Software Design Principles and UML Diagrams Explained

Dimensions of Software Design

Software design involves several key dimensions:

• Problem solving
• Modeling
• Specification
• Organizing the solution
• Communication
• Economics and reuse
• Maintenance and evolution

Design Approaches

Divide-and-Conquer

A problem is decomposed into subproblems that are worked on independently. This approach can be applied recursively for complex problems and allows managers to allocate work to multiple people.

Procedural Design

Design is viewed as a set of procedures that are composed:

• Sequentially
• Conditionally
• Iteratively
• Hierarchically

Borderline Procedural Design

In this approach, the main steps are not fixed. The user has choices with few constraints, and there is typically a main loop to find the next operation.

Event-Driven Design

This approach is good for systems driven by external sources of events. It uses event handlers, and events can occur and be handled concurrently. Designers must think in terms of listeners; responding to an event can require complex processing.

Alternative Designs

The designer explores alternatives. To evaluate them:

• Identify alternatives
• Sketch each alternative
• Discuss the options
• Find advantages and disadvantages

Recognition-Primed Decision Model [Klein]

A tentative plan is formed through associative memory, mentally simulated, and modified as needed.

Quality Factors in Design

Key quality factors include:

• Completeness
• Efficiency
• Maintainability
• Portability
• Reliability
• Security
• Simplicity
• Understandability

Metrics for these factors can be computed by analyzing formal design descriptions, through data collected when using a prototype, or through subjective judgment (e.g., user ratings).

Modularization Principles

Create modules with a clear interface and implementation. Modules should exhibit:

• Low Coupling: Change in one module has minimal impact on others.
• High Cohesion: Elements within a module are closely related and work together.

Modularization encapsulates design decisions.

Object-Oriented Design

Focuses on objects with state (values) and behaviors (methods), organized into classes and instances.

• Analysis: The initial phase involves identifying real-world objects and their services.

Graphic Models for Design

Graphic models are used to aid understanding. Types include:

• Object and Class Diagrams
• Data Flow Diagrams
• Finite-State Machines

The Design Document

Provides essential guidance. Characteristics of a good design document:

• Concise and easy to understand
• Specific
• References requirements
• High-level overview
• Describes major components and relationships
• Modular structure
• Includes a history of changes

UML Diagrams

UML (Unified Modeling Language) is a collection of diagrams used in business processes and software design.

Class Diagram

Shows the static structure, related classes and interfaces with their attributes, relationships, and constraints.

Package Diagram

Shows the structure at the level of packages; packages can import other packages.

Object Diagram

Depicts objects and their relationships at a specific point in time; it is an instance of a class diagram.

Sequence Diagram

An interaction diagram that depicts the lifelines of objects and the sequence of messages exchanged between them.

State Machine Diagram

Shows discrete behavior through state transitions.

Component Diagram

Shows how components are connected; depicts components, interfaces, and ports.

Activity Diagram

A behavior diagram that shows the flow of control and emphasizes the sequence and conditions of the flow.

UML Limitations

UML is not adequate for all purposes; custom diagrams may be needed.