State-based testing
State-based testing is a testing style that verifies observable state produced or changed by the code under test.
Unit Testing Principles, Practices, and Patterns
A unit-testing style that verifies the state of the system under test, a collaborator, or an out-of-process dependency after an operation.
- How it is used
- Verifies post-operation state when behavior is not captured solely by a return value.
- How deep it goes
- Defines the style, compares trade-offs, demonstrates assertions, and stops short of requiring total conversion to output-based testing.
- Examples
- Uses an Order collection, Article comments, and a filesystem-backed audit system to demonstrate state assertions and refactoring.
- What you can do after
- Apply it by asserting observable system, collaborator, or dependency state while managing brittleness and maintenance cost.
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The Art of Unit Testing 3e
A state-based test verifies behavior by examining the observable state produced or changed by the code under test.
- How it is used
- Verifies indirect outputs by checking changed state through another call or sibling property rather than the entry point’s return value.
- How deep it goes
- Defines indirect outputs, explains follow-up calls and sibling observables, and contrasts them with internal-call verification.
- Examples
- Uses follow-up calls, sibling functions or properties, and PasswordVerifier4’s protected findFailedRules() example.
- What you can do after
- Apply it by asserting observable state after invoking the entry point, while avoiding internal-call verification when possible.
Read how The Art of Unit Testing 3e explains state-based testing →
Related concepts
- output-based testingcontrasts with · Output-based testing evaluates software by comparing explicitly produced outputs with expected outputs rather than verifying internal interactions.
- testkind of · A test is a procedure or check that evaluates behavior using specified inputs or evidence against expected results or criteria.
- classical schoolcontrasts with · A unit-testing approach, also called the Detroit school or classical style, that isolates tests from one another while allowing the unit under test to use real collaborator objects.
- value objectuses · A value object is an object whose meaning and equality derive from its attribute values rather than an independent identity, typically with immutable state.
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