 0; } /** Implementation of bulk remove methods. */ private boolean bulkRemove(Predicate<? super E> filter) { final int expectedModCount = ++modCount; final Object[] es = queue; final int end = size; int i; // Optimize for initial run of survivors for (i = 0; i < end && !filter.test((E) es[i]); i++) ; if (i >= end) { if (modCount != expectedModCount) throw new ConcurrentModificationException(); return false; } // Tolerate predicates that reentrantly access the collection for // read (but writers still get CME), so traverse once to find // elements to delete, a second pass to physically expunge. final int beg = i; final long[] deathRow = nBits(end - beg); deathRow[0] = 1L; // set bit 0 for (i = beg + 1; i < end; i++) if (filter.test((E) es[i])) setBit(deathRow, i - beg); if (modCount != expectedModCount) throw new ConcurrentModificationException(); int w = beg; for (i = beg; i < end; i++) if (isClear(deathRow, i - beg)) es[w++] = es[i]; for (i = size = w; i < end; i++) es[i] = null; heapify(); return true; } /** * @throws NullPointerException {@inheritDoc} */ public void forEach(Consumer<? super E> action) { Objects.requireNonNull(action); final int expectedModCount = modCount; final Object[] es = queue; for (int i = 0, n = size; i < n; i++) action.accept((E) es[i]); if (expectedModCount != modCount) throw new ConcurrentModificationException(); } }