UML Basics: Class Diagrams

The basics

The purpose of the class diagram is to show the types being modeled within the system. Its describes the static view of the system. In most UML models these types include:

1. a class
2. an interface
3. a data type
4. a component.

Class name

The UML representation of a class is a rectangle containing three compartments stacked vertically.We can see the figure 1.0 describes the three components.  The top compartment shows the class's name. The middle compartment lists the class's attributes. The bottom compartment lists the class's operations. When drawing a class element on a class diagram, you must use the top compartment, and the bottom two compartments are optional. (The bottom two would be unnecessary on a diagram depicting a higher level of detail in which the purpose is to show only the relationship between the classifiers.) 

Figure 1.0

Class attribute list

The attribute section of a class (the middle compartment) lists each of the class's attributes on a separate line. The attribute section is optional, but when used it contains each attribute of the class displayed in a list format. The line uses the following format:

name : attribute type

length: Double

Inheritance

A very important concept in object-oriented design, inheritance, refers to the ability of one class (child class) to inherit the identical functionality of another class (super class), and then add new functionality of its own. (In a very non-technical sense, imagine that I inherited my mother's general musical abilities, but in my family I'm the only one who plays electric guitar.) To model inheritance on a class diagram, a solid line is drawn from the child class (the class inheriting the behaviour) with a closed, unfilled arrowhead (or triangle) pointing to the super class. Consider types of bank accounts: 

Figure 2 shows how both CheckingAccount and SavingsAccount classes inherit from the BankAccount class.

Figure 2: Inheritance is indicated by a solid line with a closed, unfilled arrowhead pointing at the super class

However, there is an alternative way to draw inheritance called tree notation. You can use tree notation when there are two or more child classes, as in Figure 2, except that the inheritance lines merge together like a tree branch. Figure 3 is a redrawing of the same inheritance shown in Figure 2, but this time using tree notation.

Figure 3: An example of inheritance using tree notation

Abstract classes and operations

The observant reader will notice that the diagrams in Figures 2 and 3 use italicized text for the BankAccount class name and withdrawal operation. This indicates that the BankAccount class is an abstract class and the withdrawal method is an abstract operation. In other words, the BankAccount class provides the abstract operation signature of withdrawal and the two child classes of CheckingAccount and SavingsAccount each implement their own version of that operation.

However, super classes (parent classes) do not have to be abstract classes. It is normal for a standard class to be a super class.

Associations

When you model a system, certain objects will be related to each other, and these relationships themselves need to be modelled for clarity. There are five types of associations. I will discuss two of them — bi-directional and uni-directional associations .

Bi-directional (standard) association

An association is a linkage between two classes. Associations are always assumed to be bi-directional; this means that both classes are aware of each other and their relationship, unless you qualify the association as some other type. Going back to our Flight example, Figure 4 shows a standard kind of association between the Flight class and the Plane class.

Figure 4: An example of a bi-directional association between a Flight class and a Plane class

A bi-directional association is indicated by a solid line between the two classes. At either end of the line, you place a role name and a multiplicity value. Figure 4 shows that the Flight is associated with a specific Plane, and the Flight class knows about this association. The Plane takes on the role of "assignedPlane" in this association because the role name next to the Plane class says so. The multiplicity value next to the Plane class of 0..1 means that when an instance of a Flight exists, it can either have one instance of a Plane associated with it or no Planes associated with it (i.e., maybe a plane has not yet been assigned). Figure 4 also shows that a Plane knows about its association with the Flight class. In this association, the Flight takes on the role of "assignedFlights"; the diagram in Figure 4 tells us that the Plane instance can be associated either with no flights (e.g., it's a brand new plane) or with up to an infinite number of flights (e.g., the plane has been in commission for the last five years).

For those wondering what the potential multiplicity values are for the ends of associations, Table 1 below lists some example multiplicity values along with their meanings.

Table 3: Multiplicity values and their indicators

Potential Multiplicity Values

Indicator	Meaning
0..1	Zero or one
1	One only
0..*	Zero or more
*	Zero or more
1..*	One or more
3	Three only
0..5	Zero to Five
5..15	Five to Fifteen

Uni-directional association

In a uni-directional association, two classes are related, but only one class knows that the relationship exists. Figure 5 shows an example of an overdrawn accounts report with a uni-directional association.

