Aggregation is a collection or formation number of things to cluster. In Object Oriented Programming , Aggregation is whole part relationship (You should have the association between classes)
Aggregation is a subset of the association
In the following example Class B is part of Class A and also this has the association
We can name the Class A as Whole Class B as Part
When you draw the class diagram the hollow diamond is next to the whole
Now lets look at some real world representation
You can think of the Car and Wheel relationship. Wheel is a part of the Car
In this UML diagram you can see that there is association between Car and Wheel. You can say, Car has wheels . So you have “has” relationship. It means you have the Association
Wheel is part of the Car. Now car is Whole wheel is Part . You can draw hollow diamond next to the Car. You can see now Aggregation
Now I will code this UML diagram in VB.NET so that you can understand association comprehensively
Car Class in VB.NET
Public Class Car
Private m_wheels() As Wheel
Public Sub SomeMethod()
End Sub
End Class
Wheel Class in VB.NET
Public Class Wheel
Private m_size As Integer
Public Sub SomeMethod()
End Sub
End Class