Smalltalk and Object Orientation: an Introduction - Free

or return something. It has since become more widely used with languages such as CLOS and
Java also using the term. Two methods are define d in Figure 2.2, one calculates the miles per
gallon while the other one sets the name of the car object.
class Car extends Vehicle
int milage;
int totalFuelUsed;
String name;
define mpg()
begin
return milage / totalFuelUsed;
end
Class
Instance
variables/
fields
define name(aName)
begin
self name = aName;
end
endClass;
Methods
Reference to
the object within
which the method
executes
Figure 2.2: A pseudo code definition of a class
Message. This is a request from one object to another object requesting some operation or data. It is
traditional to say that one object sends a message to another object requesting it to do
something. The idea is that objects are polite well behaved entities which carry out functions
by sending messages to each other. In other languages it might be consider akin to a procedure
call (again this issue will be discussed later in part 2).
Self. This is a reference to the object within which the method is executin g (see Figure 2.2). This means
that it is possible to send messages to this object (i.e. ones’ self).
Single/Multiple inheritance. Single and multiple inheritance refer to the number of superclasses that a
class can inherit from. Smalltalk is a single inheritance system. This means that a class can
only inherit from one superclass. C++ is a multiple inheritance system. This means that C++
classes can inherit from one or more classes.
2.3 Types of hierarchies in object orientation
This can be an area of confusion. In most object oriented systems there are two types of hierarchy , one
refers to inheritance (whether single or multiple) while th e other refers to instantiation. The inheritance
hierarchy (or is -a hierarchy) has already been described. It is the way in which an object “inherits”
features from a superclass.
The instantiation hierarchy relates to instances rather than classes and is important during the
execution of the object. There are in fact two types of instance hierarchy, one indicates a part -of
relationship while the other relates to a using relationship. Figure 2.3 illustrates the differences between
the two.
Part of relation
or
Reference / Using Relation
Figure 2.3: Instance relationships
The difference between an is-a relation and a part-of relationship is often conf using for new
programmers (and sometimes for those who are experienced in one language but are new to an object
oriented programming language such as Smalltalk). Figure 2.4 should make it clear. This figure
illustrates that a student is-a type of person where as an engine is part-of a car. That is, it does not
makes sense to say that a student is part-of a person nor that an engine is-a type of car!
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