1. Design for Ease in Contraction and Extension
CSE870 Discussion
April 16, 2003

2. Key Ideas Design for change
Maintainability
Identified characteristics of “non-adaptable” programs
Undesirable properties
Distributing related information over numerous programs
Chaining of data transformations
Designing multi-function components
Circular-uses
Subset:
Smallest useful collection of functions
High cohesion among the functions

3. Key Ideas (cont’d) Uses hierarchy: Virtual Machines:
Degree in which one program uses other programs
Levels:
0: programs that do NOT use any other programs
Single-purpose programs
I: programs that use at least one program at (I-1) level, not above I-1.
Multi-purpose
Virtual Machines:
SW abstraction/instructions for instructions provided by the HW.
May have several layers of abstractions that can be customized for specific applications

4. Key Ideas (cont’d) General vs Specific General: Specific:
SW could work without modification
Not as much concern for designing for flexibility
Run-time costs: customization can be expensive
Specific:
Solution to specific problem
SW footprint can be reduced because it does not contain extraneous functionality
Program can be more efficient
Narrow focus of program

5. Key Ideas (cont’d) Flexible design: Information Hiding:
Takes specific approach
Provide general services with WELL-DEFINED interfaces
Have specific implementations for these services that are kept ``secret’’
Information Hiding:
Implementation details of modules (or functions within modules)

6. Comparison to Program Families
Extension/Contraction paper gives specific techniques for how to build program families

7. Impact on Modern Technology
Frameworks:
Possible relationship to Graybox (have ability to extend) while adhering closely to superclass interfaces

8. Impact on Modern Tech (cont’d)
Design Patterns:
Iterator (operations can be indep of data type)
Model View Controller
Bridge: decouples the abstract services from its implementation
Strategy pattern:
Add strategies
Decouple interface for a strategy’s purpose from the specific algorithm.
Factory Pattern:
Composes the abstract components that can then be realized by concrete components
Chain of responsibility:
Each unit should have minimal functionality that can then be extended to provide additional capabilities.

9. Impact on Modern Day Tech (cont’d)
Distinguishes specification from implementation
Specification:
Implementation:
Later clarified by Lamport, Lynch, Abadi
Potential impact on Aspect-Oriented Programming
Advocate collection of like-purpose functionalities into single module
Interdependence with compiler (def-use) and Russell and Whitehead’s mathematical formulation for set theory (early 19XX’s)