Java tutorial
/* * Written by Josh Bloch of Google Inc. and released to the public domain, * as explained at http://creativecommons.org/licenses/publicdomain. */ //package xbird.util.concurrent.jsr166; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; import java.util.AbstractCollection; import java.util.Collection; import java.util.ConcurrentModificationException; import java.util.Deque; import java.util.Iterator; import java.util.LinkedList; import java.util.NoSuchElementException; import java.util.Queue; import java.util.Stack; /** * Resizable-array implementation of the {@link Deque} interface. Array * deques have no capacity restrictions; they grow as necessary to support * usage. They are not thread-safe; in the absence of external * synchronization, they do not support concurrent access by multiple threads. * Null elements are prohibited. This class is likely to be faster than * {@link Stack} when used as as a stack, and faster than {@link LinkedList} * when used as a queue. * * <p>Most <tt>ArrayDeque</tt> operations run in amortized constant time. * Exceptions include {@link #remove(Object) remove}, {@link * #removeFirstOccurrence removeFirstOccurrence}, {@link #removeLastOccurrence * removeLastOccurrence}, {@link #contains contains }, {@link #iterator * iterator.remove()}, and the bulk operations, all of which run in linear * time. * * <p>The iterators returned by this class's <tt>iterator</tt> method are * <i>fail-fast</i>: If the deque 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 <tt>ConcurrentModificationException</tt> 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> * * <p>This class and its iterator implement all of the * optional methods of the {@link Collection} and {@link * Iterator} interfaces. This class is a member of the <a * href="{@docRoot}/../guide/collections/index.html"> Java Collections * Framework</a>. * * @author Josh Bloch and Doug Lea * @since 1.6 * @param <E> the type of elements held in this collection */ public class ArrayDeque<E> extends AbstractCollection<E> implements Deque<E>, Cloneable, Serializable { /** * The array in which the elements of in the deque are stored. * The capacity of the deque is the length of this array, which is * always a power of two. The array is never allowed to become * full, except transiently within an addX method where it is * resized (see doubleCapacity) immediately upon becoming full, * thus avoiding head and tail wrapping around to equal each * other. We also guarantee that all array cells not holding * deque elements are always null. */ private transient E[] elements; /** * The index of the element at the head of the deque (which is the * element that would be removed by remove() or pop()); or an * arbitrary number equal to tail if the deque is empty. */ private transient int head; /** * The index at which the next element would be added to the tail * of the deque (via addLast(E), add(E), or push(E)). */ private transient int tail; /** * The minimum capacity that we'll use for a newly created deque. * Must be a power of 2. */ private static final int MIN_INITIAL_CAPACITY = 8; // ****** Array allocation and resizing utilities ****** /** * Allocate empty array to hold the given number of elements. * * @param numElements the number of elements to hold. */ private void allocateElements(int numElements) { int initialCapacity = MIN_INITIAL_CAPACITY; // Find the best power of two to hold elements. // Tests "<=" because arrays aren't kept full. if (numElements >= initialCapacity) { initialCapacity = numElements; initialCapacity |= (initialCapacity >>> 1); initialCapacity |= (initialCapacity >>> 2); initialCapacity |= (initialCapacity >>> 4); initialCapacity |= (initialCapacity >>> 8); initialCapacity |= (initialCapacity >>> 16); initialCapacity++; if (initialCapacity < 0) // Too many elements, must back off initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements } elements = (E[]) new Object[initialCapacity]; } /** * Double the capacity of this deque. Call only when full, i.e., * when head and tail have wrapped around to become equal. */ private void doubleCapacity() { assert head == tail; int p = head; int n = elements.length; int r = n - p; // number of elements to the right of p int newCapacity = n << 1; if (newCapacity < 0) throw new IllegalStateException("Sorry, deque too big"); Object[] a = new Object[newCapacity]; System.arraycopy(elements, p, a, 0, r); System.arraycopy(elements, 0, a, r, p); elements = (E[]) a; head = 0; tail = n; } /** * Copy the elements from our element array into the specified array, * in order (from first to last element in the deque). It is assumed * that the array is large enough to hold all elements in the deque. * * @return its argument */ private <T> T[] copyElements(T[] a) { if (head < tail) { System.arraycopy(elements, head, a, 0, size()); } else if (head > tail) { int headPortionLen = elements.length - head; System.arraycopy(elements, head, a, 0, headPortionLen); System.arraycopy(elements, 0, a, headPortionLen, tail); } return a; } /** * Constructs an empty array deque with the an initial capacity * sufficient to hold 16 elements. */ public ArrayDeque() { elements = (E[]) new Object[16]; } /** * Constructs an empty array deque with an initial capacity * sufficient to hold the specified number of elements. * * @param numElements lower bound on initial capacity of the deque */ public ArrayDeque(int