Aggregation, Generalization, and Association

Aggregation association is relationship between two classes when an object of one class ‘is a part of’ object of another class. Aggregation describes a special type of an association. It is represented as hallow diamond on the class with line connects to the classes that serves as its parts.

Example 1: Engine, door and tire classes having ‘is a part of’ relation with car class.
Example 2: Chapter, Index classes are ‘part of’ book class.

Generalization association is the properties of one class (sub class) acquired from properties of another class (super class). In generalization association, there is ‘a kind of’ relationship exist between the classes. It is depicted as hallow arrow pointing to the super class and its solid line connected to sub classes.

Example 1: Structural diagram (subclass 1) and behavioral diagram (subclass 2) class having ‘a kind of’ relationship with diagram class (super class).
Example 2: Student class and professor class having ‘a kind of’ relation with person class.

Association represents the relationship between one or more classes. It is represented as direct link containing multiplicity symbols between the classes. 

Example 1: A department has one or more employees.
Example 2: An employee has zero or many children.