# Aggregation vs. Association: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on February 20, 2024
Aggregation is a special form of association where one class is a part of another class, implying a whole-part relationship, while association is a general relationship between two classes without implying ownership.

## Key Differences

Aggregation is a specialized form of association that represents a "part-of" or "has-a" relationship, indicating that one object is a constituent part of another. Association, on the other hand, is a broader relationship that simply implies a linkage between two objects in a model, without specifying a part-whole relationship.
In aggregation, the lifetime of the part can be independent of the whole; the part can exist independently of the whole. In contrast, association does not inherently imply any lifetime dependency between the linked objects.
Aggregation implies a stronger relationship as compared to a general association. It indicates a more integral connection where one class is a component of another. In association, the relationship can be more flexible and less integral.
Both aggregation and association can have varying multiplicities (one-to-one, one-to-many, many-to-many) and can be either unidirectional or bidirectional. However, the "part-of" nature of aggregation often leads to a more structured multiplicity.
In object-oriented modeling, aggregation is used to represent more cohesive and structured relationships, while association is used for more generic connections between objects, where no strong ownership is implied.

## Comparison Chart

### Relationship Type

Part-of relationship (stronger)

Parts can exist independently of the whole

### Strength

Stronger, implies integral connection
Weaker, more flexible

### Multiplicity

Often structured (e.g., one-to-many)
Flexible (any type)

### Usage in Modeling

Represents cohesive structures
Represents general connections

## Aggregation and Association Definitions

#### Aggregation

Aggregation implies a whole-part relationship in object-oriented design.
A university aggregation contains departments and faculties.

#### Association

Association represents a general relationship between two separate classes.
In an association, a student is linked to a library.

#### Aggregation

Aggregation represents a 'has-a' relationship between objects.
A computer system's aggregation has a keyboard, a monitor, and a CPU.

#### Association

Associations can be one-to-one, one-to-many, or many-to-many.
A teacher association with students is typically one-to-many.

#### Aggregation

Aggregation is a relationship where an object is made up of other objects.
A car aggregation includes wheels and an engine as parts.

#### Association

Association can be bidirectional or unidirectional.
A customer and order association can be bidirectional, showing mutual interaction.

#### Aggregation

In aggregation, the whole can exist without its parts.
A tree object can exist in aggregation even if leaves are removed.

#### Association

In association, the linked objects can exist independently.
A doctor's association with a hospital doesn't affect their independent existence.

#### Aggregation

Aggregation allows individual parts to be shared among different wholes.
In a library aggregation, a book can belong to multiple categories.

#### Association

Association does not imply ownership between objects.
An employee association with a department doesn't mean ownership.

#### Aggregation

Constituting or amounting to a whole; total
Aggregate sales in that market.

#### Association

The act of associating or being connected with
My parents disapproved of my association with my friends from across town.

#### Association

An organized body of people who have an interest, activity, or purpose in common; a society.

## FAQs

#### What is aggregation in object-oriented programming?

Aggregation is a special form of association representing a whole-part relationship.

#### What types of multiplicity are possible in aggregation?

Aggregation can have one-to-one, one-to-many, or many-to-many relationships.

#### How does association differ from aggregation?

Association is a general link between two classes, without implying a part-whole relationship.

#### Is association a strong relationship?

Association is a flexible relationship and not necessarily strong.

#### What is an example of an association?

A teacher associated with multiple students in a class.

#### How does aggregation affect object lifetimes?

In aggregation, object lifetimes are independent.

#### Is association limited to specific types of classes?

No, association can occur between any types of classes.

#### What is an example of an aggregation?

A computer system as an aggregation of a keyboard, monitor, and CPU.

#### Does aggregation imply a stronger connection than association?

Yes, aggregation implies a stronger, more integral connection.

#### Can an aggregation exist without its parts?

Yes, in aggregation, the whole can exist independently of its parts.

#### Are aggregation relationships bidirectional?

Aggregation can be either unidirectional or bidirectional.

#### How does aggregation support object-oriented design?

Aggregation supports structured and cohesive object models.

#### Can association represent ownership?

No, association doesn’t imply ownership between objects.

#### Can parts in an aggregation be shared?

Yes, parts in an aggregation can be shared among different wholes.

#### Can an association exist without both objects being present?

In association, objects can exist independently of the relationship.

#### In what scenarios is association used?

Association is used to link classes in general, flexible relationships.

#### Can association be temporary?

Yes, associations can be temporary or long-term.

#### What kind of relationship does aggregation imply?

Aggregation implies a 'has-a' or part-whole relationship.

#### How does association differ in terms of directionality?

Association can be either unidirectional or bidirectional.

#### What is the significance of aggregation in modeling?

Aggregation helps in representing realistic and structured relationships in models.