numElements) { allocateElements(numElements); } /** * Constructs a deque containing the elements of the specified * collection, in the order they are returned by the collection's * iterator. (The first element returned by the collection's * iterator becomes the first element, or <i>front</i> of the * deque.) * * @param c the collection whose elements are to be placed into the deque * @throws NullPointerException if the specified collection is null */ public ArrayDeque(Collection<? extends E> c) { allocateElements(c.size()); addAll(c); } // The main insertion and extraction methods are addFirst, // addLast, pollFirst, pollLast. The other methods are defined in // terms of these. /** * Inserts the specified element to the front this deque. * * @param e the element to insert * @throws NullPointerException if <tt>e</tt> is null */ public void addFirst(E e) { if (e == null) throw new NullPointerException(); elements[head = (head - 1) & (elements.length - 1)] = e; if (head == tail) doubleCapacity(); } /** * Inserts the specified element to the end this deque. * This method is equivalent to {@link Collection#add} and * {@link #push}. * * @param e the element to insert * @throws NullPointerException if <tt>e</tt> is null */ public void addLast(E e) { if (e == null) throw new NullPointerException(); elements[tail] = e; if ((tail = (tail + 1) & (elements.length - 1)) == head) doubleCapacity(); } /** * Retrieves and removes the first element of this deque, or * <tt>null</tt> if this deque is empty. * * @return the first element of this deque, or <tt>null</tt> if * this deque is empty */ public E pollFirst() { int h = head; E result = elements[h]; // Element is null if deque empty if (result == null) return null; elements[h] = null; // Must null out slot head = (h + 1) & (elements.length - 1); return result; } /** * Retrieves and removes the last element of this deque, or * <tt>null</tt> if this deque is empty. * * @return the last element of this deque, or <tt>null</tt> if * this deque is empty */ public E pollLast() { int t = (tail - 1) & (elements.length - 1); E result = elements[t]; if (result == null) return null; elements[t] = null; tail = t; return result; } /** * Inserts the specified element to the front this deque. * * @param e the element to insert * @return <tt>true</tt> (as per the spec for {@link Deque#offerFirst}) * @throws NullPointerException if <tt>e</tt> is null */ public boolean offerFirst(E e) { addFirst(e); return true; } /** * Inserts the specified element to the end this deque. * * @param e the element to insert * @return <tt>true</tt> (as per the spec for {@link Deque#offerLast}) * @throws NullPointerException if <tt>e</tt> is null */ public boolean offerLast(E e) { addLast(e); return true; } /** * Retrieves and removes the first element of this deque. This method * differs from the <tt>pollFirst</tt> method in that it throws an * exception if this deque is empty. * * @return the first element of this deque * @throws NoSuchElementException if this deque is empty */ public E removeFirst() { E x = pollFirst(); if (x == null) throw new NoSuchElementException(); return x; } /** * Retrieves and removes the last element of this deque. This method * differs from the <tt>pollLast</tt> method in that it throws an * exception if this deque is empty. * * @return the last element of this deque * @throws NoSuchElementException if this deque is empty */ public E removeLast() { E x = pollLast(); if (x == null) throw new NoSuchElementException(); return x; } /** * Retrieves, but does not remove, the first element of this deque, * returning <tt>null</tt> if this deque is empty. * * @return the first element of this deque, or <tt>null</tt> if * this deque is empty */ public E peekFirst() { return elements[head]; // elements[head] is null if deque empty } /** * Retrieves, but does not remove, the last element of this deque, * returning <tt>null</tt> if this deque is empty. * * @return the last element of this deque, or <tt>null</tt> if this deque * is empty */ public E peekLast() { return elements[(tail - 1) & (elements.length - 1)]; } /** * Retrieves, but does not remove, the first element of this * deque. This method differs from the <tt>peek</tt> method only * in that it throws an exception if this deque is empty. * * @return the first element of this deque * @throws NoSuchElementException if this deque is empty */ public E getFirst() { E x = elements[head]; if (x == null) throw new NoSuchElementException(); return x; } /** * Retrieves, but does not remove, the last element of this * deque. This method differs from the <tt>peek</tt> method only * in that it throws an exception if this deque is empty. * * @return the last element of this deque * @throws NoSuchElementException if this deque is empty */ public E getLast() { E x = elements[(tail - 1) & (elements.length - 1)]; if (x == null) throw new NoSuchElementException(); return x; } /** * Removes the first occurrence of the specified element in this * deque (when traversing the deque from head to tail). If the deque * does not contain the element, it is unchanged. * * @param e element to be removed from this deque, if present * @return <tt>true</tt> if the deque contained the specified element */ public boolean removeFirstOccurrence(Object e) { if (e == null) return false; int mask = elements.length - 1; int i = head; E x; while ((x = elements[i]) != null) { if (e.equals(x)) { delete(i); return true; } i = (i + 