Lecture 4:
Class diagrams

An object diagram is a "blueprint" of how storage is managed when a system executes. It is similar to the "traffic diagram" an architect makes of a building that shows the movement of people through the building while the building is used.

But an architect also makes a "floor plan" of the building, which shows the layout of rooms, halls, and stairs that will be constructed for use.

A class diagram is a blueprint of the components needed to build an executing software system --- it is the "floor plan" of the software system.

UML class notation is used to draw a class diagram. We use the variant that is documented in the Visual Studio lecture notes. There is a related tool, Nclass, which helps you draw class diagrams and generate Visual Studio projects from them. Nclass was used to make the diagram below. (There are more in the VS Notes.)

Example

The computer simulation must construct objects for the entities just described; here is an object diagram of what the exam simulation might look like while it is executing:

The object diagram gives big clues about what classes must be written for the simulation. Here is a class diagram that could generate the above object diagram:

The meanings of boxes and arrows are documented in the Visual Studio lecture notes. Here is a quick summary:

• The boxes represent entities: classes, interfaces, and code fragments (the latter can also be comments). Within a class box, the fields and methods are listed. Within an interface box, the methods are listed.

• The arrows define relationships. For the above example:
  • A solid arrow defines dependency (coupling): "A refers to B", or "A needs B to compile correctly" and "A retains a handle to B in a field."
  • The dashed arrow defines dependency also, but where there is not a field to remember the entity depended upon.
  • A solid arrow with a diamond at its base defines composition: "A owns B" or "B is part of A" or "if A dies, so does B". This is a stronger form of dependency and occurs when A constructs B.
  • A dashed line with a white arrowhead defines realization: implementation of an interface.

• The word label on a solid arrow gives the name of the attribute that is declared within the class that depends on an entity, and the numerical-star information states the "multiplicity" --- how many of the object are remembered. Names of collections are enclosed in braces, and the - means private.

There is a tool, Nclass, which helps you draw class diagrams and generate Visual Studio projects from them.

Exercise