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Test Driven Development (TDD). Presented by Victor Goldberg, Ph.D. The Nature of our Problem. Then a miracle occurs. Good work, but I think we need more detail right here. What is Test Driven Development?. It’s a practice that adds reliability to the development process.

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test driven development tdd

Test Driven Development (TDD)

Presented by

Victor Goldberg, Ph.D.

Smalltalk Connections vmgoldberg@earthlink.net

the nature of our problem
The Nature of our Problem

Then a miracle occurs

Good work, but I think we need more detail right here.

Smalltalk Connections vmgoldberg@earthlink.net

what is test driven development
What is Test Driven Development?
  • It’s a practice that adds reliability to the development process.

Smalltalk Connections vmgoldberg@earthlink.net

why is tdd important
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.

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how does it work
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.

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there are different kinds of tests
There are different kinds of tests
  • Experiments (a.k.a. spikes),
  • Acceptance tests,
  • Code Behavior tests, and
  • Development code TDD’s main functional tests realm.

  We will also touch on experiments.

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a possible initial tdd test in smalltalk st
A Possible Initial TDD Test(in Smalltalk [ST])


| number |

number := card draw numericalValue .

self assert: ( number > 0 ) & ( number < 14 ) .

Method Name

Local variable

Getter – retrieves a result of the action



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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.

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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.

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(The Card class)

Fig 3. The Card class and card instance are created.

The new instance of Card is assigned to the variable card.

(The card instance)

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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.

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experiment example in the smalltalk workspace
Experiment Example in the Smalltalk Workspace

r := Random new . frequency := Bag new. "Variables typing"

1 to: 1000000

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.

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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.

Fig 7. After the experiment we feel comfortable writing #draw.

In #draw we assign the result of the calculation to the instance variable numericalValue

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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.

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tdd is fun
TDD is fun!!!

Passing the test

(the green bar)

is the feedback

that makes it fun.

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summary 1
Summary (1)


  • 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.

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summary 2
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.

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summary 3
Summary (3)

In addition to all its technical contributions to a project,

Test Driven Development succeeds because …

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