14.3. The Third Store

14.3. The Third Store

After Tim and I delivered to Sam the release containing the changes accommodating a second store, he gave me a call. "Well, I've got even better news," he said.

"The second store has been successful, so I'm thinking about expanding into either Canada or Mexico. How much of an effort will be required to make that change?"

"Are you talking Cozumel?" I asked.

"Well, I was thinking about that or Whistler. It'd be tough to have to go on a business trip to check out how the stores were doing," he replied.

"I think these systems will need a developer to install them," I stated.

"Only if it comes out of your pocketbook," he answered .

14.3.1. Currency Flexibility

If Sam picks Whistler (in British Columbia), Tim and I will not have to worry about translating the program into another language. We might even get away without any changes, since Canadians use dollars, albeit different ones from the U.S.

For an initial examination, we looked at all the places where the Dollar class is used. Since we have the separate class (and have not defined it as a double), it is easy to determine which attributes or variables represent dollars. We could add a couple of classes such as CandianDollar and MexicanPeso . However, that does not feel right. This sounds like a place to start adding some flexibility ("More Is Sometimes Less"). As long as we are going to add two variations, we might as well be prepared for more.

Tim and I change the Dollar class into a Money class. That adjustment should provide the limberness to adapt to either of the proposed expansion locations. We start by creating a single class that parallels the Dollar class:

enumeration CurrencyID

        {US_DOLLARS, CANADIAN_DOLLARS, MEXICAN_PESO}

    class Money

        CurrencyID id

        Money(CurrencyID id)

        // Same operations and attributes as for Dollar.

Currencies differ only by data, not by behavior. So creating a single class makes sense (" Don't Overclassify"). The data differences include the symbol used for display, the side (left or right) on which the symbol is placed, and the format of the number itself (decimal point and thousands separator).

Sam's systems operate independently, so each system need only deal with the local currency. The actual currency can be read in from a configuration file, or we might obtain it from the locale. If we have to worry about exchanging currencies, we can create a CurrencyExchange class that transforms a Money amount in one currency to a Money amount in another currency.

Tim and I started by making up identifiers for the individual currencies. Since currencies are universal, we did a quick search on Google? to see if a standard set of identifiers exists. We found the ISO 4217 set of currency identifiers. So the enumeration changes to this:

enumeration CurrencyID {USD, CAD,MXN}

At the same time, we found a Currency class for Java. Since we began the implementation in Java, we will use that class ("Don't Reinvent the Wheel") to designate the currency in the Money class. We also found a Java CurrencyFormat class to which we can delegate the job of properly formatting Money . If we were coding in another language, we might still use those class interfaces and create an implementation for them in that language.

Now all we need to do is to substitute Money for Dollar everywhere we find Dollar in the current system. Then we try the system using tests for Money that parallel the Dollar tests.

If we had started our design by dealing with Money in the beginning, we might have been bogged down in the details of exchange rates and so forth. Since Sam's systems were being used in only one country, we coded for that currency only. Creating a more generalized monetary package would have been premature. Using the experience gained in developing the specific Dollar class helps us to design the Money class.

String ERROR_CD_DISC_NOT_IN_COLLECTION =

            "CDDisc PhysicalID not in collection";.

CommonString error_string = get_string(CD_DISC_NOT_IN_COLLECTION);

enumeration StringID {

        CD_DISC_NOT_IN_COLLECTION,

        CUSTOMER_NOT_IN_COLLECTION};

    CommonString get_error_string (StringID which_string)

        {

        static CommonStrings strings [][NUMBER_LANGUAGES] =

            {"English CD Not..", "French CD Not..", "Spanish CD Not..."},

            {"English Customer Not...", "French Customer Not ..",

                "Spanish Customer Not..."};

        index  = Configuration.get_index_for_current_language( );

        return strings[which_string][index]

        }