Aggregation vs. Composition — What's the Difference?
Edited by Tayyaba Rehman — By Urooj Arif — Updated on May 14, 2024
Aggregation and composition are both UML relationships in object-oriented programming, where aggregation implies a weaker "has-a" association, and composition a stronger, inseparable "part-of" relationship.
Difference Between Aggregation and Composition
Table of Contents
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Key Differences
Aggregation represents a relationship where the child can exist independently of the parent. For example, a library can have multiple books, but the books are not dependent on the library for their existence. Conversely, in composition, the child cannot exist independently of the parent; for instance, a house consists of rooms, and the rooms do not exist separately from the house.
Aggregation allows for the sharing of objects, meaning that an aggregated object can be associated with multiple parent objects. For instance, a single teacher might be part of multiple departments. On the other hand, in composition, a composed object is exclusively owned by a single parent object. If the parent is destroyed, the composed objects are also destroyed, as in the case where deleting a house would also delete its rooms.
The lifecycle of the parent and child in aggregation is not tightly coupled, which means the child object can remain in existence even if the parent object is deleted. Whereas in composition, the lifecycle of the child is strongly tied to the parent; if the parent object is destroyed, the child objects are also automatically destroyed.
In terms of code implementation, aggregation might be implemented by simply storing a reference to the child object, indicating a less strict ownership. While composition typically involves directly creating instances of child objects within the parent object’s class, reflecting stronger ownership and control.
Both aggregation and composition are used to model relationships in systems but choosing between them depends on the required strength of the association and the intended system design. Aggregation is chosen when the relationship is less dependent, while composition is used for a stronger, dependent relationship.
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Comparison Chart
Dependency
Child can exist independently
Child cannot exist independently
Ownership
Partial or shared ownership
Full and exclusive ownership
Lifespan Coupling
Weak coupling
Strong coupling
Object Sharing
Possible
Not possible
Example
Library and books
House and rooms
Compare with Definitions
Aggregation
Allows sharing of objects between different parents.
A teacher might teach in both the math and science departments.
Composition
Directly controls the lifecycle and existence of the child objects.
Destroying a smartphone would also destroy its integrated battery.
Aggregation
A type of association that represents a relationship where the child can exist independently of the parent.
A university can have multiple departments, yet departments can exist independently of the university.
Composition
The combining of distinct parts or elements to form a whole.
Aggregation
Does not strictly manage the lifecycle of the aggregated object.
Removing a course from a curriculum does not eliminate the course itself.
Composition
The manner in which such parts are combined or related.
Aggregation
Used when the relationship does not imply full ownership.
A pet owner might own a dog, but the dog could also belong to a specific breed club.
Composition
General makeup
The changing composition of the electorate.
Aggregation
Constituting or amounting to a whole; total
Aggregate sales in that market.
Composition
The result or product of composing; a mixture or compound.
Aggregation
(Botany) Crowded or massed into a dense cluster.
Composition
Arrangement of artistic parts so as to form a unified whole.
Aggregation
Composed of a mixture of minerals separable by mechanical means.
Composition
The art or act of composing a musical or literary work.
Aggregation
A total considered with reference to its constituent parts; a gross amount
"An empire is the aggregate of many states under one common head" (Edmund Burke).
Composition
A work of music, literature, or art, or its structure or organization.
Aggregation
The mineral materials, such as sand or stone, used in making concrete.
Composition
A short essay, especially one written as an academic exercise.
Aggregation
To gather into a mass, sum, or whole
Aggregated the donations into one bank account.
Composition
(Law) A settlement whereby the creditors of a debtor about to enter bankruptcy agree, in return for some financial consideration, usually proffered immediately, to the discharge of their respective claims on receipt of payment which is in a lesser amount than that actually owed on the claim.
Aggregation
To amount to; total
Revenues will aggregate more than one million dollars.
Composition
(Linguistics) The formation of compounds from separate words.
Aggregation
To collect (content from different sources on the internet) into one webpage or newsreader.
Composition
(Printing) Typesetting.
Aggregation
To come together or collect in a mass or whole
"Some [bacteria]aggregate so closely as to mimic a multicellular organism" (Gina Kolata). "The first stars began to form when hydrogen and helium gas left over from the Big Bang aggregated into dense clouds" (Paul Davies).
Composition
The act of putting together; assembly.
Aggregation
The act of collecting together (aggregating).
Composition
A mixture or compound; the result of composing.
Aggregation
The state of being collected into a mass, assemblage, or sum (aggregated).
Composition
The proportion of different parts to make a whole.
Aggregation
A collection of particulars; an aggregate.
Composition
The general makeup of a thing or person.
Aggregation
(networking) Summarizing multiple routes into one route.
Composition
(obsolete) An agreement or treaty used to settle differences; later especially, an agreement to stop hostilities; a truce.
Aggregation
(epidemiology) The majority of the parasite population concentrated into a minority of the host population.
Composition
(obsolete) A payment of money in order to clear a liability or obligation; a settling or fine.
Aggregation
(object-oriented programming) Kind of object composition which does not imply ownership.
Composition
A payment of fine in order to settle a (usually minor) criminal charge.
