1. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser Objectives Lecture # 17 Good Software Design SWE 316: Software Design and Architecture  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”, 2 nd Ed. By Timothy C. Lethbridge and Robert Laganière

2. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser Topics: Principles leading to good design  DP 1: divide and conquer  DP 2: increase cohesion where possible  DP 3: reduce coupling where possible  DP 4: keep the level of abstraction as high as possible  DP 5: increase reusability where possible  DP 6: reuse existing designs and code where possible  DP 7: design for flexibility  DP 8: anticipate obsolescence  DP 9: design for portability  DP 10: design for testability  DP 11: design defensively Intro Principles (1-2)Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 2/35

3. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 UsabilityEfficiency ReliabilityMaintainability Reusability Intro Principles (1-2)Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 3/35

4. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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. Intro Principles (1-2) Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 4/35

5. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 Intro Principles (1-2) Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 5/35

6. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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-Layer -Communicational-Sequential -Procedural-Temporal -Utility Intro Principles (1-2) Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 6/35

7. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser DP 2: increase cohesion where possible (2) Intro Principles (1-2) Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 7/35

8. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser DP 2: increase cohesion where possible (3) Intro Principles (1-2) Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 8/35

9. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser DP 2: increase cohesion where possible (4)  Mixing different types of cohesion Intro Principles (1-2) Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 9/35

10. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2) Principles (3-4) Principles (5-7)Principles (8-11)Design Decisions 10/35

11. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser DP 3: Reduce coupling where possible IntroPrinciples (1-2) Principles (3-4) Principles (5-7)Principles (8-11)Design Decisions 11/35

12. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser DP 3: Reduce coupling where possible IntroPrinciples (1-2) Principles (3-4) Principles (5-7)Principles (8-11)Design Decisions 12/35

13. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2) Principles (3-4) Principles (5-7)Principles (8-11)Design Decisions 13/35

14. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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. IntroPrinciples (1-2) Principles (3-4) Principles (5-7)Principles (8-11)Design Decisions 14/35

15. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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? IntroPrinciples (1-2) Principles (3-4) Principles (5-7)Principles (8-11)Design Decisions 15/35

16. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2) Principles (3-4) Principles (5-7)Principles (8-11)Design Decisions 16/35

17. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2) Principles (3-4) Principles (5-7)Principles (8-11)Design Decisions 17/35

18. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2)Principles (3-4) Principles (5-7) Principles (8-11)Design Decisions 18/35

19. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2)Principles (3-4) Principles (5-7) Principles (8-11)Design Decisions 19/35

20. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2)Principles (3-4) Principles (5-7) Principles (8-11)Design Decisions 20/35

21. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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  E.g., constants should be in a configuration file  Leave all options open  Do not restrict the options of people who have to modify the system later  Use reusable code and make code reusable IntroPrinciples (1-2)Principles (3-4) Principles (5-7) Principles (8-11)Design Decisions 21/35

22. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2)Principles (3-4)Principles (5-7) Principles (8-11) Design Decisions 22/35

23. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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. IntroPrinciples (1-2)Principles (3-4)Principles (5-7) Principles (8-11) Design Decisions 23/35

24. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2)Principles (3-4)Principles (5-7) Principles (8-11) Design Decisions 24/35

25. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2)Principles (3-4)Principles (5-7) Principles (8-11) Design Decisions 25/35

26. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser DP 11: Design defensively  What’s wrong with this implementation?  Design by Contract  Each method has a specific contract with its caller IntroPrinciples (1-2)Principles (3-4)Principles (5-7) Principles (8-11) Design Decisions 26/35

27. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser DP 11: Design defensively  Using Asserting  identify possible logic errors during development. IntroPrinciples (1-2)Principles (3-4)Principles (5-7) Principles (8-11) Design Decisions 27/35

28. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser Techniques for making good design decisions  Using priorities and objectives to decide among alternatives  Step 1: List and describe the alternatives for the design decision.  Step 2: List the advantages and disadvantages of each alternative with respect to your objectives and priorities.  Step 3: Determine whether any of the alternatives prevents you from meeting one or more of the objectives.  Step 4: Choose the alternative that helps you to best meet your objectives.  Step 5: Adjust priorities for subsequent decision making. IntroPrinciples (1-2)Principles (3-4)Principles (5-7)Principles (8-11) Design Decisions 28/35

29. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2)Principles (3-4)Principles (5-7)Principles (8-11) Design Decisions 29/35

30. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser Example evaluation of alternatives IntroPrinciples (1-2)Principles (3-4)Principles (5-7)Principles (8-11) Design Decisions 30/35

31. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser Example evaluation of alternatives IntroPrinciples (1-2)Principles (3-4)Principles (5-7)Principles (8-11) Design Decisions 31/35

32. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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 IntroPrinciples (1-2)Principles (3-4)Principles (5-7)Principles (8-11) Design Decisions 32/35

33. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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. IntroPrinciples (1-2)Principles (3-4)Principles (5-7)Principles (8-11) Design Decisions 33/35

34. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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. IntroPrinciples (1-2)Principles (3-4)Principles (5-7)Principles (8-11) Design Decisions 34/35

35. SWE 316: Software Design and Architecture – Dr. Khalid Aljasser 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  Reuse existing designs and code where possible  Design for flexibility  Anticipate obsolescence  Design for portability  Design for testability  Design defensively IntroPrinciples (1-2)Principles (3-4)Principles (5-7)Principles (8-11)Design Decisions 35/35