Learning Test-Driven Development

Chapter 10. Error Handling

What error drives our eyes and ears amiss?

William Shakespeare (through the tongue of Antipholus of Syracuse), The Comedy of Errors

Mistakes are a part of life. One of the reasons for adopting test-driven development is to ensure that we can go as fast as we safely can, minimizing bugs in code.

The next item on our feature list is to improve error handling:

5 USD × 2 = 10 USD

10 EUR × 2 = 20 EUR

4002 KRW / 4 = 1000.5 KRW

5 USD + 10 USD = 15 USD

Separate test code from production code

Remove redundant tests

5 USD + 10 EUR = 17 USD

1 USD + 1100 KRW = 2200 KRW#

Determine exchange rate based on the currencies involved (from → to)

Improve error handling when exchange rates are unspecified

Allow exchange rates to be modified

Error Wish List

The way our code currently handles missing exchange rates is buggy. Let’s address this shortcoming. Table 10-1 shows our wish list for handling errors due to missing exchange rates.

Table 10-1. Wish list for handling errors due to missing exchange rates

Item	Description
1	The Evaluate method should signal an explicit error when one or more necessary exchange rates are missing.
2	The error message should be “greedy”—that is, it should indicate all the missing exchange rates that prevent a Portfolio from being evaluated, not just the first missing exchange rate.
3	To prevent the error from being ignored by the caller, no valid Money should be returned when an error happens due to missing ...