@Override
    public int hashCode() {
        if (!this.isPresent) {
            return -1;
        }
        return Objects.hashCode(this.value);
    }
Returns the hash code value of the present value, if any, or -1 if
no value is present.

@return hash code value of the present value or 0 if no value is present
@see Objects#hashCode(Object)
helper/src/main/java/me/lucko/helper/utils/NullableOptional.java