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