Object-Oriented Programming

Before diving into the details of Smalltalk which is a pure object-oriented programming language, let’s review the basics of object-oriented programming.

Alan Kay, one of the primary creators the Smalltalk, coined the term “object-oriented programming” in 1967.

An object encapsulates related data and methods that operate on the data. For example, an object can hold the center and radius of a circle. In Smalltalk, these are referred to as instance variables.

A class is often used as template for creating instances of the class which are referred to as objects. For example, a Circle class can be defined.

A class can define:

class methods that are used to create instances

For example, as class method can take a name and breed and return a new object that uses those.
class variables whose values are shared by all instances

For example, a class variable can hold the timestamp at which the last Circle object was created.
class methods that operate on class variables

For example, a class method can return a formatted string representation of the timestamp held in a class variable.
instance variables whose values are unique to each instances
instance methods that operate on instance variables

Classes can inherit from other classes to define a “kind of” relationship and share both data and methods. For example, the classes Square and Circle can inherit from the class Shape.

Most OO languages, such as Smalltalk, only allow a class to inherit from one other class. But there are languages that support multiple inheritance.

When a class A inherits from a class B, we say that A is a subclass of B and B is a superclass of A.

When a class A is a subclass of a class B:

Instance methods in A can access all the instance variables in B.
Instance methods in A can invoke all the instance methods in B.
Class methods in A can access all the class variables in B.
Class methods in A can invoke all the class methods in B.

Polymorphism is the ability to select the implementation of a method to invoke based on the kind of object on which it is invoked. For example, the Shape class can require its subclasses to implement an area method. The classes Square and Circle can each define the area method to compute the value differently.

A variable can hold a reference to a Shape object which can be an instance of any subclass. When the area method is invoked on that variable, the actual method invoked is determined by the type of the object.

Some languages, such as Smalltalk, determine the method to invoke at run-time instead of at compile-time. This is referred to as “late binding”.

A somewhat unique feature of Smalltalk is that methods are invoked through message passing. Rather that directly invoking a method, a message is passed to an object and a lookup process is used to find a corresponding method. Details about this lookup process are presented later.