Figure 5: An example of a uni-directional association: The OverdrawnAccountsReport class knows about the BankAccount class, but the BankAccount class does not know about the association

A uni-directional association is drawn as a solid line with an open arrowhead (not the closed arrowhead, or triangle, used to indicate inheritance) pointing to the known class. Like standard associations, the uni-directional association includes a role name and a multiplicity value, but unlike the standard bi-directional association, the uni-directional association only contains the role name and multiplicity value for the known class. In our example in Figure 5, the OverdrawnAccountsReport knows about the BankAccount class, and the BankAccount class plays the role of "overdrawnAccounts." However, unlike a standard association, the BankAccount class has no idea that it is associated with the OverdrawnAccountsReport. [Note: It may seem strange that the BankAccount class does not know about the OverdrawnAccountsReport class. This modeling allows report classes to know about the business class they report, but the business classes do not know they are being reported on. This loosens the coupling of the objects and therefore makes the system more adaptive to changes.]

Aggregation

Aggregation is a special type of association used to model a " whole to its parts" relationship. In basic aggregation relationships, the life cycle of a part class is independent from the whole class's lifecycle.

For example, we can think of Car as a whole entity and Car Wheel as part of the overall Car. The wheel can be created weeks ahead of time, and it can sit in a warehouse before being placed on a car during assembly. In this example, the Wheel class's instance clearly lives independently of the Car class's instance. However, there are times when the part class's lifecycle is not independent from that of the whole class — this is called composition aggregation. Consider, for example, the relationship of a company to its departments. Both Company and Departments are modeled as classes, and a department cannot exist before a company exists. Here the Department class's instance is dependent upon the existence of the Company class's instance.

Let's explore basic aggregation and composition aggregation further.

Basic aggregation
An association with an aggregation relationship indicates that one class is a part of another class. In an aggregation relationship, the child class instance can outlive its parent class. To represent an aggregation relationship, you draw a solid line from the parent class to the part class, and draw an unfilled diamond shape on the parent class's association end. Figure 6 shows an example of an aggregation relationship between a Car and a Wheel.

Figure 6: Example of an aggregation association

Composition aggregation
The composition aggregation relationship is just another form of the aggregation relationship, but the child class's instance lifecycle is dependent on the parent class's instance lifecycle. In Figure 7, which shows a composition relationship between a Company class and a Department class, notice that the composition relationship is drawn like the aggregation relationship, but this time the diamond shape is filled.

Figure 7: Example of a composition relationship

In the relationship modeled in Figure 7, a Company class instance will always have at least one Department class instance. Because the relationship is a composition relationship, when the Company instance is removed/destroyed, the Department instance is automatically removed/destroyed as well. Another important feature of composition aggregation is that the part class can only be related to one instance of the parent class (e.g. the Company class in our example).

Reflexive associations

We have now discussed all the association types. As you may have noticed, all our examples have shown a relationship between two different classes. However, a class can also be associated with itself, using a reflexive association. This may not make sense at first, but remember that classes are abstractions. Figure 8 shows how an Employee class could be related to itself through the manager/manages role. When a class is associated to itself, this does not mean that a class's instance is related to itself, but that an instance of the class is related to another instance of the class.

Figure 8: Example of a reflexive association relationship

The relationship drawn in Figure 8 means that an instance of Employee can be the manager of another Employee instance. However, because the relationship role of "manages" has a multiplicity of 0..*; an Employee might not have any other Employees to manage.

Visibility

In object-oriented design, there is a notation of visibility for attributes and operations. UML identifies four types of visibility: public, protected, private, and package.

The UML specification does not require attributes and operations visibility to be displayed on the class diagram, but it does require that it be defined for each attribute or operation. To display visibility on the class diagram, you place the visibility mark in front of the attribute's or operation's name. Though UML specifies four visibility types, an actual programming language may add additional visibilities, or it may not support the UML-defined visibilities. Table 2 displays the different marks for the UML-supported visibility types.

Table 2: Marks for UML-supported visibility types

Mark	Visibility type
+	Public
#	Protected
-	Private
~	Package

Now, let's look at a class that shows the visibility types indicated for its attributes and operations. In Figure 9, all the attributes and operations are public, with the exception of the updateBalance operation. The updateBalance operation is protected.

Figure 9: A BankAccount class that shows the visibility of its attributes and operations

Interfaces
An interface is a specification of behavior that implementers agree to meet; it is a contract. By realizing an interface, classes are guaranteed to support a required behavior, which allows the system to treat non-related elements in the same way – that is, through the common interface.

Interfaces may be drawn in a similar style to a class, with operations specified, as shown below. They may also be drawn as a circle with no explicit operations detailed. When drawn as a circle, realization links to the circle form of notation are drawn without target arrows.

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