Smalltalk and Object Orientation: an Introduction - Free

8. An Example Smalltalk Class
8.1 Introduction
You should now be ready to write some Smalltalk code. So this chapter will take you through a worked
example. It is a very simple example, in which you will create a new class, define some instance
variables and write a couple of methods.
8.2 The class Person
The Person class will provide a very basic set of features. It will record a person’s name and their age.
It will also allow a person to have a birthday (and thus increment their age).
8.2.1 Creating the class
The first thing you should do is to create a new class category to put your class into. This is done in the
class category w indow of the System Browser. Use the right button menu (middle on a three button
mouse) and select the add option. This option will prompt for a category name. It is best if you use a
meaningful category name. I have used the category name “Example Class”.
Once you have provided a category name it will be immediately created. However, note that it will
add the category at the bottom of the list of categories or, if you have selected (highlighted) an existing
category, immediately above that category. This means that if you have accidentally selected a category
in the middle of the list, your new category will be added in the middle of the category list. It is easier to
find your own categories if they are not mixed up with other categories. It is therefore advisable to
make sure that no categories are selected when you create a new one.
Next define a new class. You can do this by filling out the template in the bottom window
(commonly called the code view) of the System Browser to mirror the following:
Object subclass: #Person
instanceVariableNames: ' name age '
classVariableNames: ''
poolDictionaries: ''
category: 'Example Class'
That is, define the class Person as a subclass of Object and give it two instance variables name
and age. (Note that if you have called your class category something other than “ Example Class”
the category field will be different).
Notice in Visualworks, that the template fills in the superclass, the class name and the variable fields
with default values, e.g.:
NameOfSuperclass subclass: #NameOfClass
instanceVariableNames: 'instVarName1 instVarName2'
classVariableNames: 'ClassVarName1 ClassVarName2'
poolDictionaries: ''
If you do not delete these fields, they will be included in the compiled class. For example, it is easy
to find that you have four instance variables and two class variable s. The additional variables having
names such as instVarName1 and ClassVarName1.
If you have defined the new class correctly, then at this point your browser should l ook like that in
Figure 8.1. Do not worry if the list of class categories in the right most window is different, the
important points are that the class definitions match and that the highlighted text is the same.
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