Smalltalk and Object Orientation: an Introduction - Free

encapsulation. Attempting to introduce the same level of encapsulation in Ada can be done, but it is not
natural to the language.
Object1
Object 2
Key
Interface
Procedures
Data
Figure 1.3: Object structure and interaction
Figure 1.3 illustrates the way in which encapsulation works within object oriente d languages. It
shows that anything outside the object can only gain access to the data the object holds via a specific
interface (the black squares). In turn these interfaces trigger procedures which are internal to the object.
These procedures may then a ccess the data directly, use a second procedure as an intermediary or call
an interface on another object.
1.8 Inheritance
1.8.1 What is inheritance?
A class is an example of a particular type of thing (for example mammals are a class of animal). In the
object oriented world a class is a definition of the characteristics of that thing. Thus in the case of
mammals, we might define that they have fur, are warm blooded and produce live young. Animals such
as dogs and cats are then instances of the class mammal. This is all quite obvious and should not present
a concept problem for anyone. However, in most object oriented languages (the language Self being an
exception) the concept of the class is tightly linked to the concept of inheritance.
Inheritance allows us to state that one class is similar to another class but with a specified set of
differences. Another way of putting it, is that we can define all the things which are common about a
class of things, and then define what is special about each sub grouping within a subclass.
For example, if we have a class defining all the common traits of mammals we can define how
particular categories of mammals differ. Take for example the Duck -billed platypus. This is a quite
extraordinary mammal which differs from other mammals in a number of important ways. However,
we do not want to have to define all the things which it has in common with mammals twice. Not only
is this extra work, but we then have two places in which we have to maintain t his information. We can
therefore state that a Duck -billed platypus is a class of mammal that differs in that it does not produce
live young (we might also want to mention its beak etc. but for now we will ignore these issues).
1.8.2 An example of inheritance
An example which is rather closer to home for most computer scientists is illustrated in Figure 1.4. For
this example we will assume that we have been tasked with the job of designing and implementing an
administration sy stem for our local University. This system needs to record both employees of, and
students attending, the university. For students we need to record what department they are in, what
subjects/classes they are taking (referred to as modules) or what their t hesis is about. For employees we
need to record the department they work in, what their salary is, what subjects they lecture or research.
If they are a professor we need to record that fact. In the case of professors we might also want to
record the government bodies they work for.
Figure 1.4 illustrates a class hierarchy diagram for this application. That is, it illustrates each of the
classes we have defined and from where they inherit their information. There are a number of points
you should note about this diagram:
20