There are three main class relationships we need to know:

IS A

Part-of

Has-a

Part-of

In this relationship, one class object is a component of another class object. Given two classes, class A and class B, they are in a part-of relation if a class A object is a part of class B object, or vice-versa.

An instance of the component class can only be created inside the main class. In the example to the right, class B and class C have their own implementations, but their objects are only created once a class A object is created. Hence, part-of is a dependent relationship.