Aggregation
The act of aggregating, or the state of being aggregated; collection into a mass or sum; a collection of particulars; an aggregate.
Each genus is made up by aggregation of species.
A nation is not an idea only of local extent and individual momentary aggregation, but . . . of continuity, which extends in time as well as in numbers, and in space.
Composition
(legal) an agreement or compromise by which a creditor or group of creditors accepts partial payment from a debtor.
Aggregation
Several things grouped together or considered as a whole
Composition
An essay.
Aggregation
The act of gathering something together
Composition
(linguistics) The formation of compound words from separate words.
Aggregation
Denotes a "has-a" relationship with a weaker association.
A car has an engine, but the engine can be swapped out and the car still exists.
Composition
A work of music, literature or art.
Composition
(printing) Typesetting.
Composition
(mathematics) Applying a function to the result of another.
Composition
(physics) The compounding of two velocities or forces into a single equivalent velocity or force.
Composition
(obsolete) Consistency; accord; congruity.
Composition
Synthesis as opposed to analysis.
Composition
The arrangement and flow of elements in a picture.
Composition
(object-oriented programming) Way to combine simple objects or data types into more complex ones.
Composition
The act or art of composing, or forming a whole or integral, by placing together and uniting different things, parts, or ingredients.
Composition
The state of being put together or composed; conjunction; combination; adjustment.
View them in composition with other things.
The elementary composition of bodies.
Composition
A mass or body formed by combining two or more substances; as, a chemical composition.
A composition that looks . . . like marble.
Composition
A literary, musical, or artistic production, especially one showing study and care in arrangement; - often used of an elementary essay or translation done as an educational exercise.
Composition
Consistency; accord; congruity.
There is no composition in these newsThat gives them credit.
Composition
Mutual agreement to terms or conditions for the settlement of a difference or controversy; also, the terms or conditions of settlement; agreement.
Thus we are agreed:I crave our composition may be written.
Composition
The adjustment of a debt, or avoidance of an obligation, by some form of compensation agreed on between the parties; also, the sum or amount of compensation agreed upon in the adjustment.
Compositions for not taking the order of knighthood.
Cleared by composition with their creditors.
Composition
Synthesis as opposed to analysis.
The investigation of difficult things by the method of analysis ought ever to precede the method of composition.
Composition
A mixture of ingredients
Composition
The way in which someone or something is composed
Composition
The spatial property resulting from the arrangement of parts in relation to each other and to the whole;
Harmonious composition is essential in a serious work of art
Composition
A musical work that has been created;
The composition is written in four movements
Composition
Musical creation
Composition
The act of creating written works;
Writing was a form of therapy for him
It was a matter of disputed authorship
Composition
Art and technique of printing with movable type
Composition
An essay (especially one written as an assignment);
He got an A on his composition
Composition
Something that is created by arranging several things to form a unified whole;
He envied the composition of their faculty
Composition
A stronger form of association that implies ownership where the child object cannot exist without the parent.
A house consists of rooms that cease to exist independently without the house.
Composition
Represents a "part-of" relationship with strong coupling.
A body consists of organs, where organs do not exist separately from the body.
Composition
Enforces exclusive ownership and control over the components.
Deleting a bookshelf will also delete all books placed in it if they are modeled as part of the shelf.
Composition
Essential for modeling systems where components are fundamentally inseparable from the whole.
A company consists of departments that cannot function independently outside the company structure.
Common Curiosities
What is the main difference between aggregation and composition in UML?
Aggregation implies a weaker association where child objects can exist independently, while composition implies a stronger, dependent relationship where child objects cannot exist without the parent.
Can a child object in an aggregation be part of multiple parents?
Yes, in aggregation, a child object can be shared among multiple parent objects.
How should I decide whether to use aggregation or composition?
Choose aggregation if the relationship is flexible and the child can exist independently; choose composition if the child is dependent and strictly managed by the parent.
What happens to child objects in composition when the parent is destroyed?
In composition, destroying the parent object also destroys all its child objects, as they are dependent on the parent for their existence.
Why is composition considered stronger than aggregation?
Because in composition, the existence and lifecycle of the child are controlled by the parent, reflecting a stronger bond and dependency.
Are aggregation and composition mutually exclusive?
While both represent "has-a" relationships in object-oriented design, they are used based on the degree of ownership and lifecycle dependency, thus are applied according to different scenarios and are not interchangeable.
Is aggregation common in real-world modeling?
Yes, aggregation is commonly used in scenarios where systems or components need to share resources or functionalities but are operationally independent.
Can aggregation lead to more flexible system designs?
Yes, because it allows for components to be reused in different contexts without strict dependency, facilitating more modular and adaptable designs.
What is an example of composition in software systems?
An operating system and its core modules, where the modules cannot function without the operating system.
What are some practical implications of using composition in design?
It simplifies management but requires careful consideration of how components are created and destroyed, affecting system design and resource management.
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Written by
Urooj ArifUrooj is a skilled content writer at Ask Difference, known for her exceptional ability to simplify complex topics into engaging and informative content. With a passion for research and a flair for clear, concise writing, she consistently delivers articles that resonate with our diverse audience.
Edited by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
