Question

Explain association, aggregation and composition relationships of class-based modelling approach with one-one suitable example [no need to write any code]. Represent each relationship via UML diagram? What will be the impact of low cohesion in class-based modelling?

Answer 1

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 (connector).

Association can be represented by a line between these classes with an arrow indicating the navigation direction. In case an arrow is on both sides, the association is known as a bidirectional association.

We can indicate the multiplicity of an association by adding multiplicity adornments to the line denoting the association. The example indicates that a Student has one or more Instructors:

A single student can associate with multiple teachers:

The example indicates that every Instructor has one or more Students:

Student 1.. Instructor

We can also indicate the behavior of an object in an association (i.e., the role of an object) using role names.

Student leams from Instructor teaches

Aggregation and Composition are subsets of association meaning they are specific cases of association. In both aggregation and composition object of one class "owns" object of another class. But there is a subtle difference:

• 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 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.

Composition Example:

We should be more specific and use the composition link in cases where in addition to the part-of relationship between Class A and Class B - there's a strong lifecycle dependency between the two, meaning that when Class A is deleted then Class B is also deleted as a result

Aggregation Example:

It's important to note that the aggregation link doesn't state in any way that Class A owns Class B nor that there's a parent-child relationship (when parent deleted all its child's are being deleted as a result) between the two. Actually, quite the opposite! The aggregation link is usually used to stress the point that Class A instance is not the exclusive container of Class B instance, as in fact the same Class B instance has another container/s.

Enginelns pection Engine Wheel Engine

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Cohesion is an indication of how related and focused the responsibilities of an software element are.

Cohesion refers to what the class (or module) can do. Low cohesion would mean that the class does a great variety of actions - it is broad, unfocused on what it should do. High cohesion means that the class is focused on what it should be doing, i.e. only methods relating to the intention of the class.