1. INFS 6225 – Object-Oriented Systems Analysis & Design
Chapter 8:
Class & Method Design

2. Class & Method Design
Criteria, activities, & techniques used to design classes & methods
Part of Design Phase where work is actually accomplished
Refinement of UML models to create the most efficient system possible that still meets the users’ requirements
The steps just before the actual coding of the system

3. Review of Terms
Object – basic building block of a system; instances of classes
Class – defines both data & processes that each object contains
Attributes – describe data about the object
Methods – specify processes that objects can perform
Message – procedural call from one object to another

4. More Terms
Inheritance – superior classes pass attributes & methods to subordinate classes
Encapsulation – combining processes & data into a single object
Information Hiding – only information required to use an object should be available outside of an object
Polymorphism – same message is interpreted differently by different objects
Dynamic binding – interpretation of message by an object takes place at run time

5. New Terms
Cohesion – how specific a module is within a system (i.e., a class or object should represent only one thing)
Coupling – level of interdependence among modules (classes, objects, & methods) in a system
The higher the degree of coupling, the more likely that changes in part of a system will require changes in other parts
Connascence – two modules are so intertwined that a change in one requires a change in the other
Ideally, a system should have high Cohesion and low (i.e., loose) Coupling

6. Opportunities for Reuse
Class Library
Set of implemented classes that are designed for reuse
Pre-built “template” classes with Attributes and Methods already defined
Can be utilized using Inheritance
Support Foundation Layer
Component
Self-contained piece of software that can be “plugged” into a system
Simplify development of objects on Problem Domain & Human-Computer Interaction Layers

7. Optimizing the Design Shorten/simplify paths between objects
Move attributes to provide more direct access
Reduce the number of messages (Fan-out) sent by an object
Reorder execution of statements
Cache (temporarily save) computational results

8. Constraints & Contracts
formalizes the interactions between client and server objects
Document the message passing between objects
Contain information necessary for programmer to understand method
Contracts may contain:
Pre-conditions – constraints that must be met for a method to execute
Post-conditions – constraints that must be met after the method executes (or method is undone)

9. Contract Example Method Name: Class Name: ID: Clients (Consumers):
Associated Use Cases:
Description of Responsibilities:
Arguments Received:
Type of Value Returned:
Pre-Conditions:
Post-Conditions:

10. Method Specification
Method Specifications – written documents that include explicit instructions on how to write the code to implement a method
No standard syntax for Method Specification
Written in Structured English – short sentences
4 Components:
General Information about Method
Events – items that trigger method
Message Passing – what is being passed, and returned
Algorithm Specification (calculations, formula, etc.)