Aggregation vs. Composition: What's the Difference?
Edited by Aimie Carlson || By Harlon Moss || Published on February 3, 2024
Aggregation is a weak association where entities can exist independently, while composition is a strong association where the life cycle of the contained entity depends on the container.
Key Differences
Aggregation refers to a relationship where the child can exist independently of the parent. For example, a team and a player; a player can exist without a team. Composition, on the other hand, implies a relationship where the child cannot exist without the parent. For example, a house and a room; a room doesn't typically exist without a house.
In aggregation, the entities have their own lifecycle and ownership. This means that destruction of the parent entity does not necessarily destroy the child entities. In composition, the child entity's lifecycle is tightly coupled with the parent's lifecycle. The destruction of the parent entity leads to the destruction of the child entities.
Aggregation represents a “has-a” relationship with less dependency, where association is more loosely coupled. For instance, a university and its departments can be seen as an aggregation; departments can theoretically exist independently. Composition, however, is more rigidly coupled, representing a part-of relationship. For example, a car and its engine; the engine is an integral part of the car.
Considering ownership, in aggregation, the child can be associated with multiple parent entities. A book can belong to multiple libraries (aggregation). In composition, the child entity is owned by a single parent entity. A heart is part of a specific body (composition).
In terms of lifetime, aggregation allows for the independent existence of entities. A class and its students; students will continue to exist even if the class is dissolved. Composition denotes a scenario where if the parent entity ceases to exist, its child entities also cease to exist. A company and its departments; if the company shuts down, the departments also cease to exist.
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Comparison Chart
Dependency
Entities can exist independently.
Child entity's existence is dependent on the parent.
Lifecycle
Separate lifecycles for parent and child entities.
Child's lifecycle is tied to the parent's lifecycle.
Ownership
Child can be associated with multiple parents.
Child is owned by a single parent.
Relationship Type
“Has-a” relationship, loosely coupled.
“Part-of” relationship, tightly coupled.
Existence
Child entities can exist independently of the parent.
Child entities do not exist independently of the parent.
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Aggregation and Composition Definitions
Aggregation
It refers to a group or mass of distinct or varied things, people, or elements.
An aggregation of diverse cultures enriches the community.
Composition
Composition is the act or process of combining parts or elements to form a whole.
The composition of the committee was carefully chosen to reflect diverse opinions.
Aggregation
In business, it's the gathering of individuals or companies into a group.
The aggregation of small firms created a more formidable market presence.
Composition
In chemistry, it's the nature and proportion of elements in a substance.
The chemical composition of water is two hydrogen atoms bonded to one oxygen atom.
Aggregation
Aggregation is a collection or assembly of items.
The aggregation of data from different sources ensured a comprehensive analysis.
Composition
It refers to the arrangement or structure of elements in a work of art or literature.
The novel’s composition was praised for its unique narrative structure.
Aggregation
In biology, it's the clustering of organisms in a population.
The aggregation of bees around the hive is a defensive mechanism.
Composition
In music, it's the creation of a musical work.
Mozart's compositions are renowned for their complexity and beauty.
Aggregation
In computing, it's the process of combining multiple data entities into a single one.
Aggregation of multiple datasets is common in statistical analysis.
Composition
In photography and visual arts, it's the arrangement of visual elements in a work.
The composition of the photograph highlighted the contrast between light and shadow.
Aggregation
Constituting or amounting to a whole; total
Aggregate sales in that market.
Composition
The combining of distinct parts or elements to form a whole.
FAQs
What is composition?
Composition refers to a relationship where one entity is composed of, or dependent on, the other.
Is a composition entity independent of its parent?
No, in composition, the child entity is dependent on the parent and cannot exist independently.
What is aggregation?
Aggregation is a concept where a relationship between two entities is established such that one represents a collection of the other.
Can an aggregation exist without its parent entity?
Yes, in aggregation, the child entity can exist independently of the parent.
How does aggregation affect the lifecycle of an entity?
In aggregation, the lifecycle of entities is not mutually dependent.
How does composition affect the lifecycle of an entity?
In composition, the destruction of the parent often means the destruction of the child entities.
Can a child entity in composition belong to multiple parents?
No, in composition, a child entity is typically associated with a single parent.
Is composition a strong or weak association?
Composition is a strong association, indicating a higher degree of dependency.
What is an example of aggregation?
A library and books; books can exist outside of the library.
Can an entity be part of multiple aggregations?
Yes, an entity can be part of multiple aggregations.
What is an example of composition?
A house and rooms; rooms are integral parts of a house.
What kind of relationship does aggregation imply?
Aggregation implies a “has-a” relationship.
Are composition relationships easily altered?
No, composition relationships are not easily altered due to the dependency of the entities.
Is aggregation commonly used in database relationships?
Yes, aggregation is commonly used in databases to define relationships between tables.
Can aggregation relationships be easily broken?
Yes, relationships in aggregation can be more easily altered or broken.
What kind of relationship does composition imply?
Composition implies a “part-of” relationship.
Does aggregation allow shared ownership?
Yes, in aggregation, entities can have shared ownership.
Does composition allow shared ownership?
No, in composition, the parent entity has exclusive ownership of the child entities.
Is composition relevant in software development?
Yes, composition is highly relevant in software development, particularly in object-oriented programming.
Is aggregation a strong or weak association?
Aggregation is considered a weaker association compared to composition.
About Author
Written by
Harlon MossHarlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Aimie CarlsonAimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.
