test-driven development
TRANSCRIPT
Matteo [email protected]
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
Write a test
public class AdderTest { @Test
public void testTwoPlusThree() {Adder a = new Adder();assertEquals(5, a.add(2, 3));
}}
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; }}
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
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; }}
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; }}
The procedure
1. Write a test
2. Make it compile
3. Make it pass
4. Remove duplication
Expected 5, was 0
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!
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
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
The Bowling ScoreBy Robert Martin “Uncle Bob”
http://butunclebob.com/ArticleS.UncleBob.TheBowlingGameKata
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.
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.
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
• ...
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!
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
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
Do the simplest thing
1. Build the quickest code that will pass the tests
2. Refactor the code to have the simplest design possible
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
TDD is a key practice
• Defects kill predictability
no predictability, no planning!
• Test-driven is predictable
• Hardly ever use the debugger
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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