Java tutorial
/* * Copyright (C) 2010 The Guava Authors * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package com.google.common.collect; import static com.google.common.base.Preconditions.checkArgument; import static com.google.common.base.Preconditions.checkNotNull; import static com.google.common.base.Preconditions.checkPositionIndex; import static com.google.common.base.Preconditions.checkState; import static com.google.common.collect.CollectPreconditions.checkRemove; import com.google.common.annotations.Beta; import com.google.common.annotations.VisibleForTesting; import com.google.common.math.IntMath; import com.google.j2objc.annotations.Weak; import com.google.j2objc.annotations.WeakOuter; import java.util.AbstractQueue; import java.util.ArrayDeque; import java.util.ArrayList; import java.util.Collection; import java.util.Collections; import java.util.Comparator; import java.util.ConcurrentModificationException; import java.util.Iterator; import java.util.List; import java.util.NoSuchElementException; import java.util.PriorityQueue; import java.util.Queue; /** * A double-ended priority queue, which provides constant-time access to both * its least element and its greatest element, as determined by the queue's * specified comparator. If no comparator is given at creation time, the * natural order of elements is used. If no maximum size is given at creation time, * the queue is unbounded. * * <p>Usage example: <pre> {@code * * MinMaxPriorityQueue<User> users = MinMaxPriorityQueue.orderedBy(userComparator) * .maximumSize(1000) * .create();}</pre> * * <p>As a {@link Queue} it functions exactly as a {@link PriorityQueue}: its * head element -- the implicit target of the methods {@link #peek()}, {@link * #poll()} and {@link #remove()} -- is defined as the <i>least</i> element in * the queue according to the queue's comparator. But unlike a regular priority * queue, the methods {@link #peekLast}, {@link #pollLast} and * {@link #removeLast} are also provided, to act on the <i>greatest</i> element * in the queue instead. * * <p>A min-max priority queue can be configured with a maximum size. If so, * each time the size of the queue exceeds that value, the queue automatically * removes its greatest element according to its comparator (which might be the * element that was just added). This is different from conventional bounded * queues, which either block or reject new elements when full. * * <p>This implementation is based on the * <a href="http://portal.acm.org/citation.cfm?id=6621">min-max heap</a> * developed by Atkinson, et al. Unlike many other double-ended priority queues, * it stores elements in a single array, as compact as the traditional heap data * structure used in {@link PriorityQueue}. * * <p>This class is not thread-safe, and does not accept null elements. * * <p><i>Performance notes:</i> * * <ul> * <li>If you only access one end of the queue, and do use a maximum size, * this class will perform significantly worse than a {@code PriorityQueue} * with manual eviction above the maximum size. In many cases * {@link Ordering#leastOf} may work for your use case with significantly * improved (and asymptotically superior) performance. * <li>The retrieval operations {@link #peek}, {@link #peekFirst}, {@link * #peekLast}, {@link #element}, and {@link #size} are constant-time. * <li>The enqueing and dequeing operations ({@link #offer}, {@link #add}, and * all the forms of {@link #poll} and {@link #remove()}) run in {@code * O(log n) time}. * <li>The {@link #remove(Object)} and {@link #contains} operations require * linear ({@code O(n)}) time. * <li>If you only access one end of the queue, and don't use a maximum size, * this class is functionally equivalent to {@link PriorityQueue}, but * significantly slower. * </ul> * * @author Sverre Sundsdal * @author Torbjorn Gannholm * @since 8.0 */ // TODO(kevinb): GWT compatibility @Beta public final class MinMaxPriorityQueue<E> extends AbstractQueue<E> { /** * Creates a new min-max priority queue with default settings: natural order, * no maximum size, no initial contents, and an initial expected size of 11. */ public static <E extends Comparable<E>> MinMaxPriorityQueue<E> create() { return new Builder<Comparable>(Ordering.natural()).create(); } /** * Creates a new min-max priority queue using natural order, no maximum size, * and initially containing the given elements. */ public static <E extends Comparable<E>> MinMaxPriorityQueue<E> create(Iterable<? extends E> initialContents) { return new Builder<E>(Ordering.