Smalltalk Connections 1 Test Driven Development (TDD) Presented by Victor Goldberg, Ph.D.

Smalltalk Connections 2 The Nature of our Problem Then a miracle occurs Good work, but I think we need more detail right here.

Smalltalk Connections 3 What is Test Driven Development? It’s a practice that adds reliability to the development process.

Smalltalk Connections 4 Why is TDD important? Many projects fail because they lack a good testing methodology. It’s common sense, but it isn’t common practice. The sense of continuous reliability and success gives you a feeling of confidence in your code, which makes programming more fun.

Smalltalk Connections 5 How does it work?  Have a requirement. Let’s say “create a new random card, as for a card game”.  Think about what could be a reasonable test to verify compliance with the requirement.  Write the test before writing the code. Of course, the test will fail, and that’s ok.  Keep things simple.  Then, write only the minimally necessary code to make the test pass.  This is a process of discovery; as you identify possible improvements, refactor your code relentlessly to implement them.  Build, keep, and frequently run your cumulative collection of tests.

Smalltalk Connections 6 There are different kinds of tests Experiments (a.k.a. spikes), Acceptance tests, Code Behavior tests, and Development code TDD’s main functional testsrealm. We will also touch on experiments.

Smalltalk Connections 7 A Possible Initial TDD Test (in Smalltalk [ST]) testCardDrawnIsWithinRange | number | number := card draw numericalValue. self assert: ( number > 0 ) & ( number < 14 ). Local variable Object Action Getter – retrieves a result of the action Method Name

Smalltalk Connections 8 Fig 1. The same code in the ST browser. card is in red because the system doesn’t recognize such an object. self is an instance of the class TestCards. assert: is a message sent to self; its argument is a Boolean. TestCards is a child of TestCase, a class in the SUnit framework.

Smalltalk Connections 9 Fig 2. When run, the test identifies an error. The assertion didn’t have the opportunity to fail, because an error was identified before the assertion was applied.

Smalltalk Connections 10 Fig 3. The Card class and card instance are created. The new instance of Card is assigned to the variable card. (The Card class) (The card instance)

Smalltalk Connections 11 Fig 4. The test still identifies an error. The card instance doesn’t recognize the method #draw. Designing #draw requires some skill. We will develop the skill through an experiment.

Smalltalk Connections 12 Experiment Example in the Smalltalk Workspace r := Random new.frequency := Bag new."Variables typing" 1 to: do: [ :index | |result | result := (r next * 13) floor + 1. result >0 & result < 14 ifTrue: [ frequency add: result] ifFalse: [ ^ Dialog warn: 'Result out of range' ]. ]. ^ frequency contents. Fig. 5 The ST Workspace is like scratch paper, a place to experiment. The experiment here is to find whether our formula for random numbers is acceptable.

Fig 6. The results of running the code twice. The distribution of values is in the same range for each outcome, but different for each run. All are within [1, 13]. So, we are confident that our coding of the math is correct.

Smalltalk Connections 14 Fig 7. After the experiment we feel comfortable writing #draw. In #draw we assign the result of the calculation to the instance variable numericalValue

Smalltalk Connections 15 Fig 8. Now the test passes. In this case numericalValue is a getter, a message sent to the card object to retrieve the contents of the instance variable numericalValue.

Smalltalk Connections 16 TDD is fun!!! Passing the test (the green bar) is the feedback that makes it fun.

Smalltalk Connections 17 Summary

Smalltalk Connections 18 Summary (1) In TDD: Requirements drive the tests. Tests drive the development of the application code. No application code is written without writing a failing test first. Tests are collected in a suite and the suite is run frequently, like every time after code is written. Test and code are written in elementary increments. Refactoring is a continuous operation, and is supported by a passing battery of tests.

Smalltalk Connections 19 Summary (2) TDD is good because it: Reduces the number of bugs by orders of magnitude, Increases development speed, because less time is spent chasing bugs. Improves code quality because of the increased modularity, and continuous and relentless refactoring. Decreases maintenance costs because the code is easier to follow.

Smalltalk Connections 20 Summary (3) In addition to all its technical contributions to a project, Test Driven Development succeeds because …