Most {@code ArrayDeque} 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. * *

The iterators returned by this class's {@code iterator} method are * fail-fast: If the deque is modified at any time after the iterator * is created, in any way except through the iterator's own {@code 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. * *

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: the fail-fast behavior of iterators * should be used only to detect bugs. * *

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 * * Java Collections Framework. * * @author Josh Bloch and Doug Lea * @since 1.6 * @param the type of elements held in this collection */ public class ArrayDeque extends AbstractCollection implements Deque, Cloneable, Serializable { /** * The array in which the elements of 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. */ transient Object[] elements; // non-private to simplify nested class access /** * 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. */ 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)). */ 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 ****** /** * Allocates 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 = new Object[initialCapacity]; } /** * Doubles 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 = a; head = 0; tail = n; } /** * Copies 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[] 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 an initial capacity * sufficient to hold 16 elements. */ public ArrayDeque() { elements = 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 front 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 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 at the front of this deque. * * @param e the element to add * @throws NullPointerException if the specified element 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 at the end of this deque. * *

This method is equivalent to {@link #add}. * * @param e the element to add * @throws NullPointerException if the specified element 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(); } /** * Inserts the specified element at the front of this deque. * * @param e the element to add * @return {@code true} (as specified by {@link Deque#offerFirst}) * @throws NullPointerException if the specified element is null */ public boolean offerFirst(E e) { addFirst(e); return true; } /** * Inserts the specified element at the end of this deque. * * @param e the element to add * @return {@code true} (as specified by {@link Deque#offerLast}) * @throws NullPointerException if the specified element is null */ public boolean offerLast(E e) { addLast(e); return true; } /** * @throws NoSuchElementException {@inheritDoc} */ public E removeFirst() { E x = pollFirst(); if (x == null) throw new NoSuchElementException(); return x; } /** * @throws NoSuchElementException {@inheritDoc} */ public E removeLast() { E x = pollLast(); if (x == null) throw new NoSuchElementException(); return x; } public E pollFirst() { int h = head; @SuppressWarnings("unchecked") E result = (E) 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; } public E pollLast() { int t = (tail - 1) & (elements.length - 1); @SuppressWarnings("unchecked") E result = (E) elements[t]; if (result == null) return null; elements[t] = null; tail = t; return result; } /** * @throws NoSuchElementException {@inheritDoc} */ public E getFirst() { @SuppressWarnings("unchecked") E result = (E) elements[head]; if (result == null) throw new NoSuchElementException(); return result; } /** * @throws NoSuchElementException {@inheritDoc} */ public E getLast() { @SuppressWarnings("unchecked") E result = (E) elements[(tail - 1) & (elements.length - 1)]; if (result == null) throw new NoSuchElementException(); return result; } @SuppressWarnings("unchecked") public E peekFirst() { // elements[head] is null if deque empty return (E) elements[head]; } @SuppressWarnings("unchecked") public E peekLast() { return (E) elements[(tail - 1) & (elements.length - 1)]; } /** * 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. * More formally, removes the first element {@code e} such that * {@code o.equals(e)} (if such an element exists). * Returns {@code true} if this deque contained the specified element * (or equivalently, if this deque changed as a result of the call). * * @param o element to be removed from this deque, if present * @return {@code true} if the deque contained the specified element */ public boolean removeFirstOccurrence(Object o) { if (o == null) return false; int mask = elements.length - 1; int i = head; Object x; while ( (x = elements[i]) != null) { if (o.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. * More formally, removes the last element {@code e} such that * {@code o.equals(e)} (if such an element exists). * Returns {@code true} if this deque contained the specified element * (or equivalently, if this deque changed as a result of the call). * * @param o element to be removed from this deque, if present * @return {@code true} if the deque contained the specified element */ public boolean removeLastOccurrence(Object o) { if (o == null) return false; int mask = elements.length - 1; int i = (tail - 1) & mask; Object x; while ( (x = elements[i]) != null) { if (o.equals(x)) { delete(i); return true; } i = (i - 1) & mask; } return false; } // *** Queue methods *** /** * Inserts the specified element at the end of this deque. * *

This method is equivalent to {@link #addLast}. * * @param e the element to add * @return {@code true} (as specified by {@link Collection#add}) * @throws NullPointerException if the specified element is null */ public boolean add(E e) { addLast(e); return true; } /** * Inserts the specified element at the end of this deque. * *

This method is equivalent to {@link #offerLast}. * * @param e the element to add * @return {@code true} (as specified by {@link Queue#offer}) * @throws NullPointerException if the specified element is null */ public boolean offer(E e) { return offerLast(e); } /** * Retrieves and removes the head of the queue represented by this deque. * * This method differs from {@link #poll poll} only in that it throws an * exception if this deque is empty. * *

This method is equivalent to {@link #removeFirst}. * * @return the head of the queue represented by this deque * @throws NoSuchElementException {@inheritDoc} */ public E remove() { return removeFirst(); } /** * Retrieves and removes the head of the queue represented by this deque * (in other words, the first element of this deque), or returns * {@code null} if this deque is empty. * *

