Essential Test-Driven Development

9/9/13

As you enter the room…
1.  Take a 3x5 card or piece of paper, create two columns.
2.  Write these skills in the first column: TDD, Refactoring,
OO, Java (or C#), Eclipse (or IDEA or VisualStudio)
3.  For each skill, rate yourself from 0 (never heard of it) to
10 (invented it), and record in the second column.
4.  Find someone (a) whom you don’t usually work with; and
(b) who has somewhat complementary skill levels (with
them, your average is 4 to 6). Write down this person’s
name, but only if both a and b are true.
5.  Repeat step 4 until you have two names.
9 September 2013

© Agile Institute 2008-2013

1

…get comfortable.
1.  Choose someone from
your list to work with.
2.  Gather your belongings
and select a workstation
for you and your new
lab partner.
3.  Settle in at that
workstation.

9 September 2013


2

1

9/9/13

Café
Essential
Test-Driven
Development
Rob Myers

for
Agile Development Practices
East
12 November 2013

9 September 2013


3


4

Unit testing
is soooo
DEPRESSING

9 September 2013

2

9/9/13

TDD is fun,
and provides
much more than
just unit-tests!

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5

“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
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3

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TDD Demo

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7

No Magic
import org.junit.*;

public class FooTests {
@Test
public void fooBarIsBaz() {
Foo foo = new Foo();
Assert.assertEquals("Baz", foo.bar());
}
}

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4

9/9/13

Requirements & Conventions
import org.junit.*;

public class FooTests {
@Test
public void fooBarIsBaz() {
Foo foo = new Foo();
Assert.assertEquals("Baz", foo.bar());
}
}
Expected

9 September 2013

Actual


9

Basic UML Class Diagrams
Bar

Foo
- int privateVariable

+ void AbstractMethod(
object parameter)

+ int PublicMethod()

Baz
- void PrivateMethod()

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5

9/9/13

Global Currency Money-Market
Account

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Global
ATM
Access
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6

9/9/13

Stories
•  As 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).
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13

Primary Objects

Currency
Account

Holding

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7

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More Detail

Currency

Holding
value
currency
currencyConverter
convert value to
another currency

9 September 2013

CurrencyConverter


15

Start Simple
To Do
q When currencies are the same.
q When value is zero.

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8

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Specification, and Interface
@Test
public void givesSameValueWhenSameCurrencyRequested() {
CurrencyConverter converter = new CurrencyConverter();
String sameCurrency = "USD";
double sameValue = 100.0000;
double converted = converter.convert(
sameValue, sameCurrency, sameCurrency);
Assert.assertEquals(sameValue, converted, 0.00004);
}

Will not compile. L
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17

“Thank You, But…”
public class CurrencyConverter {
public double convert(
double value, String from, String to) {
throw new RuntimeException(
"D'oh! convert() not YET implemented!");
}
}

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9

9/9/13

“…I Want to See it Fail Successfully”
return 0.0;
}
}

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Technique: “Fake It”
return 100.0;
}
}

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20

10

9/9/13

a. Refactor Away the Duplication
@Test
String sameCurrency = "USD";
double sameValue = 100.0000;
sameValue, sameCurrency, sameCurrency);
Assert.assertEquals(sameValue, converted, 0.00004);
}

return 100.0;
}
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b. Triangulate

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9/9/13

We don’t add
any behavior
without a
failing test…

Rodin’s The Thinker, photo by CJ on Wikipedia

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23

Technique: “Triangulation”
@Test
public void givesZeroWhenValueIsZero() {
double zero = 0.0000;
zero, "USD", "EUR");
Assert.assertEquals(zero, converted, 0.00004);
}

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Still “Fake”?
return value;
}
}

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Maintain the Tests
@Test
public void givesZeroWhenValueIsZero() {
// ...
}
@Test
// ...
}

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9/9/13

@Before Runs Before Each Test
private CurrencyConverter converter;
@Before
public void initialize() {
converter = new CurrencyConverter();
}
@Test
public
//
}
@Test
public
//
}

void givesZeroWhenValueIsZero() {
...

void givesSameValueWhenSameCurrencyRequested() {
...

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What is the Expected Answer?
@Test
public void convertsDollarsToEuros() {
100.0000, "USD", "EUR");
Assert.assertEquals(???, converted, 0.00004);
}

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28

14

9/9/13

Let’s Invent a Scenario…
@Test
public void convertsDollarsToEuros() {
100.0000, "USD", "EUR");
Assert.assertEquals(50.0000, converted, 0.00004);
}

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29

…“Fake It”…
if (to.equals(from))
return value;
return value * 0.5000;
}

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15

9/9/13

…Refactor…
if (to.Equals(from))
return value;
return value * conversionRate(from, to);
}
private double conversionRate(String from, String to) {
return 0.5000;
}

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…And Make a Note of It
To Do
ü When currencies are the same.
ü When value is zero.
q Fix the hard-coded conversionRate

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16

9/9/13

Well, What is It???

