Smalltalk and Object Orientation: an Introduction - Free

24.2.4 Adding variables to system classes
Always add class and instance variables to the end of any exist ing variables in a system class. This is
because the interpreter on some Smalltalk implementations accesses the variables of certain classes by
position rather than by name!
24.2.5 Accessing instance and class variables
In general it is always better to access in stance and class variables via intermediate methods, referred to
as accessor methods, rather than access or set them directly. This is called variable-free programming.
This promotes the modularity of your methods and insulates the methods against changes in the way the
object (or class) holds instance (or class) information. This is a very important concept, as direct access
to instance variables can limit the power of subclassing.
It is also possible to protect the instance (or class) variables from undes ired changes. For example,
you can put preconditions on an access method, or return a copy of the contents of the variable so that it
cannot be directly affected.
24.3 Classes
24.3.1 Naming classes
The naming of a class is extremely important. The class is the core el
program. Class names are always capitalized. For example:
ement in any object oriented
• Collection
• GraphicalClassBrowser
• EmployeePensionPaymentsHistory
The second and third class names above provide extremely good examples of how a class name can
describe the class. This is because the name of the class is used by most developers to indicate its
purpose or intent. This is partly due to the fact that it is the class name which will be used when
searching for appropriate classes (for example by using the find class... menu option).
The class name should therefore be descriptive, for example, classes with names such as MyClass or
ProjectClass1 are of little use. However, class names should not be so specific that they make it appear
that the class is unlikely to be of use except in one specific situation (unless of course this is the case).
For example, in an application to record details about university lecturers, a class with a name such as
ComputerScienceDepartmentLecturer is probably not appropriate unless it really does relate
only to lecturers in the computer science department. If this is the case, you need to ask yourself in what
way are computer science lecturers special over other lecturers.
24.3.2 The role of subclasses
A subclass should only be used to modify the behaviour of its parent class. This modification should be
a refinement of the class; this can be accomplished in three ways (or in combination):
• Changes to the external protocol, that is the set of messages that the instances o f the class respond
to.
• Changes in the implementation of the methods, that is changes in the way the messages are
handled.
• Additional behaviour, which references inherited behaviour.
If a subclass does not provide one or more of the above, then it is incorrectly placed. For example, if
a subclass implements a set of new methods, but no reference is made to the parent classes’ instance
variables or methods, then this class is not really a subclass of the parent (it doesn’t extend it).
197