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Unit Testing

Before a build is delivered, the person or program building the software should execute
unit tests to verify that each unit functions properly. All code is made up of a set of objects,
functions, modules, or other non-trivial units. Each unit is built to perform a certain func-
tion. The purpose of unit testing is to create a set of tests for each unit to verify that it per-
forms its function correctly. Each unit test should be automated: it should perform a test
without any input or intervention, and should result in a pass (meaning that the test gen-
erated the expected results), failure (the results of the test differed from what were
expected), or error (meaning the code reached an error condition before the test could
pass or fail). Many people require that unit tests have no dependencies on external sys-
tems (networks, databases, shared folders, etc.).

Automated unit testing is a stepping stone to test-driven development. Test-driven develop-
ment is a programming technique in which a programmer writes the unit tests before he
writes the unit that they verify. By writing the tests first, the programmer ensures that he
fully understands the requirements. It also guarantees that the tests will be in place, so
that they aren’t left until after all of the other programming activities are completed (and
then possibly dropped, due to schedule pressure).
The main activity in unit testing is creating test cases that verify the software. A test case is a
piece of code that verifies one particular behavior of the software. Each test should be able
to run without any user input; its only output is whether it passed, failed, or halted due to
an error. The test cases for a software project are generally grouped together into suites,
where there may be a number of suites that verify the entire software. It’s often useful to
design the suites so that each one verifies specific units or features; this makes the test
cases easier to maintain.

The most common (and effective) way for programmers to do unit testing is to use a
framework, a piece of software that automatically runs the tests and reports the results. A
framework typically allows a programmer to write a set of test cases for each unit. Most
frameworks provide an automated system for executing a suite of unit tests and reporting
the results. This allows a full battery of unit tests to be executed automatically at any time,
with little or no effort. Unit testing frameworks are available for most modern program-
ming languages.

The framework usually provides some sort of object model, API, or other language inter-
face that provides test cases with functionality for reporting whether the test passed or
failed. Most frameworks allow the programmer to indicate which tests are associated with
various units, and to group the test cases into suites. Table 7-9 shows some of the test
frameworks available for various languages. (This list is by no means exhaustive—there
are many other frameworks available for these and other languages.)

156 CHAPTER SEVEN