Some Important Concepts

Association:

• If two classes in a model need to communicate with each other, there must be a link between them, and that can be represented by an association
• 
• Example 1 : A single student can associate with multiple teachers.
• Example 2 : Every Instructor has one or more Students.
• Aggregation and Composition are subsets of association

Aggregation:

• Aggregation is a subset of Association.
• 
• Aggregation implies a relationship where the child can exist independently of the parent.
• Example: Class (parent) and Student (child). Delete the Class and the Students still exist.

Composition:

• Composition is a subset of Association.
• 
• Composition implies a relationship where the child cannot exist independent of the parent.
• Example: House (parent) and Room (child). Rooms don't exist separate to a House.

Generalization:

• Generalization is a mechanism for combining similar classes of objects into a single, more general class.
• Generalization is the term that we use to denote abstraction of common properties into a base class in UML.
• The UML diagram's Generalization association is also known as Inheritance.
• When we implement Generalization in a programming language, it is often called Inheritance instead. Generalization and inheritance are the same.
• The terminology just differs depending on the context where it is being used.

Specialization:

• Specialization is the reverse process of Generalization means creating new sub-classes from an existing class.