Where does it come from?

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Circumstantial Coupling?

CurrencyConverter

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17

9/9/13

Managing the Transaction
Account

CurrencyConverter

Holding
Holding
Holding
Holding

9 September 2013

ConversionRates


35

Another Object Emerges
To Do
q Fix hard-coded conversionRate().
q Write ConversionRates.
q Make a factory for ConversionRates
that uses the FOREX.com Web Service.
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18

9/9/13

New Test Class
import org.junit.Assert;
import org.junit.Test;
public class ConversionRatesTest {
@Test
public void storesAndRetrievesRates() {
ConversionRates rates = new ConversionRates();
String from = "USD";
String to = "EUR";
double rate = 0.7424;
rates.putRate(from, to, rate);
Assert.assertEquals(rate, rates.getRate(from, to),
0.0004);
}
}
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Fail
public class ConversionRates {
public void putRate(String from, String to, double rate) {
}
public double getRate(String from, String to) {
return 0;
}
}

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19

9/9/13

Technique: “Obvious Implementation”
import java.util.HashMap;
import java.util.Map;
private Map<String, Double> rates
= new HashMap<String, Double>();
rates.put(from + to, rate);
}
return rates.get(from + to);
}
}
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Isolate Behavior
private Map<String, Double> rates
= new HashMap<String, Double>();
rates.put(key(from, to), rate);
}
return rates.get(key(from, to));
}
private String key(String from, String to) {
return from + to;
}
}
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20

9/9/13

Next?
To Do
q Fix hard-coded conversionRate().
ü Write ConversionRates.
that uses the FOREX.com Web Service.
9 September 2013


41

Fix @Before Method
public class CurrencyConverterTests {
private CurrencyConverter converter;
@Before
public void initialize() {
rates.putRate("USD", "EUR", 0.5000);
converter = new CurrencyConverter(rates);
}
// ...

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9/9/13

Partially Refactored…
private ConversionRates rates;
public CurrencyConverter(ConversionRates rates) {
this.rates = rates;
}
return 0.5000;
}
// ...

9 September 2013


43

Replace the Hard-Coded Value
private ConversionRates rates;
public CurrencyConverter(ConversionRates rates) {
this.rates = rates;
}
return rates.getRate(from, to);
}
// ...

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22

9/9/13

What Remains?
To Do
ü Fix hard-coded conversionRate().
ü Write ConversionRates.
that uses the FOREX.com Web
Service.
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45

What is the Missing Piece?

ConversionRates rates =
ConversionRates.byAccountAnd???(
account,

9 September 2013


???);

46

23

9/9/13

Ask What If…?
To Do
q Make a byAccountAndDate() factory
for ConversionRates that uses the
FOREX.com Web Service.
q ConversionRates returns 1/rate if
inverse rate found.
q ConversionRates throws when rate
not found.
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47

Testing Exceptional Behavior
@Test(expected=RateNotFoundException.class)
public void throwsExceptionIfRateNotFoundII() {
String from = "BAR";
String to = "BAZ";
rates.getRate(from, to);
}

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9/9/13

A New Exception
public class RateNotFoundException extends RuntimeException {
}

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9/9/13

Back in ConversionRates
if (!rates.containsKey(key(from, to)))
throw new RateNotFoundException();
return rates.get(key(from, to));
}

9 September 2013


1.  Write one unit test.

51

steps

2.  Build or add to the object under test
until everything compiles.

3.  Red:

Watch the test fail!

4.  Green:

Make all the tests pass by
changing the object under test.

5.  Clean:

Refactor mercilessly!

6.  Repeat.
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9/9/13

Runs all the tests.
Expresses every idea required.
Says everything once and only once.
Has no superfluous parts.

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Exercise A: Password-Strength Checker

In order to be an acceptable password,
a string must:
q Have a length greater than 7
characters.
q Contain at least one alphabetic
character.
q Contain at least one digit.
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9/9/13


steps


3.  Red:


4.  Green:


5.  Clean:


6.  Repeat.
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Iteration 2
•  Admins and regular users:
•  Admin passwords must also...
•  Be > 10 chars long
•  Contain a special character

•  People want to know all the reasons why their
password has failed.
•  Other stronger rules may apply later. ;-)
•  HINT: What is varying most frequently?

Be sure to take a break when you need one!
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steps


3.  Red:


4.  Green:


5.  Clean:


6.  Repeat.
9 September 2013


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1.  _______________________________________

2.  _______________________________________

3.  _______________________________________

CLOSING

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9/9/13

9 September 2013


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Rob.Myers@agileInstitute.com
http://PowersOfTwo.agileInstitute.com/
@agilecoach

9 September 2013


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Essential Test-Driven Development

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