Smalltalk and Object Orientation: an Introduction - Free

14. The Magnitude Class Hierarchy
14.1 Introduction
This chapter considers the magnitude class and those classes which inherit from it. This includes the
Number hierarchy. We also consider the classes Character, Time, and Date.
14.2 The class Magnitude
The Magnitude class is the abstract super class of all classes which possess some concept of a size
and the comparison of objects of different size (see Figure 14.1). This means that all types of numbers
inherit (eventually) from Magnitude, although they are in fact all subclasses of another class
Number, which acts as the abstract superclass for all numbers. This abstract class inh erits all of its size
related behavior from the Magnitude class. Other classes which have some concept of size and thus
can be compared via logical operators such as include Time, Date and Character. The
subclasses of Magnitude will be considere d later in this chapter, in this section we will concentrate
on the Magnitude class.
Figure 14.1: The top of the Magnitude class hierarchy
The Magnitude class provides protocol for comparing objects whic h possess size and which can
use their size to rank themselves. The relationship of this size to their peers is class dependent and is not
defined by the Magnitude class. The comparing protocol in Magnitude defines the methods < and = :
• = all of which are defined in terms of <
and =. The protocol also defines max: and min: messages used to return either the maximum or
minimum of two numbers respectively. For example:
10 max: 15.
10 min: 15.
The first expression returns the result 15 and the second 10. Magnitude also provides a test method
which checks to see if a value is between two other values. For example:
10 between: 1 and: 9.
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