Class Diagrams

A class diagram gives an overview of a system by showing its classes and the relationships among them. Class diagrams are static: they display what interacts but not what happens during the interaction.

The class diagram that follows models a customer order from a retail catalog. The central class is the Order. Associated with it are the Customer making the purchase and the Payment. There are three types of payments: Cash, Check, or Credit. The order contains OrderDetails (line items), each with its associated Item.

UML class notation is a rectangle divided into three parts: class name, attributes, and operations. Names of abstract classes, such as Payment, are in italics. Relationships between classes are the connecting links.

NB: In Together, the rectangle is further divided with separate partitions for properties and inner classes or inner interfaces.

The class diagram used in this example has three kinds of relationships.

Association: A relationship between instances of the two classes. There is an association between two classes if an instance of one class must know about the other in order to perform its work. In a diagram, an association is a link connecting two classes.
Aggregation: An association in which one class belongs to a collection. An aggregation has a diamond end pointing to the part containing the whole. In this diagram, Order has a collection of OrderDetails.
Generalization: An inheritance link indicating one class is a superclass of the other. A generalization has a triangle pointing to the superclass. Payment is a superclass of Cash, Check, and Credit.

An association has two ends. An end may have a role name to clarify the nature of the association. For example, an OrderDetail is a line item of each Order.

A navigation arrow on an association shows which direction the association can be traversed or queried. An OrderDetail can be queried about its Item, but not the other way around. The arrow also lets you know who "owns" the association's implementation; in this case, OrderDetail has an Item. Associations with no navigation arrows are bi-directional.

NB: In Together, by default, all associations are drawn without arrows, and bi-directional associations are currently not supported.

The multiplicity of an association end is the number of possible instances of the class associated with a single instance of the other end. Multiplicities are single numbers or ranges of numbers. In our example, there can be only one Customer for each Order, but a Customer can have any number of Orders.

This table lists the most common multiplicities:

Multiplicities	Meaning
0..1	zero or one instance. The notation n . . m indicates n to m instances.
0..* or *	no limit on the number of instances (including none).
1	exactly one instance
1..*	at least one instance

Every class diagram has classes and associations. Navigability, roles, and multiplicities are optional items placed in a diagram to provide clarity.

Using Class Diagrams as Views

Class diagrams can also be used to create subviews of the project.

By creating a new class diagram and using the Shortcut option, you can easily and quickly build subsets views for easier management. Using this feature, you can create views of distributed classes into one diagram, with Together automatically displaying any relationships that the gathered classes may have with each other.

Note that changes made here also update the source code, keeping diagram and source in sync.