Aggregation vs. Association

Attribute	Aggregation	Association
Definition	Aggregation represents a whole-part relationship where the parts can exist independently of the whole.	Association represents a relationship between two or more objects.
Type of Relationship	Has-a relationship	Uses-a relationship
Dependency	Aggregated objects can exist independently of the whole.	Associated objects are dependent on each other.
Ownership	The whole object owns the aggregated objects.	No ownership between associated objects.
Cardinality	Can have one-to-one, one-to-many, or many-to-many cardinality.	Can have one-to-one, one-to-many, or many-to-many cardinality.
Navigation	Aggregated objects can be accessed through the whole object.	Associated objects can be accessed directly.
Lifespan	Aggregated objects can exist even if the whole object is destroyed.	Associated objects are typically destroyed if one of the objects is destroyed.
Composition	Aggregation can be part of composition.	Association is not part of composition.

Introduction

When designing object-oriented systems, it is crucial to understand the different relationships between classes. Two common types of relationships are aggregation and association. While both represent connections between classes, they have distinct attributes and purposes. In this article, we will explore the characteristics of aggregation and association, highlighting their differences and similarities.

Aggregation

Aggregation is a type of relationship where one class represents a part of another class. It is often described as a "has-a" relationship. In aggregation, the associated objects have an independent lifecycle, meaning they can exist even if the owning object is destroyed. The aggregated objects can be shared among multiple owners, and they can exist independently.

Aggregation is typically represented by a diamond-shaped arrow pointing from the owning class to the aggregated class. This arrow indicates that the owning class has a reference to the aggregated class. However, it is important to note that aggregation does not imply ownership or control over the aggregated class.

For example, consider a university system where a Department class aggregates multiple Professor objects. Each Professor can exist independently and can be associated with different departments. If a department is closed, the professors can still exist and be associated with other departments or even work independently.

Aggregation allows for flexibility and reusability in object-oriented design. It enables the creation of complex systems by composing smaller, reusable components. By separating the lifecycle of the aggregated objects from the owning object, changes in one do not necessarily affect the other.

Association

Association represents a relationship between two or more classes. It is often described as a "knows-about" relationship. Unlike aggregation, the associated objects do not have an independent lifecycle and are typically dependent on the existence of the other class. The association can be one-way or bidirectional, depending on the need.

In association, the classes are connected, but they do not necessarily have a direct reference to each other. The relationship can be represented by a simple line connecting the classes. The line may include multiplicity indicators to specify the number of instances involved in the association.

For example, consider a library system where a Library class is associated with multiple Book objects. The Library class may have methods to add, remove, or search for books, while the Book class may have methods to retrieve information about the library it belongs to. The existence of a book is dependent on the library, and vice versa.

Association allows for modeling complex interactions between classes. It represents the dependencies and interactions between objects in a system. By establishing associations, classes can communicate and collaborate to achieve specific functionalities.

Comparison

Now that we have explored the attributes of aggregation and association, let's compare them in various aspects:

1. Lifecycle

In aggregation, the aggregated objects have an independent lifecycle and can exist even if the owning object is destroyed. On the other hand, in association, the associated objects are typically dependent on the existence of the other class and do not have an independent lifecycle.

2. Ownership

Aggregation does not imply ownership or control over the aggregated class. The aggregated objects can be shared among multiple owners. In contrast, association does not imply ownership either, but it represents a relationship where the associated objects are dependent on each other.

3. Flexibility

Aggregation provides flexibility and reusability in object-oriented design. It allows for the composition of smaller, reusable components to create complex systems. Association, on the other hand, enables modeling complex interactions and dependencies between classes.

4. Representation

In terms of representation, aggregation is often depicted by a diamond-shaped arrow pointing from the owning class to the aggregated class. Association, on the other hand, is represented by a simple line connecting the classes, with optional multiplicity indicators.

5. Dependency

Aggregation allows the aggregated objects to exist independently, even if the owning object is destroyed. This reduces the dependency between the objects. In association, the associated objects are typically dependent on each other, as their existence is intertwined.

Conclusion

Aggregation and association are two important types of relationships in object-oriented design. While aggregation represents a "has-a" relationship with independent lifecycles, association represents a "knows-about" relationship with dependent lifecycles. Aggregation provides flexibility and reusability, while association models complex interactions and dependencies. Understanding the attributes of aggregation and association is crucial for designing effective and maintainable object-oriented systems.