Aggregation vs Composition: Difference and Comparison

In programming languages, operating system designs, and database management systems, different objects and classes are stored with an association between the objects. Both Aggregation and Composition are used to describe this relationship between the objects for storing and retrieving the datasets more efficiently by reducing the complexities of time and space.

Key Takeaways

  1. Aggregation is an association where one object is composed of multiple other objects. In comparison, composition is a type of association where one object comprises other objects and is responsible for their lifecycle.
  2. In aggregation, the composed objects can exist independently of the object that contains them, while in composition, the composed objects cannot exist independently.
  3. Aggregation is a looser form of coupling than composition.

Aggregation vs Composition

Aggregation is a relationship where a class can contain an instance of another class as one of its attributes. Composition is a relationship where a class contains an instance of another class as one of its attributes, but the contained instance cannot exist without the container class.

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Aggregation is an association between two datasets, such as classes or objects, where the objects involved do not have a direct dependence on each other for their creation or function. Thus, manipulating one object does not alter the status of the other object. This association allows changes to be made without loss.

Composition is a special type of Aggregation and it describes the relationship between two datasets, where the objects or classes involved are directly dependent on each other for their creation, storage, function, and so on. Thus, any modification to one of the datasets directly impacts the other dataset.

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Comparison Table

Parameters of ComparisonAggregationComposition
Definition Aggregation is a type of association without the dependence of objects on each otherComposition is an association with the dependence of objects on each other
Relationship There is an indirect relationship between the datasetsThe parent-child relationship between the datasets
UML symbolAggregation is denoted with an empty diamondComposition is denoted with a filled diamond
Modification Modification of one object does not alter the other objectModification of one object alters the data on the other object
Association strength It is a weak associationIt is a strong association

What is Aggregation?

Aggregation is a type of association or relationship between two datasets, such as classes, objects, and so on. The association is made to group the datasets that have similarities together.

In this type of association, the objects or classes do not have a direct relationship. The datasets have a “Has-A” relationship.

This implies that the datasets do not depend on each other for their creation, storage, or any function or operation. This is the key benefit of using this form of association for storing similar datasets.

As the objects and classes do not have a direct dependence on each other, any modification or alteration made to one of the objects or classes does not alter or modify the contents of the data stored in the other dataset.

This makes it possible to destroy one of the datasets in aggregation without having to worry about the data stored in the other dataset.

Because of this property of the Aggregation relationship, it is mostly used in testing environments where the data has to be modified constantly. Due to the “Has-A” relationship between the aggregated objects, the data is not destroyed in the other datasets.

For UML representation, an empty diamond symbol is used to specify the Aggregation relationship.

What is Composition?

Composition is a relationship between datasets and is mostly used in database management systems and designing operating systems. The datasets may include objects, classes, and so on.

The composition creates a relationship between datasets which is more direct than Aggregation. Thus the objects and classes are directly related to each other during the creation of the datasets.

As the objects and classes are directly related, they depend on each other for their creation, storage, and any operation or function. 

The relationship is sometimes represented as a “Parent-child” relationship or “owner” relationship, where one dataset is the owner of the other dataset.

The major drawback of using this type of relationship is that when one of the objects or classes is altered or modified, the change is reflected on the other dataset, and the data contained in the dataset is also modified.

This has to be noted while working with such datasets as if one of the datasets is destroyed or deleted, the other dataset which is in association with that dataset also gets destroyed.

But the data is stored more securely when using a composition relationship between the objects or classes. A filled diamond is used for UML representation of a Composition type association.

Main Differences Between Aggregation and Composition

  1. Aggregation is an association of datasets where the datasets do not depend on each other. Composition is also a type of association, but the datasets depend on each other.
  2. In Aggregation, there is an indirect relationship between the datasets. In Composition, there is a direct relationship between the datasets.
  3. For UML representation, Aggregation is denoted by an empty diamond, whereas a filled diamond represents Composition.
  4. In an Aggregation type association, any modifications made to one of the objects involved in the association do not alter the data present in the other datasets. In a Composition type association, any modification made to one of the datasets is reflected in the other datasets involved in the association.
  5. Aggregation is a weaker type of association than Composition. Data is more securely stored in a composition-type association.
References
  1. https://ieeexplore.ieee.org/abstract/document/1402291/
  2. https://ieeexplore.ieee.org/abstract/document/1342512/
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