Smalltalk and Object Orientation: an Introduction - Free

The result of sending this method is illustrated in Figure 6.4 along with the structure of a class. The
class Person receives the message new which causes the cl ass method new to execute which
generates a new instance of the class, with its own copy of the instance variables age and name.
Class: MyClass
Instance Variables
age, name
Class variables
numberOfInstances
new
MyClass new.
Instance methods
birthday
Class methods
new
anInstance
age: 31
name: 'John'
Figure 6.4: Instance creation
The issue of classes having methods, some of which are intended for an instance of the class, and
some of which are intended for the class, is not as complicated as it may at first seem. Not least because
the tools used with Smalltalk tend to keep the two sides of the classes pretty well distinct. In an a ttempt
to make it clearer here are some definitions:
• Instance variables: Defined by the class, but a copy is maintained in each instance which has its
own value for that instance variable.
• Class variables: Defined in the class with a single copy in the cl ass accessible by instances of
the class.
• Class instance variables: Defined in the class with a single copy in the class and only accessible
by the class.
• Instance methods: Defined in the class with a single copy maintained in the class but executed
within the context of an object.
• Class methods: Defined in the class with a single copy maintained in the class and executed
within the context of the class.
Some of these concepts will be considered in greater detail later.
6.3.6 Classes in Smalltalk
There are very many classes in any Smalltalk system, e.g. in VisualWorks there over 1,000 classes.
However, you will only need to become familiar with a very few of them. The remaining classes
provide facilities that you use without even realizing it.
6.4 Method definitions
Methods provide a way of defining the behavior of an object i.e. what the object does. For example, a
method may change the state of the object or it may retrieve some information. A method is the
equivalent of a procedure in most other languages. A method can only be defined within the scope of an
object (and there is no concept such as the main method as there is in C). It has a specific structure:
messagePattern arguments
"comment"
| temporaries |
statements
where the messagePattern represents the name of the method and the arguments represent the names
of arguments. These arguments are accessible within the method.
The “comment” field is a comment describing the operation performed by the method a nd any
other useful information. Note that comments cannot be nested in Smalltalk. This can be awkward if
you wish to comment out some code for later. For example, consider the following piece of Smalltalk
which I have just commented out:
“
x := 12 * 4.
66