Slides used on my Online Training Class on Test Doubles

1. Test Doubles
The idea of test double comes from the “stunt
doubles” that take the place of the actors in
action movies.
Source: xUnit Patterns, G. Meszaros http://xunitpatterns.com/Test%20Double.html

2. Types of doubles
These are used in Unit Tests:
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Test Stub
Test Spy
Mock Object
Test Dummy

3. The SUT and the double
SUT: System Under Test
(sometimes also called Subject Under Test)
DOC: Depended-on Component
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We don't tests the doubles, we
test/design the SUT !
Doubles are just the context, the dependencies

4. Stub
“To get good enough control over the indirect
inputs of the SUT, we may have to replace some of
the context with something we can control, a Test
Stub”
- http://xunitpatterns.com/Test%20Stub.html
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Stubs can't remember calls
Just return programmed answers

5. Spy
“To get good enough visibility of the indirect
outputs of the SUT, we may have to replace some
of the context with something we can use to
capture these outputs of the SUT”. http://xunitpatterns.com/Test%20Spy.html
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Spies remember calls, observ silently
Don't have any expectations

6. Mock
“In other cases, we must peer inside the SUT to be
able to determine whether the expected behavior
has occurred”. http://xunitpatterns.com/Mock%20Object.html
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Mocks expect certain calls to happen
They fail if anything else happen
They fail if expectations are not met

7. The problem with word “Mock”
In English, all these doubles are mocks (as in mock
chicken meat, the one made from gluten or soya). This is
why some frameworks use the word “mock”
even when they really use spies. That is the case
of “mockito”.
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Understand the concepts beyond the names
used by frameworks.

8. Isolation frameworks
There are tools that allow us to stub out parts
of the system even when the design doesn't
allow for dependency injection. Let us test things
that are apparently untestable.
Example: Replacing the system funtion that gets
the current Date and Time.