Applying UML and Patterns: An Introduction to ObjectOriented Analysis and Design and Iterative Development, Third Edition [Electronic resources]

32.3. Defining Conceptual Superclasses and Subclasses

Since it is valuable to identify conceptual super- and subclasses, it is useful to clearly and precisely understand generalization, superclasses, and subclasses in terms of class definition and class sets.^MO95].

Generalization and Conceptual Class Definition

What is the relationship of a conceptual superclass to a subclass?

For example, consider the superclass

Payment and its subclasses (

CashPayment , and so on). Assume the definition of

Payment is that it represents the transaction of transferring money (not necessarily cash) for a purchase from one party to another, and that all payments have an amount of money transferred. The model corresponding to this is shown in Figure 32.3.

A

CreditPayment is a transfer of money via a credit institution which needs to be authorized. My definition of

Payment encompasses and is more general than my definition of

CreditPayment .

Generalization and Class Sets

Conceptual subclasses and superclasses are related in terms of set membership.

For example, in terms of set membership, all instances of the set

CreditPayment are also members of the set

Payment . In a Venn diagram, this is shown as in Figure 32.4.

Conceptual Subclass Definition Conformance

When a class hierarchy is created, statements about superclasses that apply to subclasses are made. For example, Figure 32.5 states that all

Payments have an

amount and are associated with a

Sale .

All

Payment subclasses must conform to having an amount and paying for a

Sale . In general, this rule of conformance to a superclass definition is the

100% Rule :

Conceptual Subclass Set Conformance

A conceptual subclass should be a member of the set of the superclass. Thus,

CreditPayment should be a member of the set of

Payments .

Informally, this expresses the notion that the conceptual subclass

is a kind of superclass.

CreditPayment is a kind of Payment . More tersely,

is-a-kind-of is called

is-a .

This kind of conformance is the

Is-a Rule :

For instance, the statement

CreditPayment is a Payment makes sense, and conveys the notion of set membership conformance.

What Is a Correct Conceptual Subclass?

From the above discussion, apply the following tests^[3] to define a correct subclass when constructing a domain model:

^[3] These rule names have been chosen for their mnemonic support rather than precision.