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

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

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

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

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

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

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

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)

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:

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.)