Class Associations

What are Class Associations?

Classes and objects, alone, are not inherently useful. They are simply small bits of a larger system. The interactions between classes and how we model them is more important.

In UML documentation, the class diagram can be expanded to contain multiple classes which use one another. These are called Associations. There are 3 types of association:

• Basic Associations - One class communication with another class. Example: Students and Teachers have a basic association because students and teachers communicate with one another.

• Aggregations - A type of parent-child association where the child can exist without the parent. An example would be that of a Student (child) and a Course (parent). If we remove the course, the student still exists.

• Compositions - A type of parent-child association where the child cannot exist without the parent. Example: Teacher (parent) and Course (child). If we delete the teacher, the course cannot exist. Another example: Rooms cannot exist without a buildings.

Example: Consider the relationships between an author, publisher and book:

• Author-Publisher: Both author and publisher can exist without one another. (Basic Association)
• Author-Book: A book cannot exist without an author. (Composition)
• Book-Publisher: Both book and publisher can exist without one another. (Basic Association)

Multiplicity

It is often useful to know how many of one class another can be associated with. For example:

Exercise

Armed with your newfound knowledge, try to implement the following Class Diagram in Java. Keep in mind, the diagram may not be fully complete.