Visual Basic.NET How to Program (PDF)

306 Object-Based Programming Chapter 8
Often, programmers do not have to create classes “from scratch.” Rather, they can derive
classes from other classes that provide behaviors required by the new classes. Classes also can
include references to objects of other classes as members. Such software reuse can greatly
enhance programmer productivity. Chapter 9 discusses inheritance—the process by which
new classes are derived from existing classes. Section 8.8 discusses composition (aggregation),
in which classes include as members references to objects of other classes.
8.3 Class Scope
In Section 6.11, we discussed method scope; now, we discuss class scope. A class’s instance
variables and methods belong to that class’s scope. Within a class’s scope, class
members are accessible to all of that class’s methods and can be referenced by name. Outside
a class’s scope, class members cannot be referenced directly by name. Those class
members that are visible (such as Public members) can be accessed only through a “handle”
(i.e., members can be referenced via the format objectReferenceName.memberName).
If a variable is defined in a method, only that method can access the variable (i.e., the
variable is a local variable of that method). Such variables are said to have block scope. If
a method defines a variable that has the same name as a variable with class scope (i.e., an
instance variable), the method-scope variable hides the class-scope variable in that
method’s scope. A hidden instance variable can be accessed in a method by preceding its
name with the keyword Me and the dot operator, as in Me.mHour. We discuss keyword
Me later in this chapter.
8.4 Controlling Access to Members
The member access modifiers Public and Private control access to a class’s instance
variables and methods. (In Chapter 9, we introduce the additional access modifiers Protected
and Friend.)
As we stated previously, Public methods serve primarily to present to the class’s clients
a view of the services that the class provides (i.e., the Public interface of the class).
We have mentioned the merits of writing methods that perform only one task. If a method
must execute other tasks to calculate its final result, these tasks should be performed by a
utility method. A client does not need to call these utility methods, nor does it need to be
concerned with how the class uses its utility methods. For these reasons, utility methods are
declared as Private members of a class.
Common Programming Error 8.2
Attempting to access a Private class member from outside that class is a syntax error. 8.2
The application of Fig. 8.3 demonstrates that Private class members are not accessible
outside the class. Line 9 attempts to access Private instance variable mHour of
CTime object time. The compiler generates an error stating that the Private member
mHour is not accessible. [Note: This program assumes that the CTime class from Fig. 8.1
is used.]
Good Programming Practice 8.5
We prefer to list instance variables of a class first, so that, when reading the code, programmers
see the name and type of each instance variable before it is used in the methods of the class. 8.5