Most 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 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 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 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 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 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 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;
- 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;
- }
-
- 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;
- }
-
- /**
- * @throws NoSuchElementException {@inheritDoc}
- */
- public E getFirst() {
- E x = elements[head];
- if (x == null)
- throw new NoSuchElementException();
- return x;
- }
-
- /**
- * @throws NoSuchElementException {@inheritDoc}
- */
- public E getLast() {
- E x = elements[(tail - 1) & (elements.length - 1)];
- if (x == null)
- throw new NoSuchElementException();
- return x;
- }
-
- public E peekFirst() {
- return elements[head]; // elements[head] is null if deque empty
- }
-
- public E peekLast() {
- return 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 e such that
- * o.equals(e) (if such an element exists).
- * Returns 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 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;
- E 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 e such that
- * o.equals(e) (if such an element exists).
- * Returns 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 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;
- E 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 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 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
- * null if this deque is empty.
- *
- * This method is equivalent to {@link #pollFirst}.
- *
- * @return the head of the queue represented by this deque, or
- * 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 null if this deque is empty.
- *
- * This method is equivalent to {@link #peekFirst}.
- *
- * @return the head of the queue represented by this deque, or
- * 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 E[] 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 true if this deque contains no elements.
- *
- * @return 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 This method is equivalent to {@link #removeFirstOccurrence}.
- *
- * @param o element to be removed from this deque, if present
- * @return 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
- * 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 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 String:
- *
- *
- * String[] y = x.toArray(new String[0]);
- *
- * Note that toArray(new Object[0]) is identical in function to
- * toArray().
- *
- * @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 all of the elements in this deque
- * @throws ArrayStoreException if the runtime type of the specified array
- * is not a supertype of the runtime type of every element in
- * this deque
- * @throws NullPointerException if the specified array is null
- */
- public