1) & mask; } return false; } /** * Removes the last occurrence of the specified element in this * deque (when traversing the deque from head to tail). If the deque * does not contain the element, it is unchanged. * * @param e element to be removed from this deque, if present * @return <tt>true</tt> if the deque contained the specified element */ public boolean removeLastOccurrence(Object e) { if (e == null) return false; int mask = elements.length - 1; int i = (tail - 1) & mask; E x; while ((x = elements[i]) != null) { if (e.equals(x)) { delete(i); return true; } i = (i - 1) & mask; } return false; } // *** Queue methods *** /** * Inserts the specified element to the end of this deque. * * <p>This method is equivalent to {@link #offerLast}. * * @param e the element to insert * @return <tt>true</tt> (as per the spec for {@link Queue#offer}) * @throws NullPointerException if <tt>e</tt> is null */ public boolean offer(E e) { return offerLast(e); } /** * Inserts the specified element to the end of this deque. * * <p>This method is equivalent to {@link #addLast}. * * @param e the element to insert * @return <tt>true</tt> (as per the spec for {@link Collection#add}) * @throws NullPointerException if <tt>e</tt> is null */ public boolean add(E e) { addLast(e); return true; } /** * Retrieves and removes the head of the queue represented by * this deque, or <tt>null</tt> if this deque is empty. In other words, * retrieves and removes the first element of this deque, or <tt>null</tt> * if this deque is empty. * * <p>This method is equivalent to {@link #pollFirst}. * * @return the first element of this deque, or <tt>null</tt> if * this deque is empty */ public E poll() { return pollFirst(); } /** * Retrieves and removes the head of the queue represented by this deque. * This method differs from the <tt>poll</tt> method in that it throws an * exception if this deque is empty. * * <p>This method is equivalent to {@link #removeFirst}. * * @return the head of the queue represented by this deque * @throws NoSuchElementException if this deque is empty */ public E remove() { return removeFirst(); } /** * Retrieves, but does not remove, the head of the queue represented by * this deque, returning <tt>null</tt> if this deque is empty. * * <p>This method is equivalent to {@link #peekFirst} * * @return the head of the queue represented by this deque, or * <tt>null</tt> if this deque is empty */ public E peek() { return peekFirst(); } /** * Retrieves, but does not remove, the head of the queue represented by * this deque. This method differs from the <tt>peek</tt> method only in * that it throws an exception if this deque is empty. * * <p>This method is equivalent to {@link #getFirst} * * @return the head of the queue represented by this deque * @throws NoSuchElementException if this deque is empty */ public E element() { return getFirst(); } // *** Stack methods *** /** * Pushes an element onto the stack represented by this deque. In other * words, inserts the element to the front this deque. * * <p>This method is equivalent to {@link #addFirst}. * * @param e the element to push * @throws NullPointerException if <tt>e</tt> is null */ public void push(E e) { addFirst(e); } /** * Pops an element from the stack represented by this deque. In other * words, removes and returns the the first element of this deque. * * <p>This method is equivalent to {@link #removeFirst()}. * * @return the element at the front of this deque (which is the top * of the stack represented by this deque) * @throws NoSuchElementException if this deque is empty */ public E pop() { return removeFirst(); } /** * Remove the element at the specified position in the elements array, * adjusting head, tail, and size as necessary. This can result in * motion of elements backwards or forwards in the array. * * <p>This method is called delete rather than remove to emphasize the * that that its semantics differ from those of List.remove(int). * * @return true if elements moved backwards */ private boolean delete(int i) { // Case 1: Deque doesn't wrap // Case 2: Deque does wrap and removed element is in the head portion if ((head < tail || tail == 0) || i >= head) { System.arraycopy(elements, head, elements, head + 1, i - head); elements[head] = null; head = (head + 1) & (elements.length - 1); return false; } // Case 3: Deque wraps and removed element is in the tail portion tail--; System.arraycopy(elements, i + 1, elements, i, tail - i); elements[tail] = null; return true; } // *** Collection Methods *** /** * Returns the number of elements in this deque. * * @return the number of elements in this deque */ public int size() { return (tail - head) & (elements.length - 1); } /** * Returns <tt>true</tt> if this collection contains no elements.