The document discusses the importance of testability for developers, highlighting key concepts such as controllability, observability, and orthogonality. It covers various testing techniques, the pitfalls of anti-testability constructs, and strategies for managing dependencies and duplication in code. Additionally, it emphasizes the role of test-driven development (TDD) in enhancing design and correctness in software development.

1. Swenug 2014-04-23
Alexander Tarnowski 1
Testablity for developers

2. Why testability and testing
for developers?
Unit testing
Maintenance
Continuous integration
Automated acceptance testing
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3. But…
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4. Ongoing project
http://leanpub.com/developer_testing
alexander.tarnowski@crisp.se
alexander_tar
blog.crisp.se/author/alexandertarnowskiBlog
www www.techbookreader.com
Blog agilamyter.com

5. Alexander Tarnowski 5
Testability costs money
Misconception

6. Alexander Tarnowski 6
Requireme
nts
Specificatio
n
Architectur
al design
Detailed
design
Coding
Unit testing
Integration
testing
System
testing
Acceptance
testing
MisconceptionTesting is a criticizing activity
that can build quality in

7. Alexander Tarnowski 7
Automated Tools
Manual
Automated
& manual
Q1
Q2 Q3
Q4

8. Defining testability
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If this were our
system…

9. Defining testability
Definition 1: Testability = decoupling
Origin: The TDD community
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10. Defining testability
Definition 2: Testability =
Controllability
Observability
Origin: The testing community
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11. Defining testability
Definition 3: Testability =
Controllability
Observability
Orthogonality
Deployability
Origin: the ”Developer Testing” book
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12. Know the driving forces behind
testability
They are:
Controllability
Observability
Orthogonality
Deployability
Make concious decisions to achieve
them
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Programmer wisdom

13. Fundamental testing
techniques
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14. Why developers should know
about fundamental testing
techniques
Pass tester tests
Incorporate them in their own tests
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Image: Stuart Miles/FreeDigitalPhotos.net

15. Equivalence partitioning
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0-17
18-23
24-60
60-100
101+
Women
Men
Insurance
premiums
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16. Typical equivalence
classes
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0-10, 11-23, 24-100
Range
s
0, 3, 6, 9, 12, 15
Defined by
functions
e.g. f(x) = x
mod 3
Sharing a property
e.g. red and blue cars Induction
proofs

17. Partition data into equivalence
classes
Cover each partition with at least
one test case
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Programmer wisdom

18. Boundary values
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Ages 24-60
22 23 24 25 26 58 59 60 61 62

19. Common edge cases
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Numbers
0, -1
Integer.MAX_VALUE/int.MaxValue
n-1, n+1, m-1, m+1
Strings
”” – empty string
null
Max input length
Dates
Precision
Days per month
Different locales
Leap years
Collections
{} – empty collection
null
Indexing → iteration

20. Boundary values and equivalence
partitions drive the number of
tests
Remember to check boundary
values
Remember to check common edge
cases
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Programmer wisdom

21. Use parameterized tests to capture
boundary values
[TestCase(18, Gender.Male, Result = 1.75)]
[TestCase(23, Gender.Male, Result = 1.75)]
[TestCase(24, Gender.Male, Result = 1.0)]
// ...
public double VerifyPremiumFactor(int age, Gender gender)
{
return new PremiumRuleEngine().GetPremiumFactor(age, gender);
}
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Programmer wisdom

22. State transition testing
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Start
Reset
Press min+sec
Setting
timePress min
Press min
Press sec
Counting
down
Press min+sec
Press start/stop
Press start/stop
Press min+sec
Press min+sec
Counting
up
Press start/stop
Press min+sec

23. Use for clarifying functionality and
finding missing or incorrect
transitions
Basis for model-based testing
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Programmer wisdom

24. Decision tables
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Age 18-23 18-23 24-59 24-59 60+ 60+
Gender Male Female Male Female Male Female
Premium factor 1.0 N N N Y N N
Premium factor 1.05 N N Y N N N
Premium factor 1.25 N N N N N Y
Premium factor 1.35 N N N N Y N
Premium factor 1.65 N Y N N N N
Premium factor 1.75 Y N N N N N
Conditio
n
Condition
alternative
Action Action entry

25. Decision tables
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Age 18-23 18-23 24-59 24-59 60+ 60+
Gender Male Female Male Female Male Female
Premium factor 1.75 1.65 1.05 1 1.35 1.25

26. Decision tables translate directly
into parameterized/data-driven
tests!
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Programmer wisdom

