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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.
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.
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.