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;
    }