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
*/
@SuppressWarnings("unchecked")
public