SWE 316: Software Design and Architecture
Objectives
Lecture # 17
Good Software Design

Practical tips for creating good design

Introduce practical tips for creating good
design
The slides and the material of this chapter are adopted from:
“Object-Oriented Software Engineering: Practical Software
Development using UML and Java”, 2nd Ed. By Timothy C.
Lethbridge and Robert Laganière
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Topics: Principles leading to good design

DP 1: divide and conquer

DP 6: reuse existing designs
and code where possible

DP 2: increase cohesion
where possible

DP 7: design for flexibility

DP 3: reduce coupling
where possible

DP 8: anticipate
obsolescence

DP 4: keep the level of
abstraction as high as
possible

DP 9: design for portability

DP 10: design for testability

DP 11: design defensively

DP 5: increase reusability
where possible
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
2/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Principles leading to good design

Overall goals of good design:

Increasing profit by reducing cost and increasing revenue

Ensuring that we actually conform with the requirements

Accelerating development

Increasing qualities such as
Usability
Reliability
Reusability
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Efficiency
Maintainability
3/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 1: divide and conquer

Trying to deal with something big all at once is normally
much harder than dealing with a series of smaller things

Separate people can work on each part.

An individual software engineer can specialize.

Each individual component is smaller, and therefore easier to
understand.

Parts can be replaced or changed without having to replace or
extensively change other parts.
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
4/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Ways of dividing a software system

A distributed system is divided up into clients and servers

A system is divided up into subsystems

A subsystem can be divided up into one or more packages

A package is divided up into classes

A class is divided up into methods
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
5/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 2: increase cohesion where possible

A subsystem or module has high cohesion if it keeps
together things that are related to each other, and keeps out
other things


This makes the system as a whole easier to understand and
change
Type of cohesion:
-Functional
-Communicational
-Procedural
-Utility
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
-Layer
-Sequential
-Temporal
6/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
7/35
DP 2: increase cohesion where possible (2)
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
8/35
DP 2: increase cohesion where possible (3)
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
9/35
DP 2: increase cohesion where possible (4)

Mixing different types of cohesion
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 3: Reduce coupling where possible

Coupling occurs when there are interdependencies
between one module and another



When interdependencies exist,
changes in one place will require
changes somewhere else.
A network of interdependencies
makes it hard to see at a glance
how some component works.
Type of coupling:
-Content -Common -Control -Stamp -Data
-Routine Call -Type use -Inclusion/Import -External
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
10/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 3: Reduce coupling where possible
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
11/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 3: Reduce coupling where possible
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
12/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 3: Reduce coupling where possible

Content Coupling

A module modifies data that is internal to another
component
It should always be AVOIDED

e.g., modify public instance variable

SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
13/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 3: Reduce coupling where possible

Control Coupling

This occurs when one procedure calls another using a
‘flag’ or ‘command’ that explicitly controls what the
second procedure does.
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
14/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 3: Reduce coupling where possible

Stamp Coupling

This occurs whenever a class is declared as the type of
a method argument.
Some stamp coupling is necessary

What’s wrong in this implementation?

SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
15/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 4: Abstraction as high as possible

Abstractions --- keeping things simple !

keep the level of abstraction as high as possible

Ensure that your designs allow you to hide or defer
consideration of details, thus reducing complexity


A good abstraction is said to provide information hiding
Abstractions allow you to understand the essence of a
subsystem without having to know unnecessary details
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
16/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Abstraction and classes

Classes are data abstractions that contain procedural
abstractions





Abstraction is increased by defining all variables as private.
The fewer public methods in a class, the better the
abstraction
Superclasses and interfaces increase the level of abstraction
Attributes and associations are also data abstractions.
Methods are procedural abstractions

Better abstractions are achieved by giving methods fewer parameters
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
17/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 5: Increase reusability where possible


Design for reuse ---- Design with reuse
Design the various aspects of your system so that they can
be used again in other contexts

Generalize your design as much as possible

Follow the preceding three design principles




Increase cohesion
Reduce coupling
Increase abstraction
Simplify your design as much as possible
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
18/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 5: Increase reusability where possible


How?
Generalize your design as much as possible




Save instances of Employee class in a binary file
Instead, consider saving any class to binary file
Another example, generalize sorting
Design your system with hooks


An aspect of the design deliberately added to allow other
designers to add additional functionality
Extensible compiler example
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
19/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 6: reuse existing designs and code

Design with reuse is complementary to design for
reusability

Actively reusing designs or code allows you to take advantage
of the investment you or others have made in reusable
components
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
20/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
21/35
DP 7: design for flexibility (adaptability)

Actively anticipate changes that a design may have to
undergo in the future, and prepare for them



