java.util.ArrayDeque Maven / Gradle / Ivy
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* published by the Free Software Foundation. Oracle designates this
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*
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*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Josh Bloch of Google Inc. and released to the public domain,
* as explained at http://creativecommons.org/publicdomain/zero/1.0/.
*/
package java.util;
import java.io.*;
/**
* 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 a stack, and faster than {@link LinkedList}
* when used as a queue.
*
* 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 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.
*/
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;
}
/**
* 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 = (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 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 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 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();
E result = 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;
}
}
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);
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;
}
}
/**
* Returns true if this deque contains the specified element.
* More formally, returns true if and only if this deque contains
* at least one element e such that o.equals(e).
*
* @param o object to be checked for containment in this deque
* @return 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;
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.
* 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).
*
* 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 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 clone() {
try {
ArrayDeque result = (ArrayDeque) super.clone();
result.elements = Arrays.copyOf(elements, elements.length);
return result;
} catch (CloneNotSupportedException e) {
throw new AssertionError();
}
}
/**
* Appease the serialization gods.
*/
private static final long serialVersionUID = 2340985798034038923L;
}