Review

class MutableLong {
    /**
     * Subtracts a value.
     * 
     * @param operand
     *            the value to add
     * @throws NullPointerException
     *             if the object is null
     *
     * @since Commons Lang 2.2
     */
    public void subtract(Number operand) {
        this.value -= operand.longValue();
    }

}