Associations

When classes are connected together conceptually, that connection is called an association.

You visualize the association as a line connecting the two classes, with the name of the association just above the line.

When one class associates with another, each one usually plays a role within that association. You can show those roles on the diagram by writing them near the line next to the class that plays the role.

Association may be more complex than just one class connected to another. Several classes can connect to one class.
Sometimes an association between two classes has to follow a rule. You indicate that rule by putting a constraint near the association line.

Just like a class, an association can have attributes and operations. In this case we have an association class.

You visualize association class the same way you show a regular class, and you use dotted line to connect it to the association line.

Multiplicity is a special type of association which shows the number of objects from one class that relate with a number of objects in an associated class.

One class can be relate to another in a

	one-to-one
	one-to-many
	one-to-one or more
	one-to-zero or one
	one-to-a bounded interval (one-to-two through twenty)
	one-to-exactly n
	one-to-a set of choices (one-to-five or eight)

The UML uses an asterisk (*) to represent more and to represent many.

Sometimes, a class is in association with itself. This can happen when a class has objects that can play a variety of roles. These associations are called reflexive associations.

Reflexive association

Without relationships, a class model would be a list of rectangles that represent a vocabulary of system. Relationships show how the terms in the vocabulary connect with one another to provide a picture of the slice of the world you're modeling. The association is the fundamental conceptual connection between classes. Each class in an association plays a role, and multiplicity specifies how many objects in one class relate to one object in the associated class. Many types of associates are possible.