Learning Test-Driven Development

Chapter 9. Currencies, Currencies, Everywhere

Small change, small wonders—these are the currency of my endurance and ultimately of my life.

Barbara Kingsolver

Here’s the current state of our evaluate feature vis-à-vis Money entities in a Portfolio:

1. When converting a Money in a currency to the same currency, it returns the amount of the Money. This is correct: the exchange rate for any currency to itself is 1.

2. In all other cases, the amount of the Money is multiplied by a fixed number (1.2). This is correct in a very limited sense: this rate ensures conversions from USD to EUR only. There is no way to modify this exchange rate or specify any other rate.

Our currency conversion code does one thing correctly and another thing almost correctly. It’s time to make it work correctly in both cases. In this chapter, we’ll introduce—at long last—the conversion of money from one currency into another using currency-specific exchange rates.

Making a Hash(map) of Things

What we need is a hashmap that allows us to look up exchange rates given a “from” currency and a “to” currency. The hashmap would be a representation of an exchange rate table we regularly see in banks and currency exchange counters at airports, as shown in Table 9-1.

Table 9-1. Exchange rate table

From	To	Rate
EUR	USD	1.2
USD	EUR	0.82
USD	KRW	1100
KRW	USD	0.00090
EUR	KRW	1344
KRW	EUR	0.00073

To read this table, use this pattern: given an amount in the “from” currency, multiply by the “rate” to get ...