Software Engineering at Google: Lessons Learned from Programming Over Time

by Titus Winters

Test State, Not Interactions Another way that tests commonly depend on implementation details involves not which methods of the system the test calls, but how the results of those calls are verified. In general, there are two ways to verify that a system under test behaves as expected. With state testing, you observe the system itself to see what it looks like after invoking with it. With interaction testing, you instead check that the system took an expected sequence of actions on its collaborators in response to invoking it. Many tests will perform a combination of state and interaction validation. Interaction tests tend to be more brittle than state tests for the same reason that it’s more brittle to test a private method than to test a public method: interaction tests check how a system arrived at its result, whereas usually you should care only what the result is.