6.5. Subtyping
If instances of an object type are classified into a more specific type, this specialized type is known as a subtype. Consider, for example, the Person type, shown in Figure 6.30. For discussion purposes, this is populated with eight people, depicted by clip art shapes. Each person’s gender and birth country are recorded. The two instances of Gender are denoted by the biological symbols for male and female. Countries are denoted by their map shapes (only Australia, Italy, and Mexico are shown here).
Figure 6.30. Person classified into subtypes according to gender or birthplace.
Based on gender, we could classify Person into ...
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