This method is equivalent to {@link #pollFirst}. * * @return the head of the queue represented by this deque, or * {@code null} if this deque is empty */ public E poll() { return pollFirst(); } /** * Retrieves, but does not remove, the head of the queue represented by * this deque. This method differs from {@link #peek peek} only in * that it throws an exception if this deque is empty. * *

This method is equivalent to {@link #getFirst}. * * @return the head of the queue represented by this deque * @throws NoSuchElementException {@inheritDoc} */ public E element() { return getFirst(); } /** * Retrieves, but does not remove, the head of the queue represented by * this deque, or returns {@code null} if this deque is empty. * *

This method is equivalent to {@link #peekFirst}. * * @return the head of the queue represented by this deque, or * {@code null} if this deque is empty */ public E peek() { return peekFirst(); } // *** Stack methods *** /** * Pushes an element onto the stack represented by this deque. In other * words, inserts the element at the front of this deque. * *

This method is equivalent to {@link #addFirst}. * * @param e the element to push * @throws NullPointerException if the specified element 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 first element of this deque. * *

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 {@inheritDoc} */ public E pop() { return removeFirst(); } private void checkInvariants() { assert elements[tail] == null; assert head == tail ? elements[head] == null : (elements[head] != null && elements[(tail - 1) & (elements.length - 1)] != null); assert elements[(head - 1) & (elements.length - 1)] == null; } /** * Removes the element at the specified position in the elements array, * adjusting head and tail as necessary. This can result in motion of * elements backwards or forwards in the array. * *

This method is called delete rather than remove to emphasize * that its semantics differ from those of {@link List#remove(int)}. * * @return true if elements moved backwards */ private boolean delete(int i) { checkInvariants(); final Object[] elements = this.elements; final int mask = elements.length - 1; final int h = head; final int t = tail; final int front = (i - h) & mask; final int back = (t - i) & mask; // Invariant: head <= i < tail mod circularity if (front >= ((t - h) & mask)) throw new ConcurrentModificationException(); // Optimize for least element motion if (front < back) { if (h <= i) { System.arraycopy(elements, h, elements, h + 1, front); } else { // Wrap around System.arraycopy(elements, 0, elements, 1, i); elements[0] = elements[mask]; System.arraycopy(elements, h, elements, h + 1, mask - h); } elements[h] = null; head = (h + 1) & mask; return false; } else { if (i < t) { // Copy the null tail as well System.arraycopy(elements, i + 1, elements, i, back); tail = t - 1; } else { // Wrap around System.arraycopy(elements, i + 1, elements, i, mask - i); elements[mask] = elements[0]; System.arraycopy(elements, 1, elements, 0, t); tail = (t - 1) & mask; } 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 {@code true} if this deque contains no elements. * * @return {@code true} if this deque 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 iterator over the elements in this deque */ public Iterator iterator() { return new DeqIterator(); } public Iterator descendingIterator() { return new DescendingIterator(); } private class DeqIterator implements Iterator { /** * 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() { if (cursor == fence) throw new NoSuchElementException(); @SuppressWarnings("unchecked") E result = (E) elements[cursor]; // This check doesn't catch all possible comodifications, // but does catch the ones that corrupt traversal if (tail != fence || result == 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)) { // if left-shifted, undo increment in next() cursor = (cursor - 1) & (elements.length - 1); fence = tail; } lastRet = -1; } public void forEachRemaining(Consumer action) { Objects.requireNonNull(action); Object[] a = elements; int m = a.length - 1, f = fence, i = cursor; cursor = f; while (i != f) { @SuppressWarnings("unchecked") E e = (E)a[i]; i = (i + 1) & m; if (e == null) throw new ConcurrentModificationException(); action.accept(e); } } } private class DescendingIterator implements Iterator { /* * 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); @SuppressWarnings("unchecked") E result = (E) 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; } } /** * Returns {@code true} if this deque contains the specified element. * More formally, returns {@code true} if and only if this deque contains * at least one element {@code e} such that {@code o.equals(e)}. * * @param o object to be checked for containment in this deque * @return {@code true} 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; Object 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. * If the deque does not contain the element, it is unchanged. * More formally, removes the first element {@code e} such that * {@code o.equals(e)} (if such an element exists). * Returns {@code true} if this deque contained the specified element * (or equivalently, if this deque changed as a result of the call). * *

This method is equivalent to {@link #removeFirstOccurrence(Object)}. * * @param o element to be removed from this deque, if present * @return {@code true} if this deque contained the specified element */ public boolean remove(Object o) { return removeFirstOccurrence(o); } /** * Removes all of the elements from this deque. * The deque will be empty after this call returns. */ 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 deque * in proper sequence (from first to last element). * *

The returned array will be "safe" in that no references to it are * maintained by this deque. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * *

This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all of the elements in this deque */ public Object[] toArray() { return copyElements(new Object[size()]); } /** * Returns an array containing all of the elements in this deque in * proper sequence (from first to last element); 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. * *

If this deque fits in the specified array with room to spare * (i.e., the array has more elements than this deque), the element in * the array immediately following the end of the deque is set to * {@code null}. * *

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. * *

Suppose {@code x} is a deque known to contain only strings. * The following code can be used to dump the deque into a newly * allocated array of {@code String}: * *

 {@code String[] y = x.toArray(new String[0]);}