test-driven development

Matteo Vaccarim.vaccari@sourcesense.com

http://matteo.vaccari.name/blog

BIt Systems, 4 June 2008

Test-Driven Development

Three kinds of test

• Unit test

• Integration test

• Customer test

Unit tests• Focus on a class, method or function

• Run entirely in memory

• Extremely fast!

Unit tests run fast.A test is not a unit test if:

1. It talks to a database

2. It communicates across the network

3. It touches the file system

4. You have to do things to your environment to run it (eg, change config files)

Tests that do this are integration tests

Michael Feathers

Integration tests

• Unit tests prove our logic is correct

• Integration test prove our program communicates correctly with the outside world

System under test

DB

Billing

GUI

End-to-end tests

System under test

DB

Billing

GUI

End-to-end tests

• ...are the least convenient kind of integration tests

System under test

DB

Billing

GUI

Focused integration tests

• Each test deals with one external interaction

Customer tests• Also known as “Acceptance tests”

• Motto: We implement tricky domain concepts correctly

Example from Mugridge & Cunningham Fit For Developing Software

Customer tests• Customer test are

customer-provided examples

• Often captured in table form

• Expressed in the language of the business

• Automated

Example from Mugridge & Cunningham Fit For Developing Software

Test-Driven Development

Clean code that works.

Clean code that works

• is out of reach of even the best programmers, some of the time,

• and out of reach of most programmers (like me) most of the time

-- Kent Beck

Simple design

The code is simple enough when it:0. Runs all the tests1. Expresses every idea that we need to express2. Contains no duplication3. Has the minimum number of classes and functions

(In this order)

Adapted from Extreme Programming Installed by Ron Jeffries et al.

Clean code that works

• First we'll solve the “that works” part

• Then we'll solve the “clean code” part

m.vaccari@sourcesense.com

Write a test

public class AdderTest { @Test

public void testTwoPlusThree() {Adder a = new Adder();assertEquals(5, a.add(2, 3));

}}

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Now it compilespublic class AdderTest { @Test

public void testTwoPlusThree() {Adder a = new Adder();assertEquals(5, a.add(2, 3));

}}

public class Adder { public int add(int a, int b) { return 0; }}

m.vaccari@sourcesense.com

Red bar!public class AdderTest { @Test

public void testTwoPlusThree() {Adder a = new Adder();assertEquals(5, a.add(2, 3));

}}

public class Adder { public int add(int a, int b) { return 0; }}

Expected 5, was 0

m.vaccari@sourcesense.com

Just pretendpublic class AdderTest { @Test

public void testTwoPlusThree() {Adder a = new Adder();assertEquals(5, a.add(2, 3));

}}

public class Adder { public int add(int a, int b) { return 5; }}

m.vaccari@sourcesense.com

Remove the duplicated “5”public class AdderTest {

@Testpublic void testTwoPlusThree() {

Adder a = new Adder();assertEquals(5, a.add(2, 3));

}}

public class Adder { public int add(int a, int b) { return a+b; }}

m.vaccari@sourcesense.com

The procedure

1. Write a test

2. Make it compile

3. Make it pass

4. Remove duplication

Expected 5, was 0

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Red

GreenRefactor

Repeat every 2-10 min.

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Clean code, why?

• Design is the great accelerator:

• If you drop quality for speed, you will get neither

• If you aim for quality...

• ... and you know how to get it...

• ... you will also be fast!

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Test first, why?

• You think code from the point of view of the caller

• This perspective makes for better design

• Test coverage is a useful byproduct

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Refactor, why?

• Because I can: the tests support refactoring

• Refactoring is when I do design

• I don’t claim I can guess the right design at first

• Design emerges, with thought, care and small steps

m.vaccari@sourcesense.com

The Bowling ScoreBy Robert Martin “Uncle Bob”

http://butunclebob.com/ArticleS.UncleBob.TheBowlingGameKata

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Il punteggio del bowling

Ci sono 10 frame. In ogni frame il giocatore ha due possibilità di abbattere 10 birilli (pins). Il punteggio per il frame è il numero di birilli abbattuti, più i bonus per stike o spare.

Uno spare è quando il giocatore abbatte 10 birilli in due tiri. Il bonus per quel frame è il numero di birilli abbattuti al tiro successivo. Nel frame 3 dell'esempio, il punteggio è 10 (i birilli abbattuti) più il bonus di 5 (abbattuti nel tiro successivo.)

Uno strike è quando il giocatore abbatte tutti i birilli in un solo tiro. Il bonus per quel frame è il numero di birilli abbattuti nei due tiri successivi

Nel decimo frame, se il giocatore fa uno strike o spare può fare i tiri necessari per completare il frame. In ogni caso, al decimo frame non vengono fatti più di tre tiri.

m.vaccari@sourcesense.com

The requirements

• Write class “Game” with two methods:

• void roll(int pins); call when the player rolls a ball. The argument is the number of pins knocked down.

• int score(); called when the game is ended. Returns the final score.

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Let’s think about design?

• A quick object-oriented analysis leads us to think we need

• class Game

• class Frame

• class Roll

• class TenthFrame extending Frame

• ...

m.vaccari@sourcesense.com

Let’s think about design?

• A quick object-oriented analysis leads us to think we need

• class Game

• class Frame

• class Roll

• class TenthFrame extending Frame

• ... forget about all that!

m.vaccari@sourcesense.com

Demo time

• ...eclipse!

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Ancora, perché test first?• per concentrarsi su quello che serve veramente (no gold

plating)

• good enough! quando il test passa so che posso fermarmi

• perché penso al codice come un cliente di questo codice

• perché ottengo codice testabile, e il codice testabile

• ha uno scopo preciso

• è disaccoppiato dal resto del sistema

• è più generale

• il design emerge mano a mano che capisco meglio il problema

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Ancora, perché refactoring?

• simplicity is key

• il design nel tempo si imbastardisce

• fare il design prima significa farlo nel momento peggiore: quando ne so di meno

• molto meglio fare design mentre sviluppo

m.vaccari@sourcesense.com

Do the simplest thing that can possibly work

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Do the simplest thing

1. Build the quickest code that will pass the tests

2. Refactor the code to have the simplest design possible

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What is simple design?

• The code passes all tests

• There is no duplication

• The code expresses the programmer’s intention

• Using the smallest number of classes and methods

In this order

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TDD is a key practice

• Defects kill predictability

no predictability, no planning!

• Test-driven is predictable

• Hardly ever use the debugger

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No silver bullet

• Needs lots of practice

• Requires discipline

• Must think and be alert at all times!

Debugging Sucks

Testing Rocks

Supermarket checkout

• Compute the total price

• Scan items one at a time

• In any order

Source: Dave Thomas, http://codekata.pragprog.com/2007/01/kata_nine_back_.html

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