Test-Driven Development Overview

Realistic Test-Driven Development:Paying and PreventingTechnical DebtRob MyersAgile Development Practices11 November 200911 November 2009© Rob Myers 20091

11 November 2009© Rob Myers 20092Unit testing is sooooDEPRESSING

11 November 2009© Rob Myers 20093TDD is fun,and provides much more than just unit-tests!

11 November 2009© Rob Myers 20094fromXPtoATDD&BDD

11 November 2009© Rob Myers 20095waste

11 November 2009© Rob Myers 20096#1automationpublic static void Main() {Console.WriteLine( “I sure hope this works...” +myObject.Foo());}

11 November 2009© Rob Myers 20097

11 November 2009© Rob Myers 20098#2Test-FirstWe write a test before writing the code that makes it pass.

Why Wait?11 November 2009© Rob Myers 20099CodeTestMonthsCodeTestWeeksCodeTestDaysCodeTest

What If?11 November 2009© Rob Myers 200910CodeRun TestTestWrite TestRun TestRun TestRun Test

Benefits of Test-FirstJust-In-Time analysis.We know when something is……done.…done correctly.…broken.…and fixed!11 November 2009© Rob Myers 200911

11 November 2009© Rob Myers 200913#3Any behavior-preserving change that improves maintainability.

11 November 2009© Rob Myers 200916#4Mock Objects

Prep road-data for new Mars Colony Road/Canal Service Build ‘Earth’ adapter around map-data service

Refactor all callers to use Earth (all?)

Extract abstract parent ‘Planet’

Test that Mars adapter calls new Mars-map service (mock the service)

Test that Planet factory determines which planet client is on (mock config file)11 November 2009© Rob Myers 200917#5A To-Do List

11 November 2009© Rob Myers 200918Analogies & Metaphors

11 November 2009© Rob Myers 200921“The results of the case studies indicate that the pre-release defect density of the four products decreased between 40% and 90% relative to similar projects that did not use the TDD practice. Subjectively, the teams experienced a 15–35% increase in initial development time after adopting TDD.”http://research.microsoft.com/en-us/projects/esm/nagappan_tdd.pdf, Nagappan et al, © Springer Science + Business Media, LLC 2008

11 November 2009© Rob Myers 200922real value

11 November 2009© Rob Myers 200923architecturalperspective

11 November 2009© Rob Myers 200924roadblocks

11 November 2009© Rob Myers 200925discipline

11 November 2009© Rob Myers 200926stepsWrite oneunit test.Build or add to the object under test until everything compiles.Red: Watch the test fail!Green: Make all the tests pass by changing the object under test.Clean: Refactormercilessly!Repeat.

11 November 2009© Rob Myers 200927 given when then

11 November 2009© Rob Myers 200928TDD Demo

11 November 2009© Rob Myers 200929Basic UML Class DiagramsFoo- intprivateVariable+ intPublicMethod()BarBaz+ void AbstractMethod( object parameter)- void PrivateMethod()

11 November 2009© Rob Myers 200930Global Currency Money-Market Account

11 November 2009© Rob Myers 200931Global ATMAccess

StoriesAs Rob the US account holder, I want to be able to withdraw USD from a US ATM, but select which currency holdings to withdraw from.As Rob, traveling in Europe, I want to be able to withdraw EUR from either my EUR holdings or my USD holdings. The default should be the most beneficial to me at the time.Shortly after the end of each month, I want to receive a report of my holdings and balance. The total balance should appear in the currency of my account address (USD).11 November 2009© Rob Myers 200932

Primary Objects11 November 2009© Rob Myers 200933CurrencyAccountHolding

More Detail11 November 2009© Rob Myers 200934CurrencyHoldingvaluecurrencycurrencyConverterCurrencyConverterconvert value to another currency

Start SimpleTo DoWhen currencies are the same.

When value is zero.11 November 2009© Rob Myers 200935

Specification, and Interface11 November 2009© Rob Myers 200936[TestMethod]public void GivesSameValueWhenSameCurrencyRequested() {CurrencyConverter converter = new CurrencyConverter(); string sameCurrency = "USD"; double sameValue = 100.0000; double converted = converter.convert(sameValue, sameCurrency, sameCurrency);Assert.AreEqual(sameValue, converted, 0.00004);}Will not compile. 

