Computer Programming with GNU Smalltalk - Free

58 Computer Programming with GNU Smalltalk
Multiple Inheritance and Single Inheritance
Some programming languages allow programmers to create classes which inherits properties and
behavior from more than one class. So, some classes have more than one superclass. This is called
multiple inheritance. Other inheritance type, which allows only one superclass per class is called single
inheritance.
The examples we have given so far are of single inheritance and the examples we will give from now
on will also be of single inheritance because Smalltalk uses single inheritance to manage classes. It is
still very arguable whether multiple inheritance is needed in object oriented design or only makes the
inheritance more complex.
Polymorphism
In a world full of different kinds of objects, some objects may response the same message although
they are different kind of objects, meaning they are instances of different classes. That's why some
programming languages allow us to define methods with same selector in different classes. Or they
allow a subclass to change a selector's behavior which it inherited from its superclass. This is called
polymorphism.
Note:
The special case where a subclass changes the behavior of a method it inherited from its superclass is
named as overriding.
We will now give a classical but very explanatory example for this concept. Think of a class called
Animal and its two subclasses Dog and Cat. A method we would implement in Animal class would
be speak because every animal communicates somehow with other livings by making some kind of
noise (although dogs prefer to bite sometimes!). But when this message is sent, dogs would bark while
cats would miaow. So they response to the same message but in a different way.
Creating Your Own Classes
GNU Smalltalk has hundreds of classes built-in but the universe has uncountable of them so GNU
Smalltalk gives us the opportunity to create the classes we want for our needs. We will sometimes call
the classes we made as custom classes.
First, we should talk about what classes are made of. Classes are the blue prints of objects so they
definitely should describe all the things that objects have. For example, they have instance variables to
define the properties of an object. They should have method definitions to make objects responsive to
some certain messages. But they have more. There are class variables. Class variables hold the
information about the state of the class. There are also class methods which only classes can execute
and not the instances of them.