SWE 205 - Introduction to
Software Engineering
Lecture 28 –
Introduction to Software Testing
1
Lecture Outline





Software testing motivation.
What is a software testing?
Why software fail?
What is cost of software failures?
What makes a good software tester?
2
Software Testing Motivation

The Lion King Animated Storybook




Disney’s first multimedia CD-ROM game
for kids. Released at Christmas season.
26th December….. Customer Support’s
Nightmare.
The CD was testing only for specific PC
platform.
It failed on many popular PC operating
system.
3
Software Testing Motivation

Intel Pentium Floating-Point Bug




(4195835/3145727)* 3145727 - 4195835
If answer is ZERO, your computer is fine.
Anything else, you have old Intel Pentium
CPU with floating-point division bug.
A software bug burned into a computer
chip and reproduced over and over in the
manufacturing process.
4
Software Testing Motivation

NASA Mars Polar Lander, 1999



3rd December 1999, Mars Polar Lander
disappeared during its landing attempt.
Failure Review Board concludes the likely
failure reason was the unexpected setting
of a single data bit.
Most alarming was why the problem was
not caught by internal tests.
5
Software Testing Motivation

Malaysia Airlines Jetliner, August 2005


Flight between Perth, Australia and Kuala
Lampur, Malaysia zoomed 3,000 feet
upwards.
A defective software program had provided
incorrect data about the aircraft’s speed
and acceleration, confusing flight
computers.
6
What is Software Testing?

The process of finding evidence of
defects in software systems.

Establishing confidence that a program
does what it is supposed to do.
1. Software testing is not debugging.
2. Software testing is not quality assurance
7
Software Testing Vs Quality
Assurance (QA)

Testing is necessary but not enough for
QA process.


Testing contributes to improve quality by
helping to identify problems.
QA sets standards that project members
(including testers) should follow in order
to build a better software.
8
What is Software Testing?




Correctness of software with respect to
requirements or intent;
Performance of software under various
conditions;
Robustness of software, its ability to handle
erroneous input and unanticipated conditions;
Installation and other facets of a software
release.
9
Basic Definitions

Failure


Fault


There is a deviation of the observed behavior of a
program or a system from its specification.
An incorrect step, process or data definition.
Error

Difference between computed, observed or
measured value and the true or theoretically
correct value or condition.
10
Trivial Example

Failure
For any integer n, square (n) = n*n.
int square (int x)
{
return x*2;
}
Square (3) = 6
Fault
11
Trivial Example

Correct Result
For any integer n, square (n) = n*n.
int square (int x)
{
return x*2;
}
Square (2) = 4
12
Important Considerations




Detect system failures by choosing test
inputs carefully.
Determine the faults leading to the
failures detected.
Repair the faults leading to the failures
detected; and
Re-test the module/system.
13
Why do Failure Occur?
Specification
Design
Code
Other
14
Root Causes of Failures

Inaccurate understanding of end user
requirements.


Late discovery of serious project flaws.


Inability to deal with changing requirements.
For example, modules that do not fit together.
Untrustworthy build & release process.

Implementation team’s chaos.
15
Failure Costs
Disney’s Lion King CD
16
Software Testers


Good understanding of the development
process and its products.
Ability to anticipate likely faults and
errors.
17
Semantics of programs

Squeeze Function


Semantics of C are such that integers and
characters are somewhat interchangeable.
Fibonacci Function


Input Domain - set of values of type ‘int’.
Input domain can vary on different types of
machines.
18
Semantics of programs

These examples are deterministic in
nature.



A unique output.
Different methods required for nondeterministic programs.
Similarly, programs may terminate or
not terminate.
19
Key Points



Software testing is the process of
discovering evidence of defects and
failures in software systems.
Test early, test often, test enough.
Testers should have good
understanding of the development
process, product.
20
Announcements

Quiz # 4

Wednesday 24/12/2008.
21