Aggregation vs. Composition

Attribute	Aggregation	Composition
Definition	Aggregation represents a "has-a" relationship where one class contains a reference to another class, but the referenced class can exist independently.	Composition represents a "part-of" relationship where one class is composed of other classes, and the composed classes cannot exist independently.
Dependency	Aggregation has a weak dependency between the containing class and the contained class.	Composition has a strong dependency between the containing class and the composed class.
Lifecycle	The lifecycle of the contained class can exist independently of the containing class.	The lifecycle of the composed class is tightly coupled with the lifecycle of the containing class.
Ownership	The containing class does not own the contained class.	The containing class owns the composed class and is responsible for its creation and destruction.
Cardinality	Aggregation can have a cardinality of 0..n, indicating that the containing class can have zero or multiple instances of the contained class.	Composition typically has a cardinality of 1, indicating that the containing class can have exactly one instance of the composed class.
Navigation	The containing class can navigate to the contained class, but the contained class does not have direct access to the containing class.	The containing class can navigate to the composed class, and the composed class can have direct access to the containing class.
Aggregation Example	A car has a wheel. The wheel can exist independently and can be shared among multiple cars.	A car has an engine. The engine is tightly coupled with the car and cannot exist without it.
Composition Example	A house has a room. The room cannot exist independently and is part of the house.	A computer has a motherboard. The motherboard is an integral part of the computer and cannot exist without it.

Introduction

When designing object-oriented software systems, it is crucial to understand the concepts of aggregation and composition. Both aggregation and composition are relationship types that allow objects to be connected in various ways. While they may seem similar at first glance, there are distinct differences between the two. In this article, we will explore the attributes of aggregation and composition, highlighting their characteristics, use cases, and implications.

Aggregation

Aggregation is a relationship where one object is composed of multiple other objects, but the objects can exist independently. In other words, the aggregated objects have their own lifecycle and can be shared among multiple parent objects. The relationship between the parent object and the aggregated objects is often described as a "has-a" relationship. For example, consider a university and its departments. The university has multiple departments, and each department can exist independently even if the university ceases to exist.

One of the key attributes of aggregation is that the aggregated objects can be shared among multiple parent objects. This means that changes made to the aggregated objects can affect multiple parent objects simultaneously. Additionally, the aggregated objects can be added or removed from the parent object dynamically during runtime. This flexibility allows for greater modularity and reusability in the design of software systems.

Aggregation is commonly represented by a diamond-shaped arrow pointing from the parent object to the aggregated objects. This notation helps to visually distinguish aggregation from other relationship types. It is important to note that aggregation does not imply ownership or exclusive containment. The aggregated objects can exist independently and may be shared among multiple parent objects.

Composition

Composition, on the other hand, is a stronger form of aggregation where the aggregated objects are part of the parent object and cannot exist independently. In composition, the parent object owns the aggregated objects and is responsible for their creation, initialization, and destruction. The relationship between the parent object and the aggregated objects is often described as a "part-of" relationship. For example, consider a car and its engine. The engine is an integral part of the car, and if the car is destroyed, the engine is also destroyed.

Unlike aggregation, composition implies exclusive ownership and strong containment. The aggregated objects cannot be shared among multiple parent objects, and their lifecycle is tightly coupled with the parent object. When the parent object is destroyed, the aggregated objects are automatically destroyed as well. This tight coupling ensures that the aggregated objects are always in a valid state and prevents them from being used outside the context of the parent object.

Composition is commonly represented by a filled diamond-shaped arrow pointing from the parent object to the aggregated objects. This notation helps to visually distinguish composition from other relationship types. It is important to note that composition is a more restrictive form of aggregation and should be used when the aggregated objects are truly part of the parent object and cannot exist independently.

Use Cases and Implications

Both aggregation and composition have their own use cases and implications in software design. Understanding these differences is crucial for making informed decisions when designing object-oriented systems.

Aggregation Use Cases

Aggregation is often used when there is a "whole-part" relationship between objects, but the parts can exist independently. Some common use cases for aggregation include:

• Modeling relationships between classes, such as a university and its departments, a company and its employees, or a library and its books.
• Creating modular and reusable components by allowing objects to be shared among multiple parent objects.
• Implementing collections or containers that hold a group of objects, such as a list of students in a class or a set of products in an inventory.

When using aggregation, it is important to consider the implications. Since the aggregated objects can be shared among multiple parent objects, changes made to the aggregated objects can have a wider impact. Care must be taken to ensure that modifications to the aggregated objects do not violate the integrity of the parent objects or other dependent objects.

Composition Use Cases

Composition is often used when there is a strong ownership relationship between objects, and the aggregated objects cannot exist independently. Some common use cases for composition include:

• Modeling relationships where the aggregated objects are an integral part of the parent object, such as a car and its engine, a house and its rooms, or a computer and its components.
• Ensuring the lifecycle management of the aggregated objects, where they are created, initialized, and destroyed along with the parent object.
• Enforcing encapsulation and preventing the aggregated objects from being used outside the context of the parent object.

When using composition, it is important to consider the implications. The tight coupling between the parent object and the aggregated objects means that any changes to the parent object can directly affect the aggregated objects. This can simplify the design and ensure the consistency of the system, but it also limits the flexibility and reusability of the aggregated objects.

Conclusion

In conclusion, aggregation and composition are two important relationship types in object-oriented design. While both involve connecting objects, they have distinct attributes and implications. Aggregation allows objects to be shared among multiple parent objects and can exist independently, while composition implies exclusive ownership and strong containment. Understanding the differences between aggregation and composition is crucial for designing modular, reusable, and maintainable software systems. By carefully considering the relationships between objects and their lifecycles, developers can make informed decisions and create robust object-oriented designs.