27. Anti-testability constructs
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28. Temporal coupling & state
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var car = new Car();
10 LOC
car.Initialize();
while(itsRaining)
{
50 LOC
}
if (sky.StarsProperlyAligned())
{
car.SetGear(1);
}
30 LOC
car.StartEngine();
car.ReleaseClutch();

29. Avoid partially initialized objects
Keep temporal coupling in mind
when designing messages and
protocols
Limit state by keeping program
elements small and short-lived
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Programmer wisdom

30. Indirect input
Direct input (e.g. pure functions):
int DoubleValue(int i)
{
return i * 2;
}
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DateTime.Now.Minute;
new Calculator.Add(10, 10);
using (StreamReader sr = new StreamReader(“Magic.txt"))
{
int.Parse(sr.ReadToEnd());
}
Indirect input
int DoMagic(int i)
{
return i +
}

31. Indirect input is natural and we
shouldn’t fear it
To cope with indirect input we need
control points and stubs
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Programmer wisdom

32. Indirect output
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Direct output (i.e. observable through the
public API):
int DoubleValue(int i)
{
return i * 2;
}
Indirect output
bool DoMagic(int i)
{
<insert code here>
return true;
}
Console.Out.WriteLine(i * 42);
RemoteEndpoint.Invoke(42, ”magic”, i);
EventLog.WriteEntry(”MagicApp”, ”value:” + i);

33. Indirect output is natural and we
shouldn’t fear it
To cope with indirect output we
need observation points and mock
objects
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Programmer wisdom

34. Domain-to-range ratio
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Input values
(domain, D)
Output
values
(range, R)
Correct computation
Faulty computation
DRR = |D| / |R|
f(0) 0
f(1-9) 1
f(11-99) 2
f(100-999) 3
f(1000-9999) 4
….

35. Compression
bool IsValidAge(int age)
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82
false
~4*109
true

36. Use high DRR as a warning flag
Be aware of information loss
Avoid using too large data types
Introduce abstractions (OO/DDD)
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Programmer wisdom

37. Dependencies
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38. Relations between objects
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public class ListWrapper
{
private IList<int> wrapped;
public int WrappedListSize
{
get { return wrapped.Count; }
}
public ListWrapper()
{
wrapped = new List<int> { 1, 2, 3 };
}
} Indirect input

39. Pass in the dependency
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public class ListWrapper
{
private IList<int> wrapped;
public int WrappedListSize
{
get { return wrapped.Count; }
}
public ListWrapper(IList<int> data)
{
wrapped = data;
}
}

40. Use factory method
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public class ListWrapper
{
private IList<int> wrapped;
public int WrappedListSize
{
get { return wrapped.Count; }
}
public ListWrapper()
{
wrapped = CreateWrappedList();
}
protected virtual IList<int> CreateWrappedList()
{
return new List<int> { 1, 2, 3 };
}
}

41. Provide external factory or
builder
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public class ListWrapper
{
private IList<int> wrapped;
public int WrappedListSize
{
get { return wrapped.Count; }
}
public ListWrapper(IntegerListBuilder builder)
{
wrapped = builder.Build();
}
}

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public class IntegerListBuilder
{
private int startingAt = 1;
private int endingWith = 10;
public IntegerListBuilder StartingAt(int startingAt)
{
this.startingAt = startingAt;
return this;
}
public IntegerListBuilder EndingWith(int endingWith)
{
this.endingWith = endingWith;
return this;
}
public IList<int> Build()
{
return Enumerable.Range(startingAt, endingWith).ToList();
}
}

43. Making code testable is about
making dependencies explicit
Generic strategies for doing so:
Passing them in
Using factory methods
Using factories or builders
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Programmer wisdom

44. System resource
dependencies
Files
System clock
Sockets, etc
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45. Solutions
Solution #1:
Your own abstraction
public interface ITimeSource
{
DateTime Now { get; }
}
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Solution #2:
Fakes
using (ShimsContext.Create()) {
System.Fakes.ShimDateTime.NowGet =
() => { return new DateTime(fixedYear, 1, 1); };

46. There isn’t a single problem in
computer science that cannot be
solved by adding another layer of
indirection. This is certainly true of
system resoruce dependencies.
Use Fakes if there’s no other way.
But first try without.
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Programmer wisdom

47. Dependencies between
layers
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Presentation
Business
Data access
Theoretic
al
dependen
cies
Actual
dependen
cies

48. Clean up messy cross-layer
dependencies
Use Dependency Inversion; depend
on abstractions
Use Dependency Injection
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Programmer wisdom

