from ArrayDeque.java file, find 2^n value. //寻找最临近的2的指数次数值,结果－1可以作为mask使用

ArrayDeque

    private static int getCapacity(int numElements) {
        int 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
        return initialCapacity;
    }