<E>natural()).create(initialContents); } /** * Creates and returns a new builder, configured to build {@code * MinMaxPriorityQueue} instances that use {@code comparator} to determine the * least and greatest elements. */ public static <B> Builder<B> orderedBy(Comparator<B> comparator) { return new Builder<B>(comparator); } /** * Creates and returns a new builder, configured to build {@code * MinMaxPriorityQueue} instances sized appropriately to hold {@code * expectedSize} elements. */ public static Builder<Comparable> expectedSize(int expectedSize) { return new Builder<Comparable>(Ordering.natural()).expectedSize(expectedSize); } /** * Creates and returns a new builder, configured to build {@code * MinMaxPriorityQueue} instances that are limited to {@code maximumSize} * elements. Each time a queue grows beyond this bound, it immediately * removes its greatest element (according to its comparator), which might be * the element that was just added. */ public static Builder<Comparable> maximumSize(int maximumSize) { return new Builder<Comparable>(Ordering.natural()).maximumSize(maximumSize); } /** * The builder class used in creation of min-max priority queues. Instead of * constructing one directly, use {@link * MinMaxPriorityQueue#orderedBy(Comparator)}, {@link * MinMaxPriorityQueue#expectedSize(int)} or {@link * MinMaxPriorityQueue#maximumSize(int)}. * * @param <B> the upper bound on the eventual type that can be produced by * this builder (for example, a {@code Builder<Number>} can produce a * {@code Queue<Number>} or {@code Queue<Integer>} but not a {@code * Queue<Object>}). * @since 8.0 */ @Beta public static final class Builder<B> { /* * TODO(kevinb): when the dust settles, see if we still need this or can * just default to DEFAULT_CAPACITY. */ private static final int UNSET_EXPECTED_SIZE = -1; private final Comparator<B> comparator; private int expectedSize = UNSET_EXPECTED_SIZE; private int maximumSize = Integer.MAX_VALUE; private Builder(Comparator<B> comparator) { this.comparator = checkNotNull(comparator); } /** * Configures this builder to build min-max priority queues with an initial * expected size of {@code expectedSize}. */ public Builder<B> expectedSize(int expectedSize) { checkArgument(expectedSize >= 0); this.expectedSize = expectedSize; return this; } /** * Configures this builder to build {@code MinMaxPriorityQueue} instances * that are limited to {@code maximumSize} elements. Each time a queue grows * beyond this bound, it immediately removes its greatest element (according * to its comparator), which might be the element that was just added. */ public Builder<B> maximumSize(int maximumSize) { checkArgument(maximumSize > 0); this.maximumSize = maximumSize; return this; } /** * Builds a new min-max priority queue using the previously specified * options, and having no initial contents. */ public <T extends B> MinMaxPriorityQueue<T> create() { return create(Collections.<T>emptySet()); } /** * Builds a new min-max priority queue using the previously specified * options, and having the given initial elements. */ public <T extends B> MinMaxPriorityQueue<T> create(Iterable<? extends T> initialContents) { MinMaxPriorityQueue<T> queue = new MinMaxPriorityQueue<T>(this, initialQueueSize(expectedSize, maximumSize, initialContents)); for (T element : initialContents) { queue.offer(element); } return queue; } @SuppressWarnings("unchecked") // safe "contravariant cast" private <T extends B> Ordering<T> ordering() { return Ordering.from((Comparator<T>) comparator); } } private final Heap minHeap; private final Heap maxHeap; @VisibleForTesting final int maximumSize; private Object[] queue; private int size; private int modCount; private MinMaxPriorityQueue(Builder<? super E> builder, int queueSize) { Ordering<E> ordering = builder.ordering(); this.minHeap = new Heap(ordering); this.maxHeap = new Heap(ordering.reverse()); minHeap.otherHeap = maxHeap; maxHeap.otherHeap = minHeap; this.maximumSize = builder.maximumSize; // TODO(kevinb): pad? this.queue = new Object[queueSize]; } @Override public int size() { return size; } /** * Adds the given element to this queue. If this queue has a maximum size, * after adding {@code element} the queue will automatically evict its * greatest element (according to its comparator), which may be {@code * element} itself. * * @return {@code true} always */ @Override public boolean add(E element) { offer(element); return true; } @Override public boolean addAll(Collection<? extends E> newElements) { boolean modified = false; for (E element : newElements) { offer(element); modified = true; } return modified; } /** * Adds the given element to this queue. If this queue has a maximum size, * after adding {@code element} the queue will automatically evict its * greatest element (according to its comparator), which may be {@code * element} itself. */ @Override public boolean offer(E element) { checkNotNull(element); modCount++; int insertIndex = size++; growIfNeeded(); // Adds the element to the end of the heap and bubbles it up to the correct // position. heapForIndex(insertIndex).bubbleUp(insertIndex, element); return size <= maximumSize || pollLast() != element; } @Override public E poll() { return isEmpty() ? null : removeAndGet(0); } @SuppressWarnings("unchecked") // we must carefully only allow Es to get in E elementData(int index) { return (E) queue[index]; } @Override public E peek() { return isEmpty() ? null : elementData(0); } /** * Returns the index of the max element. */ private int getMaxElementIndex() { switch (size) { case 1: return 0; // The lone element in the queue is the maximum. case 2: return 1; // The lone element in the maxHeap is the maximum. default: // The max element must sit on the first level of the maxHeap. It is // actually the *lesser* of the two from the maxHeap's perspective. return (maxHeap.compareElements(1, 2) <= 0) ? 1 : 2; } } /** * Removes and returns the least element of this queue, or returns {@code * null} if the queue is empty. */ public E pollFirst() { return poll(); } /** * Removes and returns the least element of this queue. * * @throws NoSuchElementException if the queue is empty */ public E removeFirst() { return remove(); } /** * Retrieves, but does not remove, the least element of this queue, or returns * {@code null} if the queue is empty. */ public E peekFirst() { return peek(); } /** * Removes and returns the greatest element of this queue, or returns {@code * null} if the queue is empty. */ public E pollLast() { return isEmpty() ? null : removeAndGet(getMaxElementIndex()); } /** * Removes and returns the greatest element of this queue. * * @throws NoSuchElementException if the queue is empty */ public E removeLast() { if (isEmpty()) { throw new NoSuchElementException(); } return removeAndGet(getMaxElementIndex()); } /** * Retrieves, but does not remove, the greatest element of this queue, or * returns {@code null} if the queue is empty. */ public E peekLast() { return isEmpty() ? null : elementData(getMaxElementIndex()); } /** * Removes the element at position {@code index}. * * <p>Normally this method leaves the elements at up to {@code index - 1}, * inclusive, untouched. Under these circumstances, it returns {@code null}. * * <p>Occasionally, in order to maintain the heap invariant, it must swap a * later element of the list with one before {@code index}. Under these * circumstances it returns a pair of elements as a {@link MoveDesc}. The * first one is the element that was previously at the end of the heap and is * now at some position before {@code index}. The second element is the one * that was swapped down to replace the element at {@code index}. This fact is * used by iterator.remove so as to visit elements during a traversal once and * only once. */ @VisibleForTesting MoveDesc<E> removeAt(int index) { checkPositionIndex(index, size); modCount++; size--; if (size == index) { queue[size] = null; return null; } E actualLastElement = elementData(size); int lastElementAt = heapForIndex(size).getCorrectLastElement(actualLastElement); E toTrickle = elementData(size); queue[size] = null; MoveDesc<E> changes = fillHole(index, toTrickle); if (lastElementAt < index) { // Last element is moved to before index, swapped with trickled element. if (changes == null) { // The trickled element is still after index. return new MoveDesc<E>(actualLastElement, toTrickle); } else { // The trickled element is back before index, but the replaced element // has now been moved after index. return new MoveDesc<E>(actualLastElement, changes.replaced); } } // Trickled element was after index to begin with, no adjustment needed. return changes; } private MoveDesc<E> fillHole(int index, E toTrickle) { Heap heap = heapForIndex(index); // We consider elementData(index) a "hole", and we want to fill it // with the last element of the heap, toTrickle. // Since the last element of the heap is from the bottom level, we // optimistically fill index position with elements from lower levels, // moving the hole down. In most cases this reduces the number