Automated testing is the practice of using a program to test another program. Much as a compiler is a program that translates a program from a higher-order language into a lower-level form, a test program executes a test plan against the program being tested. And much like you must supply the program to be compiled, for automated testing you must supply the tests that need to be executed. In many ways, the process of writing automated tests is like writing a manual test plan - you are writing instructions of what to try, and what the results should be. The difference is with a manual test plan, you are writing these instructions for a human. With an automated test plan, you are writing them for a program.

Automated tests are typically categorized as unit, integration, and system tests:

• Unit tests focus on a single unit of code, and test it in isolation from other parts of the code. In object-oriented programs where code is grouped into objects, these are the units that are tested. Thus, for each class you would have a corresponding file of unit tests.
• Integration tests focus on the interaction of units working together, and with infrastructure external to the program (i.e. databases, other programs, etc).
• System tests look at the entire program’s behavior.

The complexity of writing tests scales with each of these categories. Emphasis is usually put on writing unit tests, especially as the classes they test are written. By testing these classes early, errors can be located and fixed quickly.

Unit Tests

In this course, we’ll focus on the creation of unit tests to effectively test the software we create. At a minimum, our goal is to write enough tests to achieve a high level of code coverage of our program being tested. Recall that code coverage is a measure of the amount of code in a program that is executed by a set of unit tests.

In theory, a good set of unit tests should, at a minimum, execute every line of code in the program at least once. Of course, that doesn’t nearly guarantee that the unit tests are sufficient to find all bugs, or even a majority of bugs, but it is a great place to start and make sure that the unit tests are properly testing the entirety of the program.

On the next few pages, we’ll discuss how to write unit tests for programs written in both Java and Python. Feel free to only read about the language you are learning, but it might be interesting to see how other languages handle the same idea in different ways.