“Thank You, But…”11 November 2009© Rob Myers 200937using System;class CurrencyConverter { public double convert( double value, string from, string to) { throw new NotImplementedException(); }}

“…I Want to See it Fail Successfully”11 November 2009© Rob Myers 200938class CurrencyConverter { public double convert( double value, string from, string to) {return 0.0; }}

Just Enough to Pass11 November 2009© Rob Myers 200939public double convert( double value, string from, string to) { return 100.0;}

a. Refactor Away the Duplication11 November 2009© Rob Myers 200940[TestMethod]public void GivesSameValueWhenSameCurrencyRequested() {CurrencyConverter converter = new CurrencyConverter(); string sameCurrency = "USD"; double sameValue = 100.0000; double converted = converter.convert(sameValue, sameCurrency, sameCurrency);Assert.AreEqual(sameValue, converted, 0.00004);}public double convert( double value, string from, string to) { return 100.0;}

b. Triangulate11 November 2009© Rob Myers 200941

11 November 2009© Rob Myers 200942We don’t add any behavior without a failing test…Rodin’s The Thinker, photo by CJ on Wikipedia

Triangulating Test11 November 2009© Rob Myers 200943[TestMethod]public void GivesZeroWhenValueIsZero() {CurrencyConverter converter = new CurrencyConverter(); double zero = 0.0000; double converted = converter.convert( zero, "USD", "EUR");Assert.AreEqual(zero, converted, 0.00004);}

Still “Cheating”?11 November 2009© Rob Myers 200944class CurrencyConverter { public double convert( double value, string from, string to) { return value; }}

Maintain the Tests11 November 2009© Rob Myers 200945[TestMethod]public void GivesZeroWhenValueIsZero() {CurrencyConverter converter = new CurrencyConverter(); // ...}[TestMethod]public void GivesSameValueWhenSameCurrencyRequested() {CurrencyConverter converter = new CurrencyConverter(); // ...}

TestInitialize Runs Before Each Test11 November 2009© Rob Myers 200946private CurrencyConverter converter;[TestInitialize]public void InitConverter() { converter = new CurrencyConverter();}[TestMethod]public void GivesZeroWhenValueIsZero() { // ...}[TestMethod]public void GivesSameValueWhenSameCurrencyRequested() { // ...}

What is the Expected Answer?11 November 2009© Rob Myers 200947[TestMethod]public void ConvertsDollarsToEuros() { double converted = converter.convert( 100.0000, "USD", "EUR");Assert.AreEqual(???, converted, 0.00004);}

Let’s “Guess”…11 November 2009© Rob Myers 200948[TestMethod]public void ConvertsDollarsToEuros() { double converted = converter.convert( 100.0000, "USD", "EUR");Assert.AreEqual(50.0000, converted, 0.00004);}

…Get it Working…11 November 2009© Rob Myers 200949public double convert( double value, string from, string to) { if (to.Equals(from)) return value; return value * 0.5000;}

…Refactor…11 November 2009© Rob Myers 200950public double convert( double value, string from, string to) { if (to.Equals(from)) return value; return value * ConversionRate(from, to);}private double ConversionRate(string from, string to) { return 0.5000;}

…And Make a Note of ItTo DoWhen currencies are the same.

Managing the Transaction11 November 2009© Rob Myers 200954AccountCurrencyConverterHoldingHoldingHoldingConversionRatesHolding

Another Object EmergesTo DoWhen currencies are the same.

Fix hard-coded ConversionRate().

New Test Class11 November 2009© Rob Myers 200956using Microsoft.VisualStudio.TestTools.UnitTesting;[TestClass]public class ConversionRatesTests { [TestMethod] public void StoresAndRetrievesRates() {ConversionRates rates = new ConversionRates(); string from = "USD"; string to = "EUR"; double rate = 0.7424;rates.PutRate(from, to, rate);Assert.AreEqual(rate, rates.GetRate(from, to)); }}

Fail11 November 2009© Rob Myers 200957public class ConversionRates { public void PutRate(string from, string to, double rate) { } public double GetRate(string from, string to) { return 0.0; }}

No Need to be Coy11 November 2009© Rob Myers 200958public class ConversionRates { private Dictionary<string, double> rates = new Dictionary<string, double>(); public void PutRate(string from, string to, double rate) { rates[from + to] = rate; } public double GetRate(string from, string to) { return rates[from + to]; }}

Isolate Behavior11 November 2009© Rob Myers 200959public class ConversionRates { private Dictionary<string, double> rates = new Dictionary<string, double>(); public void PutRate(string from, string to, double rate) { rates[Key(from, to)] = rate; } public double GetRate(string from, string to) { return rates[Key(from, to)]; } private string Key(string from, string to) { return from + to; }}

Next?To DoWhen currencies are the same.

Test-Driven Development Overview

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