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10.2 Simple Constraints
Simple constraints that involve one object, or two objects and their
link, provide the foundation for working with more complex
constraints that involve multiple objects and their links.
10.2.1 Attributes
To specify a rule about the
values
of an object's
attributes, you can refer to an object's attribute
from the object itself by using the name of the attribute.Within the context of an organization, the following expression
indicates that the name of an organization may not be an empty
string:
Name <> ''
Using the keyword self, this rule may be described
as follows:
self.Name <> ''
Notice that a literal string is enclosed in single quotes.Figure 10-1 to
show how the rule is captured. Look specifically at the top
compartment for Organization.
Figure 10-2. Organizations with an OCL expression indicating that organizations must have names
Within the context of a person, the following expression indicates
that a person must work more than 10 hours and less than or equal to
40 hours:
HoursAvailable > 10 and HoursAvailable <= 40
Using the keyword self, this rule may be described
as follows:
self.HoursAvailable > 10 and self.HoursAvailable <= 40
Notice that the keyword self is used at the
beginning of each occurrence of the HoursAvailable
attribute. Also notice that I can access this attribute even though
it is private, because the expression is written within the context
of a person. If the context of the rule is not a person object, I
would not be able to access the attributes directly in the
expression.
10.2.2 Operations
To specify a rule that uses an
object's
operations, you can refer to an object's operation
from the object itself by using the name of the operation followed by
parentheses containing an optional comma-separated list that
specifies the actual parameters passed to the operation. If the
operation does not have any parameters, the parentheses are mandatory
and left empty.Within the context of a project, the following expression uses the
validStartEndDates operation to check that the
start and end dates are valid:
validStartEndDates ( )
Using the keyword self, this rule may be described
as follows:
self.validStartEndDates ( )
Notice that because the operation returns a Boolean value, I did not
have to compare it to anything. If the operation returns a value of
True, the rule is satisfied. If the operation
returns a value of False, the rule is not
satisfied.
10.2.3 Link Ends
As discussed in Chapter 3, a link
end is
an endpoint of a link that connects the
link to an object.To refer to an object across a link, use the rolename attached to the
link end. If the link end does not have a rolename, use the class
name of the object of the link end, but start the class name with a
lowercase character.
10.2.3.1 Link ends with a multiplicity of 1
Within the context of a person, the
following partial expression returns the
organization related to the person using the rolename attached to the
link end:
self.employer
Within the context of a project, the following partial expression
returns the organization related to the project using the class name
of the object of the link end, but starting with a lowercase
character:
self.organization
In these two expressions referring to a person's
organization and a team's organization, because the
multiplicity on the association end related to the link end attached
to the Organization class has a maximum of 1, the
result is an object of the class connected to the link end. That is,
each previous expression results in a single object of the
Organization class. Also notice that I used the
class name to refer to the organization from the project, but I used
the rolename to refer to the organization from the person. You can
use the class name if no rolename is specified; otherwise, you can
use the rolename.To refer to a characteristic of a single object, use a dot. Within
the context of a person, the following expression indicates that the
name of the organization related to a person cannot be an empty
string:
self.employer.Name <> ''
Within the context of a project, the following expression indicates
that the name of the organization related to a project cannot be an
empty string:
self.organization.Name <> ''
10.2.3.2 Link ends with a multiplicity of more than 1
Within the context of an organization, the following partial expression returns
the collection of people related to an organization using the
rolename attached to the link end:
self.employee
Within the context of an organization, the following partial
expression returns the collection of projects related to an
organization using the class name (starting with a lowercase
character) of the objects at the link end:
self.project
In these two expressions referring to an
organization's people and an
organization's projects, because the multiplicity on
the association ends related to the link ends attached to the
People class and Project class
have no maximum or have a maximum of more than 1, the result is a set
collection of objects of the class connected to the link end. Such a
set is a collection of unique and unordered objects. That is, the
first expression above results in a set of objects of the
Person class and the second expression results in
a set of objects of the Project class.To refer to a characteristic of a collection, use an arrow
(->). Within the context of an organization,
the following expression indicates that the names of the people
related to an organization may not be an empty string:
self.employee->forAll (e : Person | e.Name <> '')
The forAll in this expression indicates a rule
that must be satisfied by each element of the set of employees. The
first part of the expression enclosed in the parentheses indicates
that each element of the set is a person that is referenced using the
variable e. The second part of the expression,
which is separated from the first part by a vertical bar
(|), indicates the rule that each element of the
set must satisfy. If any employee has an empty name, the result of
the forAll is False; otherwise,
the result is True.Within the context of an organization, the following expression
indicates that the names of the projects related to an organization
cannot be an empty string:
self.project->forAll (p : Project | p.Name <> '')
The forAll in this expression indicates a rule
that must be satisfied by each element of the set of projects. The
first part of the expression enclosed in the parentheses indicates
that each element of the set is a project that is referenced using
the variable p. The second part of the expression,
which is separated from the first part by a vertical bar
(|), indicates the rule that each element of the
set must satisfy. If any project has an empty name, the result of the
forAll is False; otherwise, the
result is True.Within the context of an organization, the following expression
indicates that each person who is employed by an organization must
work more than 10 hours and less than or equal to 40 hours:
self.employee->forAll (e : Person | e.isHoursAvailableInRange (10, 40))
Notice that I must use the isHoursAvailableInRange
operation, and that I cannot use the
HoursAvailable attribute because it is private to
the person objects and not accessible from the context of an
organization.
10.2.4 Link Objects
As discussed in Chapter 3, a link
object is a
specific
instance of an association that links multiple objects.Given a link and the objects it relates, you can refer to a link
object from the objects that are linked, and you can refer to the
objects that are linked from the link object.
10.2.4.1 Referring to link objects
To refer to link objects from an object on
one end of a link, use the class name
of the link object, but starting with a lowercase character, which
results in a set collection of objects of the link's
class.Within the context of a team or a person, the following expression
indicates that the title of each role object of a team or a person
may not be an empty string:
self.role->forAll (r : Role | r.Title <> '')
Notice that this expression may be used within the context of a team
or a person, because it references the links between teams and
people.
10.2.4.2 Referring to linked object
To refer to the object of a link end from
a link object, use the rolename
attached to the link end. If the link end does not have a rolename,
use the class name of the object of the link end, but starting with a
lowercase character, which results in exactly one object.Within the context of a role, the following expression indicates that
the name of the team to which a role applies may not be an empty
string using the class name:
self.team.Name <> ''
Within the context of a role, the following expression indicates that
the name of the person to which a role applies may not be an empty
string using the rolename:
self.member.Name <> ''
Notice that these are two different expressions used within the
context of a role, because each expression references a different link
end.
