When a class inherits another class, we often say that the child class is a kind of parent class. If a car takes the benefits of a vehicle class, for example, we say that a car is a kind of vehicle. In inheritance, classes always have an is-a relationship. In this section, we will discuss two more important topics related to object-oriented programming, in which classes have a has-a relationship instead. These concepts are aggregation and composition. Let's start by looking at aggregation.