Tests

Why Testing is Important

Damien Cassou, Stéphane Ducasse and Luc Fabresse http://stephane.ducasse.free.fr

Goal

why tests are important?
what are their advantages?
what are the techniques to write good tests?

Pros

Finding problems
Understanding code
Increase trust

Collateral pros

Finding Problems: Pros

find bugs when they appear
improve customer trust
reproduce complex scenari
check contracts of super types
guarantee old bugs won't come back
isolate the problem

Finding Problems: Characteristics of a Good Test Suite

check extreme cases (e.g., null, 0 and empty)
check complex cases (e.g., exceptions, network pbs)
1 test for each bug (at least)
good coverage
check abstractions
check units independently

Understanding Code

convert
   | s |
   s := '#000000' copy.
   s at: 2 put: (Character digitValue: ((rgb bitShift: -6 - RedShift) bitAnd: 15)).
   s at: 3 put: (Character digitValue: ((rgb bitShift: -2 - RedShift) bitAnd: 15)).
   s at: 4 put: (Character digitValue: ((rgb bitShift: -6 - GreenShift) bitAnd: 15)).
   s at: 5 put: (Character digitValue: ((rgb bitShift: -2 - GreenShift) bitAnd: 15)).
   s at: 6 put: (Character digitValue: ((rgb bitShift: -6 - BlueShift) bitAnd: 15)).
   s at: 7 put: (Character digitValue: ((rgb bitShift: -2 - BlueShift) bitAnd: 15)).
   ^ s

Understanding Code

testConvert
   self assert: Color white convert = '#FFFFFF'.
   self assert: Color red convert = '#FF0000'.
   self assert: Color black convert = '#000000'

Understanding Code

testConvert2
   | table aColorString |
   table := #('0' '1' '2' '3' '4' '5' '6' '7' '8' '9' 'A' 'B' 'C' 'D' 'E' 'F').

   table do: [ :each |
      aColorString := '#', each, each, '0000'.
      self assert: ((Color fromString: aColorString) convert sameAs: aColorString)].

   table do: [ :each |
      aColorString := '#', '00', each, each, '00'.
      self assert: ((Color fromString: aColorString) convert sameAs: aColorString)].

   table do: [ :each |
      aColorString := '#', '0000', each, each.
      self assert: ((Color fromString: aColorString) convert sameAs: aColorString)].

Understanding Code

testBitShift
   self assert: (2r11 bitShift: 2) equals: 2r1100.
   self assert: (2r1011 bitShift: -2) equals: 2r10.

testShiftOneLeftThenRight
   "Shift 1 bit left then right and test for 1"
   1 to: 100 do: [:i | self assert: ((1 bitShift: i) bitShift: i negated) = 1].

Understanding Code: Pros

give simple and reproducible examples
explain an API
give up-to-date documentation
check conformity of new code
offer a first client to new code
force a modular design

Understanding Code: Characteristics of a Good Test Suite

deterministic
automatic
self-explained
simple
unit

Increasing Trust: Pros

accelerate bug detection
accelerate new code checking
ease refactorings
prevent regressions

Increasing Trust: Characteristics of a Good Test Suite

change less frequently than the rest
good code coverage
deterministic

Collateral Pros

improve feeling of customers who care
allow for automatic bug fixing
improve type inference
provide examples to variable values

Testing Abstractions

How do you test contracts of abstract types?

Testing Abstractions

How do you test that one and only one state is active at any time?

Testing Abstractions

testOnlyOneValidStateAtEachMoment
   | action |
   action := self createAction.
   self assert: action isReady.
   self deny: action isInProgress. self deny: action isFinished.

   [ action isFinished ] whileFalse: [
      action doStep.
      self deny: action isReady.
      self assert: action isFinished = action isInProgress not ].

   self deny: action isReady. self deny: action isInProgress.
   self assert: action isFinished

Testing Abstractions

parallel hierarchies
test must be in the highest abstraction
factory method

Mocking

How do you test that a questionnaire only accepts compatible answers from the user?

Mocking

How do you test that a questionnaire only accepts compatible answers from the user?

Mocking

readAnswerAsLongAsItIsNotCompatible
   | nbRejectsBeforeAccept question ui |
   nbRejectsBeforeAccept := 3.
   question := MockQuestion new nbRejects: nbRejectsBeforeAccept.
   ui := MockQuestionnaireUI new.
   self assert: ui nbReadAnswers equals: 0.
   self assert: question nbAcceptAnswerCalls equals: 0.
   questionnaire runQuestion: question on: ui.
   self assert: ui nbReadAnswers equals: nbRejectsBeforeAccept + 1.
   self assert: question nbAcceptAnswerCalls equals: nbRejectsBeforeAccept + 1.

Mocking

mocks are reusable across tests
mocks can be generated with mocking frameworks

Different Testing Levels

http://martinfowler.com/bliki/TestPyramid.html

← →