Software Engineering at Google

Chapter 12. Unit Testing

Written by Erik Kuefler

Edited by Tom Manshreck

The previous chapter introduced two of the main axes along which Google classifies tests: size and scope. To recap, size refers to the resources consumed by a test and what it is allowed to do, and scope refers to how much code a test is intended to validate. Though Google has clear definitions for test size, scope tends to be a little fuzzier. We use the term unit test to refer to tests of relatively narrow scope, such as of a single class or method. Unit tests are usually small in size, but this isn’t always the case.

After preventing bugs, the most important purpose of a test is to improve engineers’ productivity. Compared to broader-scoped tests, unit tests have many properties that make them an excellent way to optimize productivity:

• They tend to be small according to Google’s definitions of test size. Small tests are fast and deterministic, allowing developers to run them frequently as part of their workflow and get immediate feedback.

• They tend to be easy to write at the same time as the code they’re testing, allowing engineers to focus their tests on the code they’re working on without having to set up and understand a larger system.

• They promote high levels of test coverage because they are quick and easy to write. High test coverage allows engineers to make changes with confidence that they aren’t breaking anything.

• They tend to make it easy to understand what’s wrong when they fail because ...