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A property is a characteristic 1of a
model element. Use properties to define attributes and rules for a
given type of modeling element. For example, properties can define
the following: attributes representing a project’s
start and end dates, a specific textual description of the
relationship between a project and the things that make up the
project, and a rule that the start date must precede the end date.
Properties are shown as a comma-delimited list of text strings inside
a pair of braces ({}) after or below the name of a model element.
Each property may be expressed in any natural or computer language.
Each text string may be a tag or constraint, both of which are
discussed in the next sections.
Tags
A tag is an attribute of a
model element and its
corresponding value — for example, attributes representing a
project’s start and end dates.
Defining tags
In the UML, you create
a tag
definition when defining a stereotype by showing a name
for the attribute, called
a keyword,
followed by a colon followed by the type of the attribute. The tags
defined for a stereotype apply to each model element to which the
stereotype is applied.
For example, Figure 9-4 updates Figure 9-1 and shows a tag named StartDate (which is a string representing a
project’s start date), a tag named
EndDate (which is a string
representing a project’s end date), and a tag named
Descripton (which is a string representing a
description of the relationship between a project and a thing that
makes up the project). The StartDate and EndDate tags are defined for the
Project stereotype, and the
Descripton tag is defined for the
MadeOf stereotype.
Figure 9-4. Defining stereotypes and tags
Applying tags
In the UML, applying a
tag (called a
tagged value) is done when applying a stereotype
by showing the name of the attribute followed by an equal sign
followed by its value, together called
a keyword-value
pair. When no equal sign or default value is used, it is
assumed that the keyword represents a Boolean value of
True, and the absence of the keyword represents a
Boolean value of False.
For example, Figure 9-5 updates Figure 9-3 using the tags in Figure 9-4, and shows that the Proj Mngmnt Sys project starts on January 1, 2003 and ends on December
31, 2003. The Java Deployment project starts on
December 31, 2003 and ends on January 1, 2003, which is obviously
wrong, because the start date is more recent than the end date.
Constraints are used to ensure that such errors are not allowed and
are discussed in the next section. Figure 9-5 also
shows a description for the ProjMngmnt Sys project’s MadeOf stereotyped link and indicates that the
description for the Java Deployment
project’s Made Of stereotyped
link is empty or blank.
Figure 9-5. Applying stereotypes and tags
Constraints
A constraint is a
rule for a model
element—for example, a rule that a project’s
start date must precede the project’s end date. The
rules defined for a stereotype apply to each model element to which
the stereotype is applied, and each model element to which the
stereotype is applied must adhere to the rules.
In the UML, a constraint is shown when defining a stereotype as a
text string that may be expressed using the
Object Constraint Language (OCL), which is discussed in Chapter 10. Constraints may also be expressed using
pseudocode or another language. For example, you can express a
constraint using Java, C++, C#, or some other programming or
nonprogramming language.
Figure 9-6 also shows the following constraint on
the relationships between a project and the things that make up a
project:
The Description must not be empty.
According to these rules, the Proj Mngmnt Sys
project described in Figure 9-5 is considered
valid. It’s start date precedes its end date. On the
other hand, the JavaDeployment
project described in Figure 9-5 is considered
invalid or erroneous because it starts
after it ends.
Figure 9-6. Defining stereotypes, tags, and constraints