<p> * * @return <tt>true</tt> if this collection contains no elements. */ public boolean isEmpty() { return head == tail; } /** * Returns an iterator over the elements in this deque. The elements * will be ordered from first (head) to last (tail). This is the same * order that elements would be dequeued (via successive calls to * {@link #remove} or popped (via successive calls to {@link #pop}). * * @return an <tt>Iterator</tt> over the elements in this deque */ public Iterator<E> iterator() { return new DeqIterator(); } private class DeqIterator implements Iterator<E> { /** * Index of element to be returned by subsequent call to next. */ private int cursor = head; /** * Tail recorded at construction (also in remove), to stop * iterator and also to check for comodification. */ private int fence = tail; /** * Index of element returned by most recent call to next. * Reset to -1 if element is deleted by a call to remove. */ private int lastRet = -1; public boolean hasNext() { return cursor != fence; } public E next() { E result; if (cursor == fence) throw new NoSuchElementException(); // This check doesn't catch all possible comodifications, // but does catch the ones that corrupt traversal if (tail != fence || (result = elements[cursor]) == null) throw new ConcurrentModificationException(); lastRet = cursor; cursor = (cursor + 1) & (elements.length - 1); return result; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); if (delete(lastRet)) cursor--; lastRet = -1; fence = tail; } } /** * Returns <tt>true</tt> if this deque contains the specified * element. More formally, returns <tt>true</tt> if and only if this * deque contains at least one element <tt>e</tt> such that * <tt>e.equals(o)</tt>. * * @param o object to be checked for containment in this deque * @return <tt>true</tt> if this deque contains the specified element */ public boolean contains(Object o) { if (o == null) return false; int mask = elements.length - 1; int i = head; E x; while ((x = elements[i]) != null) { if (o.equals(x)) return true; i = (i + 1) & mask; } return false; } /** * Removes a single instance of the specified element from this deque. * This method is equivalent to {@link #removeFirstOccurrence}. * * @param e element to be removed from this deque, if present * @return <tt>true</tt> if this deque contained the specified element */ public boolean remove(Object e) { return removeFirstOccurrence(e); } /** * Removes all of the elements from this deque. */ public void clear() { int h = head; int t = tail; if (h != t) { // clear all cells head = tail = 0; int i = h; int mask = elements.length - 1; do { elements[i] = null; i = (i + 1) & mask; } while (i != t); } } /** * Returns an array containing all of the elements in this list * in the correct order. * * @return an array containing all of the elements in this list * in the correct order */ public Object[] toArray() { return copyElements(new Object[size()]); } /** * Returns an array containing all of the elements in this deque in the * correct order; the runtime type of the returned array is that of the * specified array. If the deque 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 deque. * * <p>If the deque fits in the specified array with room to spare (i.e., * the array has more elements than the deque), the element in the array * immediately following the end of the collection is set to <tt>null</tt>. * * @param a the array into which the elements of the deque 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 the elements of the deque * @throws ArrayStoreException if the runtime type of a is not a supertype * of the runtime type of every element in this deque */ public <T> T[] toArray(T[] a) { int size = size(); if (a.length < size) a = (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), size); copyElements(a); if (a.length > size) a[size] = null; return a; } // *** Object methods *** /** * Returns a copy of this deque. * * @return a copy of this deque */ public ArrayDeque<E> clone() { try { ArrayDeque<E> result = (ArrayDeque<E>) super.clone(); // These two lines are currently faster than cloning the array: result.elements = (E[]) new Object[elements.length]; System.arraycopy(elements, 0, result.elements, 0, elements.length); return result; } catch (CloneNotSupportedException e) { throw new AssertionError(); } } /** * Appease the serialization gods. */ private static final long serialVersionUID = 2340985798034038923L; /** * Serialize this deque. * * @serialData The current size (<tt>int</tt>) of the deque, * followed by all of its elements (each an object reference) in * first-to-last order. */ private void writeObject(ObjectOutputStream s) throws IOException { s.defaultWriteObject(); // Write out size int size = size(); s.writeInt(size); // Write out elements in order. int i = head; int mask = elements.length - 1; for (int j = 0; j < size; j++) { s.writeObject(elements[i]); i = (i + 1) & mask; } } /** * Deserialize this deque. */ private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException { s.defaultReadObject(); // Read in size and allocate array int size = s.readInt(); allocateElements(size); head = 0; tail = size; // Read in all elements in the proper order. for (int i = 0; i < size; i++) elements[i] = (E) s.readObject(); } public Iterator<E> descendingIterator() { return new DescendingIterator(); } private class DescendingIterator implements Iterator<E> { /* * This class is nearly a mirror-image of DeqIterator, using * tail instead of head for initial cursor, and head instead of * tail for fence. */ private int cursor = tail; private int fence = head; private int lastRet = -1; public boolean hasNext() { return cursor != fence; } public E next() { if (cursor == fence) throw new NoSuchElementException(); cursor = (cursor - 1) & (elements.length - 1); E result = elements[cursor]; if (head != fence || result == null) throw new ConcurrentModificationException(); lastRet = cursor; return result; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); if (!delete(lastRet)) { cursor = (cursor + 1) & (elements.length - 1); fence = head; } lastRet = -1; } } }