Class ArrayDeque

AndroidAsync/src/com/koushikdutta/async/util/ArrayDeque.java:59–844

Resizable-array implementation of the 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 pro

 * @param <E> the type of elements held in this collection
 */
public class ArrayDeque<E> extends AbstractCollection<E>
                           implements Deque<E>, Cloneable, java.io.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 Object[] 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 = new Object[initialCapacity];