49. Duplication
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50. Copy and paste
public class CustomerRepository
{
public void Create(Customer customer)
{
if (customer.Gender == Gender.Unknown || !customer.YearOfBirth.HasValue)
{
throw new ArgumentException("Incomplete customer");
}
…
}
public void Update(Customer customer)
{
if (customer.Gender == Gender.Unknown || !customer.YearOfBirth.HasValue)
{
throw new ArgumentException("Incomplete customer");
}
…
}
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51. Block copy and paste
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public class CustomerRepository
{
public void Create(Customer customer)
{
if (customer.Gender == Gender.Unknown || !customer.YearOfBirth.HasValue)
{
throw new ArgumentException("Incomplete customer");
}
if (DateTime.Now.Year - customer.YearOfBirth.Value < 18)
{
throw new ArgumentOutOfRangeException("Underage customer");
}
…
}
public void Update(Customer customer)
{
if (customer.Gender == Gender.Unknown || !customer.YearOfBirth.HasValue)
{
throw new ArgumentException("Incomplete customer");
}
if (DateTime.Now.Year - customer.YearOfBirth.Value < 18)
{
throw new ArgumentOutOfRangeException("Underage customer");
}
…
}

52. Constructor copy and
paste
public NetworkInterface(IPAddress ipAddress, NetMask netMask, IPAddress broadcast, IPAddress defaultRoute)
{
this.ipAddress = ipAddress;
this.netMask = netMask;
this.broadcast = broadcast;
this.defaultRoute = defaultRoute;
}
public NetworkInterface(IPAddress ipV6Address, NetMaskIpV6 ipV6NetMask, IPAddress ipV6DefaultRoute)
{
this.ipV6Address = ipV6Address;
this.ipV6NetMask = ipV6NetMask;
this.ipV6DefaultRoute = ipV6DefaultRoute;
}
public NetworkInterface(IPAddress ipAddress, NetMask netMask, IPAddress broadcast, IPAddress defaultRoute,
IPAddress ipV6Address, NetMaskIpV6 ipV6NetMask, IPAddress ipV6DefaultRoute)
{
this.ipAddress = ipAddress;
this.netMask = netMask;
this.broadcast = broadcast;
this.defaultRoute = defaultRoute;
this.ipV6Address = ipV6Address;
this.ipV6NetMask = ipV6NetMask;
this.ipV6DefaultRoute = ipV6DefaultRoute;
}
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53. The same method in different
contexts
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x 8
public static long DiffTime(DateTime t1, DateTime t2)
{
if (t1.Date != t2.Date)
{
throw new ArgumentException("Incomparable dates");
}
return (t2.Hour - t1.Hour) * 60;
}

54. Divergent duplicated
methods in different contexts
public static long DiffTime(DateTime t1, DateTime t2)
{
if (t1.Date != t2.Date)
{
throw new ArgumentException("Incomparable dates");
}
return (t2.Hour * 60 + t2.Minute) - (t1.Hour * 60 + t1.Minute);
}
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in 7/8 places…

55. Code that’s been mechanically
copied and pasted increases the size
of the code base and is annoying…
… but the true danger lies in
convergence
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Programmer wisdom

56. Different methods or classes
doing similar things
Ignorance
Fear
Laziness
Choice
Conflict
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57. Different ways of doing
similar things
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Module Alpha uses… Module Bravo uses…
Logging framework Apple Logging framework Banana
Hand-written SQL An O/R mapper
A date/time library Time computations
implemented ”by hand”
Client-side validation Server-side validation

58. Competing domain models
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Image: vectorolie,Danilo Rizzuti/FreeDigitalPhotos.net

59. Understand that mental
duplication makes
testing, understanding, and
sometimes even recruiting harder
Have a long-term strategy for
dealing with mental duplication
Duplication makes things
”hard”. If it’s hard then…
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Programmer wisdom

60. Test-driven development
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Refact
or
Green
Red

61. Properties of test-driven
code
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Code
driven by
tests
Executabl
e in
isolation
Small
Does one
thing
Indirect
creation
Pure
functions
separete
d from
side
effects

62. • Drives design, not correctness
• Negative tests are not used
• There are two kinds of TDD
Classic TDD London school
TDD and testability
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63. Supplement tests that drive design
with tests that drive correctness!
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Programmer wisdom

64. Summary
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Testability is a skill
Developers do supporting
technical testing
Testability:
 Controllability
 Observability
 Orthogonality
 Deployability
Testing techniques
Anti-testability const
 Temporal coupling
 Indirect input
 Indirect output
 High DRR
Duplication:
 Mechanical
 Mental
TDD drives design
not correctness!
Dependencies:
 Pass in
 Factory methods
 Factories/Builders
 Fakes