Requirements Modeling

Class Diagrams

What are Class Diagrams?

Pressman, R., Maxim, B. Software Engineering: A Practitioners Approach

• Built from requirements elicited from the customer.
• A representation understood by both stakeholders and software engineers.
• Represents the objects that the system will manipulate.
• Specifies the operations that will be applied to the objects
• Specifies relationships between objects/classes

Why?

Pressman, R., Maxim, B. Software Engineering: A Practitioners Approach

• Depicts a view of the system that the customer can understand.
• Typically results in useful feedback at the earliest possible time.
• With further refinements, becomes the basis for software design.

How do I build them?

Pressman, R., Maxim, B. Software Engineering: A Practitioners Approach

• Identify Classes/Objects
• Specify attributes
• Define operations
• Specify associations and dependencies

Identifying Classes

Pressman, R., Maxim, B. Software Engineering: A Practitioners Approach

• Underline each noun or noun phrase and record them somewhere.
  • Pay attention to synonyms
• Classes are typically: 
  • external entites (e.g. people, other systems)
  • things (e.g. reports, displays)
  • roles (e.g. manager, engineer)
  • any other entity that is important to the problem or solution space
• Generally, classes should never have "procedural names" e.g. DeleteUser

Identifying Classes: Selection

Pressman, R., Maxim, B. Software Engineering: A Practitioners Approach

• Review and select from your list of classes (nouns and phrases) as follows:
  • Information about the class must be remembered so that the system can function.
  • Has a set of identifiable operations that can change attributes of the class.
  • Multiple attributes: sometimes a class with a single attribute is better represented as an attribute of another class.
  • Has attributes that apply to all instances of the class.
  • Has operations that apply to all instances of the class.
  • External entities that produce or consume information.

Attributes and Operations

http://www.tutorialspoint.com/uml/uml_class_diagram.htm

• Attributes describe a class.

• Operations define the behavior of an object.

Relationships: Composition

Pressman, R., Maxim, B. Software Engineering: A Practitioners Approach

• One class is composed of another.
• The black diamond represents composition
• Arrowhead denotes the relationship is navigable in only one direction.

Relationships: Inheritance

http://www.objectmentor.com/resources/articles/umlClassDiagrams.pdf

• Depicted by triangular arrowhead that looks like a slice of pizza.
• "is-a" relationship

Relationships: Aggregation

Pressman, R., Maxim, B. Software Engineering: A Practitioners Approach

• "whole/part" relationships.
• Represented by an open diamond.
• Example: A window may contain many shape instances
• Notice that the role has been named. This is the name that Window knows its Shape instance by.
• "*" represents many.

Now, go forth into the world and build those class diagrams!