of // comparisons with toTrickle, but in some cases we will need to bubble it // all the way up again. int vacated = heap.fillHoleAt(index); // Try to see if toTrickle can be bubbled up min levels. int bubbledTo = heap.bubbleUpAlternatingLevels(vacated, toTrickle); if (bubbledTo == vacated) { // Could not bubble toTrickle up min levels, try moving // it from min level to max level (or max to min level) and bubble up // there. return heap.tryCrossOverAndBubbleUp(index, vacated, toTrickle); } else { return (bubbledTo < index) ? new MoveDesc<E>(toTrickle, elementData(index)) : null; } } // Returned from removeAt() to iterator.remove() static class MoveDesc<E> { final E toTrickle; final E replaced; MoveDesc(E toTrickle, E replaced) { this.toTrickle = toTrickle; this.replaced = replaced; } } /** * Removes and returns the value at {@code index}. */ private E removeAndGet(int index) { E value = elementData(index); removeAt(index); return value; } private Heap heapForIndex(int i) { return isEvenLevel(i) ? minHeap : maxHeap; } private static final int EVEN_POWERS_OF_TWO = 0x55555555; private static final int ODD_POWERS_OF_TWO = 0xaaaaaaaa; @VisibleForTesting static boolean isEvenLevel(int index) { int oneBased = index + 1; checkState(oneBased > 0, "negative index"); return (oneBased & EVEN_POWERS_OF_TWO) > (oneBased & ODD_POWERS_OF_TWO); } /** * Returns {@code true} if the MinMax heap structure holds. This is only used * in testing. * * TODO(kevinb): move to the test class? */ @VisibleForTesting boolean isIntact() { for (int i = 1; i < size; i++) { if (!heapForIndex(i).verifyIndex(i)) { return false; } } return true; } /** * Each instance of MinMaxPriortyQueue encapsulates two instances of Heap: * a min-heap and a max-heap. Conceptually, these might each have their own * array for storage, but for efficiency's sake they are stored interleaved on * alternate heap levels in the same array (MMPQ.queue). */ @WeakOuter private class Heap { final Ordering<E> ordering; @Weak Heap otherHeap; Heap(Ordering<E> ordering) { this.ordering = ordering; } int compareElements(int a, int b) { return ordering.compare(elementData(a), elementData(b)); } /** * Tries to move {@code toTrickle} from a min to a max level and * bubble up there. If it moved before {@code removeIndex} this method * returns a pair as described in {@link #removeAt}. */ MoveDesc<E> tryCrossOverAndBubbleUp(int removeIndex, int vacated, E toTrickle) { int crossOver = crossOver(vacated, toTrickle); if (crossOver == vacated) { return null; } // Successfully crossed over from min to max. // Bubble up max levels. E parent; // If toTrickle is moved up to a parent of removeIndex, the parent is // placed in removeIndex position. We must return that to the iterator so // that it knows to skip it. if (crossOver < removeIndex) { // We crossed over to the parent level in crossOver, so the parent // has already been moved. parent = elementData(removeIndex); } else { parent = elementData(getParentIndex(removeIndex)); } // bubble it up the opposite heap if (otherHeap.bubbleUpAlternatingLevels(crossOver, toTrickle) < removeIndex) { return new MoveDesc<E>(toTrickle, parent); } else { return null; } } /** * Bubbles a value from {@code index} up the appropriate heap if required. */ void bubbleUp(int index, E x) { int crossOver = crossOverUp(index, x); Heap heap; if (crossOver == index) { heap = this; } else { index = crossOver; heap = otherHeap; } heap.bubbleUpAlternatingLevels(index, x); } /** * Bubbles a value from {@code index} up the levels of this heap, and * returns the index the element ended up at. */ int bubbleUpAlternatingLevels(int index, E x) { while (index > 2) { int grandParentIndex = getGrandparentIndex(index); E e = elementData(grandParentIndex); if (ordering.compare(e, x) <= 0) { break; } queue[index] = e; index = grandParentIndex; } queue[index] = x; return index; } /** * Returns the index of minimum value between {@code index} and * {@code index + len}, or {@code -1} if {@code index} is greater than * {@code size}. */ int findMin(int index, int len) { if (index >= size) { return -1; } checkState(index > 0); int limit = Math.min(index, size - len) + len; int minIndex = index; for (int i = index + 1; i < limit; i++) { if (compareElements(i, minIndex) < 0) { minIndex = i; } } return minIndex; } /** * Returns the minimum child or {@code -1} if no child exists. */ int findMinChild(int index) { return findMin(getLeftChildIndex(index), 2); } /** * Returns the minimum grand child or -1 if no grand child exists. */ int