TDD (Test Driven Design)

TEST DRIVEN
DEVELOPMENT/DESIGN
Presented By:
Eralp Erat
Pragmalinq - PRG
1

ORIGIN
 Test-Driven Development is a core part of the
agile process formalized by Kent Beck called
eXtreme Programming (XP).
 XP originally had the rule to test everything that
could possibly break. Now, however, the
practice of testing in XP has evolved into Test-
Driven Development.“
 Do not need to adopt XP in order to practice TDD
and gain the benefit from it. 3

INTRODUCTION
 Traditional Approach
 Test last
 Problems with Traditional
 Errors in production
 Programmer moves onto other projects
 Test and code written by different programmers
 Tests based on outdated information
 Infrequent testing
 Fixes that create other problems
4

COST OF DEVELOPMENT
Time
C
o
s
t
Traditional
TDD
5

COST OF FIXING FAULTS
1
3
10
15
30
40
0
5
10
15
20
25
30
35
40
Reqs.
Design
Coding
Dev.Test
Accept.
Operation
Relative Cost
6

WHAT IS TDD?
 TDD is a technique whereby you write your test cases
before you write any implementation code
 Forces developers to think in terms of implementer and
user
 Tests drive or dictate the code that is developed
 “Do the simplest thing that could possibly work”
 Developers have less choice in what they write
 An indication of “intent”
 Tests provide a specification of “what” a piece of code
actually does – it goes some way to defining an interface
 Some might argue that “tests are part of the
documentation”
 Could your customers/clients write tests?
7

WHAT IS TDD?
 “Before you write code, think about what it will do.
Write a test that will use the methods you haven’t even
written yet.”
 A test is not something you “do”, it is something you
“write” and run once, twice, three times, etc.
 It is a piece of code
 Testing is therefore “automated”
 Repeatedly executed, even after small changes
 “TDD is risk averse programming, investing work in the
near term to avoid failures later on”
8

WHAT CAN BE TESTED?
 Valid Input
 In-valid Input
 Exceptions
 Boundary Conditions
 Everything that should be possible break.
10

ASPECTS OF TDD
 Features
 High level user requirements
 User story
 Customer Tests
 Customer identified acceptance tests
 Developer Tests
 Tests developed during software construction
11

METHODOLOGY
 Test first – Code last
 You may not write production code unless you’ve first
written a failing unit test
12

TDD STAGES
Write a test
Compile
Fix compile errors
Run test,
watch it fail
Write code
Run test,
watch it pass
Refactor code
(and test)
13

TDD STAGES
 The Extreme Programming Explored , Bill Wake describes
the test cycle:
1. Write a single test
2. Compile it. It shouldn’t compile because you’ve not written
the implementation code
3. Implement just enough code to get the test to compile
4. Run the test and see it fail
5. Implement just enough code to get the test to pass
6. Run the test and see it pass
7. Refactor for clarity and “once and only once”
8. Repeat
14

LIFE CYCLE
Write Test
Compile
Run & See the
Fail
Refactor As
Needed
15

WHY DOES TDD WORK?
 The (sometimes tedious) routine leads the
programmers to think about details they
otherwise don’t (because they’ve bitten off more
than they can chew)
 Specifically, test cases are thought through
before the programmer is allowed to think about
the “interesting part” of how to implement the
functionality
16

WHY DOES TDD WORK?
 Encourages “divide-and-conquer”
 Programmers are never scared to make a change
that might “break” the system
 The testing time that is often squeezed out of the
end of a traditional development cycle cannot be
squeezed out.
17

ADVANTAGES OF TDD
 TDD shortens the programming feedback loop
 TDD promotes the development of high-quality
code
 User requirements more easily understood
 Reduced interface misunderstandings
 TDD provides concrete evidence that your
software works
 Reduced software defect rates
 Better Code
 Less Debug Time.
18

DISADVANTAGES OF TDD
 Programmers like to code, not to test
 Test writing is time consuming
 TDD may not always work
19

EXAMPLE
 We want to develop a method that, given two
Integers, returns an Integer that is the sum of
parameters.
20

EXAMPLE (CONT.)
 Test
Integer i =
new Integer(5);
Integer j =
new Interger(2);
Object o = sum(i,j);
 Method
21

EXAMPLE (CONT.)
 Test
Integer i =
new Integer(5);
Integer j =
new Interger(2);
 Method
public static Object
sum(Integer i,
Integer j) {
return new
Object();
}
22

EXAMPLE (CONT.)
 Test
Integer i =
new Integer(5);
Integer j =
new Interger(2);
if (o instanceof
Integer)
return true;
else
return false;
 Method
public static Object
sum(Integer i,
Integer j) {
return new
Object();
}
23

EXAMPLE (CONT.)
 Test
Integer i =
new Integer(5);
Integer j =
new Interger(2);
if (o instanceof
Integer)
return true;
else
return false;
 Method
public static Integer
sum(Integer i,
Integer j) {
return new
Integer();
}
24

EXAMPLE (CONT.)
 Test
Integer i =
new Integer(5);
Integer j =
new Interger(2);
if ((o instanceof
Integer) &&
((new Integer(7))
.equals(o))
return true;
else
return false;
 Method
sum(Integer i,
Integer j) {
return new
Integer();
}
25

EXAMPLE (CONT.)
 Test
Integer i =
new Integer(5);
Integer j =
new Interger(2);
if ((o instanceof
Integer) &&
((new Integer(7))
.equals(o))
return true;
else
return false;
 Method
sum(Integer i,
Integer j) {
return new
Integer(
i.intValue() +
j.intValue());
}
26

TECHNIQUE
 Identify a “smallest possible” change to be made
 Implement test and (the one line of) code for that
change (see previous slide)
 Run all tests
 Save test and code together in source control
system
 Repeat
27

CONCLUSION
 More code has to be written using TDD but that isn’t
the bottleneck in Software Development
 Techniques have to be learned by developers and
enforced by managers
 User Interface testing is the hardest
 Resulting unit tests most valuable when run as part of
an automated build process
28

TDD (Test Driven Design)

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