Development III – Unit testing, good practices
Štěpán Kozák


Revision from Last Lesson
•What is the performance testing good for?
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•What is the difference between unit tests and integration tests?
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•Why the hell do we spend second lecture on testing?

Unit vs. Integration Tests
•Unit testing - basic testing of small unit functionality
–NO external dependencies
–NO need for configuration
–NO influence on other tests running in parallel
–NO change in results no matter how many times we run the test
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•Integration testing
–May have external dependencies
–May need some configuration
–May have influence on other tests running in parallel

Why do We Write Automatic tests?
•What are the benefits of having testable code covered with (unit|integration) tests?
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•Did it ever happen to you?
–You build your code on a API created by your colleague, everything was working, you made no
changes to your code and suddenly your code does not work anymore
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•What are the disadvantages of the approach?

So … Why then?
•Anytime somebody changes public interface of a code you rely on, the bug is found immediately •It
forces you to write a „better“ code following well-established patterns •Speeds up the testing
process in case of huge applications (backward compatibility, new functionality) •Speeds up
debugging process (you can identify the source of a bug faster)

What about the Disadvantages?
•Takes some time to write (good) tests
•
•Takes (significant) amount of time learn how to write (good tests)
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•Delusion of having no bugs and completely correct code when all tests pass
•

Good Practices When Writing Tests
•Confidentiality (any time you make a modification to a code, ALL the tests have to pass, no
exceptions)
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•No complex expressions (no if-else statements, no try-catch, …)
–Test cannot contain any bug J
–Test has to be readable and managable by anybody without the deep knowledge of given functionality
•

Some Questions about Automatic Tests
•Who creates the unit tests?
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•In which phase of the development process are the tests created?
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•Can I change the code of a test when I don‘t succeed in having it passed?

Test Driven Development
1.Test creation at the beginning for ALL the functionality intended for production
2.Implementation of given functionality so that all the tests pass
3.Improvements of production code (refactoring)
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•Main rule of TDD:
•Production code cannot be modified until there is a test which proves its incorrectness
•

Code Coverage
•Have it as close to 100% as possible, however … •100% code coverage does not prove 100%
correctness!

How to Fake the Tests …
•Never fake tests! J
•Fake objects you use in the tests to get rid of the external dependencies and configuration
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•The technique is called mocking
–Manual mocks
–Automatic mocks (frameworks, e.g. NSubstitute)

Testable Code
•When is it easy to write (real) unit tests for a class?
•Dependency injection
–„Is a software design pattern that allows a choice of component to be made at run-time rather than
compile time. This can be used, for example, as a simple way to load plugins dynamically or to
choose mock objects in test environments vs. real objects in production environments“