R Programming By Example

Hierarchies

Hierarchies can be formed in two ways—inheritance and composition. The idea of inheritance is to form new classes as specialized versions of old ones. The specialized classes are subclasses and the more general ones are superclasses. This type of relationship is often referred to as an is-a type of relationship, since a subclass is a type of the superclass. For example, a lion is a type of animal, so animal would be the superclass and lion the subclass. Another type of relationship is known as the has-a relation. This means that one class has instances of another class. For example, a car has wheels. We wouldn't say that wheels are a type of car, so there's no inheritance there, but we would say that they are part of a car, which ...