Containment indicates that one object contains another. Example 4-3 shows a hierarchy of contained classes, each nested within the other; the CreditCard object contains a BillingInfo object, which in turn contains an Address object.
The contained object, if it is used only within the parent object, can be declared as Private. Being restrictive (in terms of scope) at the class level is as important as being restrictive within the class itself. Don't make the class available if it is not needed. Here, Address is an object that could be used by several other classes, such as Order, ShippingInfo, or Invoice. BillingInfo is restricted to the payment and should not exist outside of it.
Public MustInherit Class Payment 'Payment data End Class Public Class Address 'Address1, Address2 'City, ST, Zip End Class Public Class CreditCard : Inherits Payment Private Class BillingInfo Private myAddress As Address 'Methods to get to address here End Class Private myBillingInfo As BillingInfo End Class
Remember that the classes represent a has-a relationship between the parent class and the child class or classes.
The exception to this rule is the various collection classes that contain a type of an object. In this case, the container's primary behavior is to insert, remove, and maintain the objects it contains.
The same rule regarding accessibility that applies to member data also applies to a contained class; it should be private. The ...