UML Weekend Crash Course™ - To Parent Directory

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Saturday Afternoon
Introducing Elements of the Object Diagram Notation
The Object diagram consists of just two elements: objects and links. You know already that
an object is a real entity created from a class, a definition of a type of object. In the same
way, a link is created from an association, the definition of a type of relationship. A link
represents a relationship between two objects. An association defines a type of relationship
and the rules that govern it.
Figure 13-1 shows an object called Tom. Like the Class notation, the Object notation has a
name compartment at the top of the box. The name includes the name of the object and the
name of the class to which the object conforms: “Customer.” This helps distinguish the object
Tom of type Customer from the object Tom of type Employee. This notation also allows you to
model an example or test case in which many objects of the same class participate (for example,
one employee supervises another employee).
Tom: Customer
custID = 123456
name = Tom Pender
address = 1234 UML Ave
Figure 13-1 UML Object notation
Use the format object name : class name to fully define the object name. Then the entire
expression is underlined. You may also use the abbreviated form, : class name, without the
object name. This form is called an anonymous object. It is used when you want to draw an
example where it doesn’t really matter which specific object participates in the example
because any object of the named class would behave in exactly the same way.
The second compartment in the object box contains facts about the attributes. Each
attribute is named and assigned a value. In Figure 13-1, you see name = Tom Pender. An
object is real. It exists. So it can have values assigned to each attribute. Even a value of
blank or null is a value that distinguishes the condition of the object from other possibilities.
Note how different this is from the class attribute compartment. The class contained
definitions for each attribute and contained no values. Again, the class is a set of rules
where the object is a set of facts. The class says that an Employee object may have a name
attribute that contains a String value 20 characters long. The object says that the name
attribute does contain the value “Tom Pender.”
Comparing the Object Diagram and the Class Diagram Notations
Figure 13-2 contains a Class diagram showing the rules regarding Shipment and Product
and the relationship between the two objects. The Class diagram defines the attributes that
must be used to define each type of object and the behaviors that each type of object must
support.