Class and Method Design Chapter 14

Key Concepts Low-level or detailed design is critical despite libraries and components Pre-existing classes need to be understood and organized Some, if not all code, is generally still needed to instantiate new classes

DESIGN CRITERIA

Coupling Interdependency among modules Interaction coupling through message passing

Types of Interactive Coupling Level Type Good No Direct Coupling Data Stamp Control Common or Global Bad Content or Pathological

Cohesion “Single-mindedness of a module” Method cohesion Class cohesion Generalization/specialization cohesion

Types of Method Cohesion Level Type Good Functional Sequential Communicational Procedural Temporal or Classical Logical Bad Coincidental

Types of Class Cohesion Level Type Good Ideal Mixed-role Mixed-domain Worse Mixed-instance

Connascence Creating the need to change another module as a result of changing one Minimize overall connascence Minimize across encapsulation boundaries Maximize within encapsulation boundary

OBJECT DESIGN ACTIVITIES

Additional Specification First review the current set of models Sufficient but only necessary classes to solve problem No missing attributes or methods No extraneous attributes or methods Examine visibility

Signatures for each method Name of the method Parameters or arguments to pass Type of value the method will return to the calling method

Identify Opportunities for Reuse Patterns Framework Class libraries Components

Simple Contract Format 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

Syntax No formal syntax specification General information Events Message Passing Algorithm Specification

Structured English

Pseudocode Example (Get CD-info module) Accept (CD_title) {Required} Accept (CD_artist) {Required} Accept (CD_category) {Required} Accept (CD_length) Return

Summary Translating the basic elements of OO analysis into design requires attention to prevent difficulties and conflicts in execution. Coupling, cohesion, and connascence are proposed for criteria for evaluating design. The designer will look for opportunities to optimize the design, will create constraints and contracts, and will specify methods.