UML Class Diagrams

Class Diagrams

A class diagram is a set of classes, interfaces, collaborations and their relationships

Graphically, a class diagram is a collection of vertices and arcs

Contents
Class diagram contains:

-Dependency
-Generalization
-Association

It also contains packages or subsystems, used to group elements in a model into larger chunks

Uses

Class diagram model the static design view of a system, which is used:

Common Modeling Techniques

Modeling Simple Collaborations

To model collaboration, identify the mechanism to model
A mechanism represents some function or behavior of the part of the system which is modeled. A mechanism results from the interaction of a society of classes, interfaces and other things
For each mechanism, identify classes, interfaces and other collaborations that participate in this collaboration
Identify relationships among these things as well
Use scenarios to walk through these things
Along the way, parts of model that went missing and parts that were just plain semantically wrong can be discovered
Be sure to populate these elements with their contents
For classes, start with getting a good balance of responsibilities
Then, over time, turn these into concrete attributes and operations

Modeling a Logical Database Schema

To model a schema,

Identify those classes in the model whose state must transcend the lifetime of their applications
Create a class diagram that contains these classes. Custom set of stereotypes and tagged values to address database-specific details can be defined
Expand the structural details of these classes
In general, this means specifying the details of their attributes and focusing on the associations and their multiplicities that relate these classes
Watch for common patterns that complicate physical database design such as cyclic associations and one-to-one associations
Where necessary, create intermediate abstractions to simplify your logical structure
Consider also the behavior of these classes by expanding operations that are important for data access and data integrity
In general, to provide a better separation of concerns, business rules concerned with the manipulation of sets of these objects should be encapsulated in a layer above these persistent classes
Where possible, use tools to help transform the logical design into a physical design

Forward and Reverse Engineering

Forward Engineering

To forward engineer a class diagram, Find out the rules for mapping to the implementation language. This can be done at the project level or organization level
Based on semantics of the languages selected, use of certain UML features may have to be avoided. For example, many object-oriented languages do not support multiple inheritance, but it is possible to model it in UML. In this case, developers can be prevented from modeling multiple inheritance, even though it confines the model to certain languages
Tagged values can be used to assist implementation choices in the target language
This can be done at the level of individual classes. This can also be done at a higher level, for example, with collaborations or packages
Tools can be used to generate code

Reverse Engineering

To reverse engineer a class diagram,

Find out the rules for mapping from the implementation language. This can be done at the project level or organization level
Tools can be used to create a new model or modify an existing model, which was previously forward engineered
One simple model may not possible from large piece of code
The code that needs to be reverse engineered can be selected using the tool
The most common diagram that is generated using reverse engineering is class diagram. This can be done starting from one or more classes and navigating to neighboring classes. The level of detail to depict can be selected based on the purpose of the model

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