UML class diagrams show the classes of the system, their
inter-relationships, and the operations and attributes of the classes.
Class diagrams are typically used, although not all at once, to:
domain concepts in the form of a domain model
requirements in the form of a conceptual/analysis model
the detailed design of object-oriented or object-based software
A class model is comprised of one or more class diagrams
and the supporting specifications that describe model elements including
classes, relationships between classes, and interfaces. There
are guidelines for:
Because class diagrams are used for a variety of purposes
– from understanding requirements to describing your detailed design – you
will need to apply a different style in each circumstance. This section describes style guidelines pertaining to
different types of class diagrams.
1. Analysis and design versions of a class.
2. Modeling association classes.
- Identify Responsibilities on Domain Class Diagrams.
- Indicate Visibility Only On Design Models.
- Indicate Language-Dependent Visibility With Property Strings.
- Indicate Types Only On Design Models.
- Indicate Types On Analysis Models Only When The Type is an Actual
- Design Class Diagrams Should Reflect Language Naming Conventions. In
Figure 1 you see that the design
version of the
class uses names that conform to common
conventions such as
- Model Association Classes On Analysis Diagrams. Figure 2 shows that association classes are depicted as class
attached via a dashed line to an association – the association line, the
class, and the dashed line are considered one symbol in the UML.
- Do Not Name Associations That Have Association Classes.
- Center The Dashed Line of an Association Class.
A class is effectively a template from which objects are
created (instantiated). Although in
the real world Doug, Wayne, John, and Bill are all student objects we would
model the class Student instead.
Classes define attributes, information that is pertinent to their
instances, and operations, functionality that the objects support.
Classes will also realize interfaces (more on this later). Note that you may need to soften some of the naming
guidelines to reflect your implementation language or software purchased from a
3. The OrderItem class with and without scaffolding code.
4. Indicating the exceptions thrown by an operation.
+ findAllInstances(): Vector
- Use Common Terminology for Names
- Prefer Complete Singular Nouns for Class Names
- Name Operations with a Strong Verb
- Name Attributes With a Domain-Based Noun
- Do Not Model Scaffolding Code. Scaffolding code refers to the attributes and operations
required to implement basic functionality within your classes, such as the code
required to implement relationships with other classes.
3 depicts the difference between the
class without scaffolding code and with it.
- Never Show Classes With Just Two Compartments
- Label Uncommon Class Compartments
- Include an Ellipsis ( … ) At The End of Incomplete Lists
- List Static Operations/Attributes Before Instance Operations/Attributes
- List Operations/Attributes in Decreasing Visibility
- For Parameters That Are Objects, Only List Their Type
- Develop Consistent Method Signatures
- Avoid Stereotypes Implied By Language Naming Conventions
- Indicate Exceptions In An Operation’s Property String. Exceptions can be indicated with a UML property string, an example of
which is shown in Figure 4.
interface is a collection of operation signature and/or
attribute definitions that ideally defines a cohesive set of behaviors.
Interfaces are implemented, “realized” in UML parlance, by classes
and components – to realize an interface a class or component must implement
the operations and attributes defined by the interface.
Any given class or component may implement zero or more interfaces and
one or more classes or components can implement the same interface.
5. Interfaces on UML class diagrams.
- Interface Definitions Must Reflect Implementation Language Constraints. In
Figure 5 you see that a standard
class box has been used to define the interface
(note the use of the
- Name Interfaces According To Language Naming Conventions
- Apply “Lollipop” Notation To Indicate That A Class Realizes an
- Define Interfaces Separately From Your Classes
- Do Not Model the Operations and Attributes of an Interface in Your
Classes. In Figure 5 you’ll notice that the
Shipment class does not include the attributes or operations defined by the two
interfaces that it realizes.
- Consider an Interface to Be a Contract
For ease of discussion the term relationships shall include
all UML concepts such as associations, aggregation, composition, dependencies,
inheritance, and realizations – in other words, if it’s a line on a UML
class diagram we’ll consider it a relationship.
6. Shipping an order.
7. Modeling an order.
8. Professors and seminars.
9. Modeling people at a university.
Do Not Model Every Single Dependency
- Model Relationships Horizontally
- Collaboration Indicates Need for a Relationship
- Model a Dependency When The Relationship is Transitory
- Depict Similar Relationships Involving A Common Class As A Tree. In
Figure 6 you see that both
Order have a
Note how the two dependencies are drawn in combination in “tree
configuration”, instead of as two separate lines, to reduce clutter in the
- Always Indicate the Multiplicity
- Avoid a Multiplicity of “*”
- Replace Relationships By Indicating Attribute Types. In
Figure 7 you see that Customer
attribute of type
part of the scaffolding code to maintain the association between customer
objects and address objects.
- Do Not Model Implied Relationships
Center Names on Associations
Write Concise Association Names In Active Voice
Indicate Directionality To Clarify An Association Name
Name Unidirectional Associations In The Same Direction
Word Association Names Left-To-Right
Indicate Role Names When Multiple Associations Between Two Classes Exist
Indicate Role Names on Recursive Associations
Make Associations Bi-Directional Only When Collaboration Occurs In Both
Directions. The lives at association of Figure
9 is uni-directional.
Redraw Inherited Associations Only When Something Changes
Question Multiplicities Involving Minimums And Maximums
Inheritance models “is a” and “is like”
relationships, enabling you to easily reuse existing data and code. When “A”
inherits from “B” we say that “A” is the subclass of “B” and that
“B” is the superclass of “A.” Furthermore,
we say that we have “pure inheritance” when “A” inherits all of the
attributes and methods of “B.” The UML modeling notation for inheritance is
a line with a closed arrowhead pointing from the subclass to the superclass.
- Apply the Sentence Rule For Inheritance
- Place Subclasses Below Superclasses
- Beware of Data-Based Inheritance
- A Subclass Should Inherit Everything
Sometimes an object is made up of other objects.
For example, an airplane is made up of a fuselage, wings, engines,
landing gear, flaps, and so on. A
delivery shipment contains one or more packages.
A team consists of two or more employees.
These are all examples of the concept of aggregation, which represents
“is part of” relationships. An
engine is part of a plane, a package is part of a shipment, and an employee is
part of a team. Aggregation is a
specialization of association, specifying a whole-part relationship between two
objects. Composition is a stronger
form of aggregation where the whole and parts have coincident lifetimes, and it
is very common for the whole to manage the lifecycle of its parts.
From a stylistic point of view, because aggregation and composition are
both specializations of association the guidelines for associations apply.
Figure 10. Examples of
aggregation and composition.
- Apply the Sentence Rule for Aggregation
- You Should Be Interested In Both The Whole And The Part
- Depict the Whole to the Left of the Part
- Apply Composition to Aggregates of Physical Items
- Apply Composition When the Parts Share The Persistence Lifecycle With the
- Don’t Worry About Getting the Diamonds Right
The Elements of UML 2.0 Style describes a collection
of standards, conventions, and
for creating effective
UML diagrams. They are based on sound, proven
software engineering principles that lead to diagrams
that are easier to understand and work with. These
conventions exist as a collection of simple, concise
guidelines that if applied consistently, represent an
important first step in increasing your productivity as
a modeler. This book is oriented towards
intermediate to advanced UML modelers, although there
are numerous examples throughout the book it would not
be a good way to learn the UML (instead, consider
The Object Primer). The book is a brief 188
pages long and is conveniently pocket-sized so it's easy
to carry around.
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