findMinGrandChild(int index) { int leftChildIndex = getLeftChildIndex(index); if (leftChildIndex < 0) { return -1; } return findMin(getLeftChildIndex(leftChildIndex), 4); } /** * Moves an element one level up from a min level to a max level * (or vice versa). * Returns the new position of the element. */ int crossOverUp(int index, E x) { if (index == 0) { queue[0] = x; return 0; } int parentIndex = getParentIndex(index); E parentElement = elementData(parentIndex); if (parentIndex != 0) { // This is a guard for the case of the childless uncle. // Since the end of the array is actually the middle of the heap, // a smaller childless uncle can become a child of x when we // bubble up alternate levels, violating the invariant. int grandparentIndex = getParentIndex(parentIndex); int uncleIndex = getRightChildIndex(grandparentIndex); if (uncleIndex != parentIndex && getLeftChildIndex(uncleIndex) >= size) { E uncleElement = elementData(uncleIndex); if (ordering.compare(uncleElement, parentElement) < 0) { parentIndex = uncleIndex; parentElement = uncleElement; } } } if (ordering.compare(parentElement, x) < 0) { queue[index] = parentElement; queue[parentIndex] = x; return parentIndex; } queue[index] = x; return index; } /** * Returns the conceptually correct last element of the heap. * * <p>Since the last element of the array is actually in the * middle of the sorted structure, a childless uncle node could be * smaller, which would corrupt the invariant if this element * becomes the new parent of the uncle. In that case, we first * switch the last element with its uncle, before returning. */ int getCorrectLastElement(E actualLastElement) { int parentIndex = getParentIndex(size); if (parentIndex != 0) { int grandparentIndex = getParentIndex(parentIndex); int uncleIndex = getRightChildIndex(grandparentIndex); if (uncleIndex != parentIndex && getLeftChildIndex(uncleIndex) >= size) { E uncleElement = elementData(uncleIndex); if (ordering.compare(uncleElement, actualLastElement) < 0) { queue[uncleIndex] = actualLastElement; queue[size] = uncleElement; return uncleIndex; } } } return size; } /** * Crosses an element over to the opposite heap by moving it one level down * (or up if there are no elements below it). * * Returns the new position of the element. */ int crossOver(int index, E x) { int minChildIndex = findMinChild(index); // TODO(kevinb): split the && into two if's and move crossOverUp so it's // only called when there's no child. if ((minChildIndex > 0) && (ordering.compare(elementData(minChildIndex), x) < 0)) { queue[index] = elementData(minChildIndex); queue[minChildIndex] = x; return minChildIndex; } return crossOverUp(index, x); } /** * Fills the hole at {@code index} by moving in the least of its * grandchildren to this position, then recursively filling the new hole * created. * * @return the position of the new hole (where the lowest grandchild moved * from, that had no grandchild to replace it) */ int fillHoleAt(int index) { int minGrandchildIndex; while ((minGrandchildIndex = findMinGrandChild(index)) > 0) { queue[index] = elementData(minGrandchildIndex); index = minGrandchildIndex; } return index; } private boolean verifyIndex(int i) { if ((getLeftChildIndex(i) < size) && (compareElements(i, getLeftChildIndex(i)) > 0)) { return false; } if ((getRightChildIndex(i) < size) && (compareElements(i, getRightChildIndex(i)) > 0)) { return false; } if ((i > 0) && (compareElements(i, getParentIndex(i)) > 0)) { return false; } if ((i > 2) && (compareElements(getGrandparentIndex(i), i) > 0)) { return false; } return true; } // These would be static if inner classes could have static members. private int getLeftChildIndex(int i) { return i * 2 + 1; } private int getRightChildIndex(int i) { return i * 2 + 2; } private int getParentIndex(int i) { return (i - 1) / 2; } private int getGrandparentIndex(int i) { return getParentIndex(getParentIndex(i)); // (i - 3) / 4 } } /** * Iterates the elements of the queue in no particular order. * * If the underlying queue is modified during iteration an exception will be * thrown. */ private class QueueIterator implements Iterator<E> { private int cursor = -1; private int expectedModCount = modCount; private Queue<E> forgetMeNot; private List<E> skipMe; private E lastFromForgetMeNot; private boolean canRemove; @Override public boolean hasNext() { checkModCount(); return (nextNotInSkipMe(cursor + 1) < size()) || ((forgetMeNot != null) && !forgetMeNot.isEmpty()); } @Override public E next() { checkModCount(); int tempCursor = nextNotInSkipMe(cursor + 1); if (tempCursor < size()) { cursor = tempCursor; canRemove = true; return elementData(cursor); } else if (forgetMeNot != null) { cursor = size(); lastFromForgetMeNot = forgetMeNot.poll(); if (lastFromForgetMeNot != null) { canRemove = true; return lastFromForgetMeNot; } } throw new NoSuchElementException("iterator moved past last element in queue."); } @Override public void remove() { checkRemove(canRemove); checkModCount(); canRemove = false; expectedModCount++; if (cursor < size()) { MoveDesc<E> moved = removeAt(cursor); if (moved != null) { if (forgetMeNot == null) { forgetMeNot = new ArrayDeque<E>(); skipMe = new ArrayList<E>(3); } forgetMeNot.add(moved.toTrickle); skipMe.add(moved.replaced); } cursor--; } else { // we must have set lastFromForgetMeNot in next() checkState(removeExact(lastFromForgetMeNot)); lastFromForgetMeNot = null; } } // Finds only this exact instance, not others that are equals() private boolean containsExact(Iterable<E> elements, E target) { for (E element : elements) { if (element == target) { return true; } } return false; } // Removes only this exact instance, not others that are equals() boolean removeExact(Object target) { for (int i = 0; i < size; i++) { if (queue[i] == target) { removeAt(i); return true; } } return false; } void checkModCount() { if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * Returns the index of the first element after {@code c} that is not in * {@code skipMe} and returns {@code size()} if there is no such element. */ private int nextNotInSkipMe(int c) { if (skipMe != null) { while (c < size() && containsExact(skipMe, elementData(c))) { c++; } } return c; } } /** * Returns an iterator over the elements contained in this collection, * <i>in no particular order</i>. * * <p>The iterator is <i>fail-fast</i>: If the MinMaxPriorityQueue is modified * at any time after the iterator is created, in any way except through the * iterator's own remove method, the iterator will generally throw a * {@link ConcurrentModificationException}. Thus, in the face of concurrent * modification, the iterator fails quickly and cleanly, rather than risking * arbitrary, non-deterministic behavior at an undetermined time in the * future. * * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification. Fail-fast iterators * throw {@code ConcurrentModificationException} on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * * @return an iterator over the elements contained in this collection */ @Override public Iterator<E> iterator() { return new QueueIterator(); } @Override public void clear() { for (int i = 0; i < size; i++) { queue[i] = null; } size = 0; } @Override public Object[] toArray() { Object[] copyTo = new Object[size]; System.arraycopy(queue, 0, copyTo, 0, size); return copyTo; } /** * Returns the comparator used to order the elements in this queue. Obeys the * general contract of {@link PriorityQueue#comparator}, but returns {@link * Ordering#natural} instead of {@code null} to indicate natural ordering. */ public Comparator<? super E> comparator() { return minHeap.ordering; } @VisibleForTesting int capacity() { return queue.length; } // Size/capacity-related methods private static final int DEFAULT_CAPACITY = 11; @VisibleForTesting static int initialQueueSize(int configuredExpectedSize, int maximumSize, Iterable<?> initialContents) { // Start with what they said, if they said it, otherwise DEFAULT_CAPACITY int result = (configuredExpectedSize == Builder.UNSET_EXPECTED_SIZE) ? DEFAULT_CAPACITY : configuredExpectedSize; // Enlarge to contain initial contents if (initialContents instanceof Collection) { int initialSize = ((Collection<?>) initialContents).size(); result = Math.max(result, initialSize); } // Now cap it at maxSize + 1 return capAtMaximumSize(result, maximumSize); } private void growIfNeeded() { if (size > queue.length) { int newCapacity = calculateNewCapacity(); Object[] newQueue = new Object[newCapacity]; System.arraycopy(queue, 0, newQueue, 0, queue.length); queue = newQueue; } } /** Returns ~2x the old capacity if small; ~1.5x otherwise. */ private int calculateNewCapacity() { int oldCapacity = queue.length; int newCapacity = (oldCapacity < 64) ? (oldCapacity + 1) * 2 : IntMath.checkedMultiply(oldCapacity / 2, 3); return capAtMaximumSize(newCapacity, maximumSize); } /** There's no reason for the queueSize to ever be more than maxSize + 1 */ private static int capAtMaximumSize(int queueSize, int maximumSize) { return Math.min(queueSize - 1, maximumSize) + 1; // don't overflow } }