Analysis: Class Diagrams

Section 0: Module Objectives or Competencies

Section 1: Overview

Previously the focus has been on modeling system behavior using business process and functional models; now the attention shifts to the objects underlying the behavior and how they are organized and presented.

A structural, or conceptual, model describes the structure of the objects that support the business processes in an organization.

Structural models represent the objects that are used and created by a business system.

• It illustrates people, places, or things about which information is captured and how they are related to one another.

Use cases form the foundation on which the business information system is created.

From an architecture-centric perspective, structural modeling supports the creation of an internal structural or static view of a business information system in that it shows how the system is structured to support the underlying business processes.

• A class diagram is a structural model that contains analysis classes, attributes, operations, and the relationships among the analysis classes.

Example Class Diagram

Intro to Class Diagrams

This video provides an introduction to Class Diagrams (start at 2:46 mark on YouTube):

Note

The class diagrams developed during the analysis phase are conceptual models that focus on the logical organization of the objects without indicating how they are stored, created, or manipulated so that analysts can focus on the real business requirements of the system, without being distracted by technical details.

• Later, in design, analysts evolve the conceptual structural model into a design model that reflects how the objects will be organized in databases and software.

If you begin experiencing deja vu, visit Introduction to Object-Oriented Paradigm.

Section 2: Classes

Section 3: Syntax

A class diagram is a static model that shows classes and their relationships to one another.

Class Diagram Syntax

Section 4: Elements

Classes

• Entities within the system (a person, place or thing).
• Stores and manages information in the system and contains:
  • Attributes—characteristics of the class
  • Operations—activities the class can perform

Attributes

Attributes are properties of the class about which we want to capture information.

Operations

Operations are actions or functions that a class can perform.

• Only operations unique to the class are included.

Operation Types

• Constructor — creates an object.
• Destructor — deletes or removes an object.
• Query or accessor — makes information about the state of an object available.
• Update or mutator — changes values of some or all of an object’s attributes.

Section 5: Relationships

Relationships are the associations between classes and describe how classes relate to one another.

• Depicted as lines between classes with the line labeled with the name of the relationship.
• May be directional (depicted with a triangle; e.g., a patient schedules an appointment).
  
• Multiplicity indicates how many of one class is/are associated with other class.
  

Relationship Types

There are multiple basic types of relationships - association, aggregation, composition, and generalization.

Association

• Miscellaneous relationships between classes.
• Depicted with a line labeled with the name of the relationship.
• May be directional (depicted with a triangle; e.g., a patient schedules an appointment).
  

Aggregation

• Relates parts to wholes or assemblies.
• Aggregation is a relationship of independently constructed and externally visible parts.
• Aggregation denotes a logical “a-part-of” or “has-parts” relationship.
  

Composition

• Composition is a relationship that encapsulates (hides) the parts of the relationship.
• Composition denotes a physical “a-part-of” relationship.
  

Generalization

• Enables inheritance of attributes and operations.
• Properties and operations of the superclass are valid for the sub-class.
• Represents relationships that are “a-kind-of”.
• Depicted as a solid line with a hollow arrow pointing at the superclass.
  

Section 6: Association Classes

There are times when a relationship itself has associated properties, especially when its classes share a many-to-many relationship.

In these cases, a class called an association class is formed, which has its own attributes and operations.

• Common in many-to-many relationships (like composite entities in ERDs).
• Used when attributes about the relationship between two classes needs to be recorded.
  • Students are related to courses; a Grade class provides an attribute to describe this relationship.
  • Illnesses are related to symptoms; a Treatment class provides an attribute to describe this relationship.
  • 

Section 7: Example

Section 8: Another Example

Recall this example class diagram from this old assignment:

Class Diagram

Section 9: Verifying and Validating the Model

The verification and validation of the structural model are accomplished during a formal review meeting using a walkthrough approach in which an analyst presents the model to a team of developers and users.

• Including people outside the development team who produced the model can bring a fresh perspective to the model and uncover missing objects.
• The analyst walks through the model, explaining each part of the model and all the reasoning behind the decision to include each of the classes in the structural model.
  • This explanation includes justifications for the attributes, operations, and relationships associated with the classes.
  • Each class should be linked back to at least one use case; otherwise, the purpose of including the class in the structural model will not be understood.

Section 10: Resources

• Video: Creating a Class Diagram (no audio)
  
• Video: UML Class Diagrams
  
• Video: UML 2.0 Class Diagrams