Chapter 17. Subtyping and Inheritance

Table of Contents

1. Introducing Subtypes by Specialization
2. Introducing Supertypes by Generalization
3. Intension versus Extension
4. Type Hierarchies
5. The Class Hierarchy Merge Design Pattern
6. Subtyping and Inheritance in Computational Languages
6.1. Subtyping and Inheritance with OOP Classes
6.2. Subtyping and Inheritance with Database Tables
7. Quiz Questions
7.1. Question 1: Statements about a Class Hierarchy
7.2. Question 2: Class Hierarchy Merge

The concept of a subtype, or subclass, is a fundamental concept in natural language, mathematics, and informatics. For instance, in English, we say that a bird is an animal, or the class of all birds is a subclass of the class of all animals. In linguistics, the noun "bird" is a hyponym of the noun "animal".

An object type may be specialized by subtypes (for instance, Bird is specialized by Parrot) or generalized by supertypes (for instance, Bird and Mammal are generalized by Animal). Specialization and generalization are two sides of the same coin.

A subtype inherits all features from its supertypes. When a subtype inherits attributes, associations and constraints from a supertype, this means that these features need not be explicitly rendered for the subtype in the class diagram, but the reader of the diagram has to know that all features of a supertype also apply to its subtypes.

When an object type has more than one direct supertype, we have a case of multiple inheritance, which is common in conceptual modeling, but prohibited in many object-oriented programming languages, such as Java and C#, where subtyping leads to class hierarchies with a unique direct supertype for each object type.

1. Introducing Subtypes by Specialization

A new subtype may be introduced by specialization whenever new features of more specific types of objects have to be captured. We illustrate this for our example model where we want to capture text books and biographies as special cases of books. This means that text books and biographies also have an ISBN, a title and a publishing year, but in addition they have further features such as the attribute subjectArea for text books and the attribute about for biographies. Consequently, we introduce the object types TextBook and Biography by specializing the object type Book, that is, as subtypes of Book.

Figure 17.1. The object type Book is specialized by two subtypes: TextBook and Biography

The object type Book is specialized by two subtypes: TextBook and Biography

When specializing an object type, we define additional features for the newly added subtype. In many cases, these additional features are more specific properties. For instance, in the case of TextBook specializing Book, we define the additional attribute subjectArea. In some programming languages, such as in Java, it is therefore said that the subtype extends the supertype.

However, we can also specialize an object type without defining additional properties (or operations/methods), but by defining additional constraints.