Reduce coupling and increase cohesion
Create abstractions
Do not hard-code anything


Leave all options open


E.g., constants should be in a configuration file
Do not restrict the options of people who have to modify the system
later
Use reusable code and make code reusable
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 8: Anticipate Obsolescence

Plan for changes in the technology or environment so the
software will continue to run or can be easily changed





Avoid using early releases of technology
Avoid using software libraries that are specific to particular
environments
Avoid using undocumented features or little-used features of
software libraries
Avoid using software or special hardware from companies that
are less likely to provide long-term support
Use standard languages and technologies that are supported by
multiple vendors
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
22/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 9: design for portability

Have the software run on as many platforms as possible

Avoid the use of facilities that are specific to one particular
environment

E.g. a library only available in Microsoft Windows

Some programming languages, such as Java, make this easy
because the language itself is designed to allow software to run
on different platforms unchanged.

Another important portability issue has to do with text files:
the characters used to terminate lines differ from platform to
platform.
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
23/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 10: Design for Testability

Take steps to make testing easier

Design a program to automatically test the software

Ensure that all the functionality of the code can by driven by an
external program, bypassing a graphical user interface

In Java, you can create a main() method in each class in order
to exercise the other methods
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
24/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 11: Design defensively


Never trust how others will try to use a component you
are designing

Handle all cases where other code might attempt to use your
component inappropriately

Check that all of the inputs to your component are valid: the
preconditions
Example: method to check if a certain date is working day
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
25/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
DP 11: Design defensively

What’s wrong with this implementation?

Design by Contract

Each method has a specific contract with its caller
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Design Decisions
26/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
DP 11: Design defensively

Using Asserting

identify possible logic errors during development.
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
27/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
28/35
Techniques for making good design decisions

Using priorities and objectives to decide among
alternatives

Step 1: List and describe the any of the alternatives
alternatives for the design
prevents you from meeting
decision.
one or more of the
objectives.

Step 2: List the advantages
and disadvantages of each
alternative with respect to
your objectives and
priorities.

Step 3: Determine whether

Step 4: Choose the
alternative that helps you to
best meet your objectives.

Step 5: Adjust priorities for
subsequent decision making.
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Example priorities and objectives

Imagine a system has the following objectives, starting
with top priority:






Security: Encryption must not be breakable within 100 hours
of computing time on a 400Mhz Intel processor, using known
cryptanalysis techniques.
Maintainability. No specific objective.
CPU efficiency. Must respond to the user within one second
when running on a 400MHz Intel processor.
Network bandwidth efficiency: Must not require
transmission of more than 8KB of data per transaction.
Memory efficiency. Must not consume over 20MB of RAM.
Portability. Must be able to run on Windows 98, NT 4 and
ME as well as Linux
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
29/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Example evaluation of alternatives
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
30/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Example evaluation of alternatives
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
31/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
32/35
Cost-benefit analysis to choose among alternatives

To estimate the costs, add up:




The incremental cost of doing the software engineering work,
including ongoing maintenance
The incremental costs of any development technology required
The incremental costs that end-users and product support
personnel will experience
To estimate the benefits, add up:


The incremental software engineering time saved
The incremental benefits measured in terms of either
increased sales or else financial benefit to users
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Some general principles of good design







Use good design patterns, patterns of call-return or
cooperative objects, event-structure should be consistent
Modular – logically partitioned into elements that perform
specific functions and sub-functions.
Should have distinct representations of data, interfaces and
components
Functions and components should be independent
Interfaces between components should be kept as simple as
possible.
Use a repeatable method that transforms the requirements
through analysis to high-level and then low-level design
Don’t reinvent the wheel.
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
33/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
CRC cards




CRC stands for Classes, Responsibilities,
Collaborations.
CRC cards allow people to break away
from the procedural mode of thought
and more fully appreciate object
technology.
One common way of checking for a
good design and guiding its refinement
is to use CRC cards.
Although CRC is not part of UML, they
add some very useful insights
throughout a development.
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser
Design Decisions
34/35
Intro
Principles (1-2)
Principles (3-4)
Principles (5-7)
Principles (8-11)
Design Decisions
Summary

Overall goals of good design:





Increasing profit by reducing cost
and increasing revenue
Ensuring that we actually conform
with the requirements
Accelerating development
Increasing qualities such as
Design principles leading to
good design:





Divide and conquer
Increase cohesion where possible
Reduce coupling where possible
Keep the level of abstraction as
high as possible
Increase reusability where
possible
SWE 316: Software Design and Architecture – Dr. Khalid Aljasser






Reuse existing designs and code
where possible
Design for flexibility
Anticipate obsolescence
Design for portability
Design for testability
Design defensively
35/35