Question

Explain the difference between aggregation and composition. Give an example to support your answer.

Short answer

Aggregation allows independent existence of parts, while composition binds parts to the whole's lifespan.

Step-by-step solution

Define Aggregation

Aggregation is a relationship where an object (the whole) contains or is composed of other objects (the parts), but the parts can exist independently of the whole. This implies a "has-a" relationship. The container can be considered to hold a reference to the objects it contains, but it does not own them.

Define Composition

Composition is a stronger relationship than aggregation where the contained objects are owned by the whole. If the whole is destroyed, the parts are also destroyed. This also implies a "has-a" relationship, but with ownership and a shared lifecycle.

Provide Example of Aggregation

An example of aggregation is a university and its departments. The university contains departments, but the departments can exist independently without the university, perhaps merging with another university or operating as a standalone institution.

Provide Example of Composition

An example of composition is a house and its rooms. The house contains rooms, and if the house is destroyed, its rooms are also destroyed and cannot exist independently.

Summarize and Contrast

In summary, aggregation indicates a "whole-part" relationship where parts can exist independently of the whole, whereas in composition, parts are entirely dependent on the whole for their existence. Aggregation represents a weaker association compared to the more rigid composition relationship.

Key concepts

Aggregation

In object-oriented programming, aggregation is a concept that describes a relationship between objects where one object serves as a container for other objects, but those parts can still exist on their own. This type of relationship is often characterized as a "has-a" relationship, but without the powerful ownership associated with it.

For example, consider a university and its departments. Here, the university aggregates its departments, meaning the university acts as a container. However, each department can still function independently—it can merge with another institution or operate alone. This illustrates the flexibility of aggregation, where the parts (departments) can continue to exist even without the whole (university).

Some key points to remember about aggregation:

• Objects can exist independently even when the containing object is deleted.
• Represents a weaker form of association compared to composition.
• Commonly used when object lifecycles are not bound to each other.

Composition

Composition offers a stronger connection between objects than aggregation. In this relationship, the containing object or the whole owns its parts, meaning if the whole is destroyed, so too are the parts.

A classic example of composition is a house and its rooms. The rooms are parts of the house and cannot exist independently from it. If the house is demolished, the rooms disappear along with it, showcasing how the lifecycle of the parts is tied directly to the whole.

Important aspects of composition to consider:

• Parts cannot exist without the whole being intact.
• It represents a stronger, more rigid "has-a" relationship.
• Best used when parts are inherently dependent on the whole for their existence.

Object Relationships

Understanding the distinctions between aggregation and composition is crucial for grasping object relationships in software design. These relationships help define how objects interact and depend on one another within a system.


Object relationships enhance the modularity and maintainability of a software system. By determining whether a component should be aggregated or composed, software engineers can better model real-world associations and ensure the system behaves as expected.

Some benefits of recognizing these relationships include:

• A clearer structure of a system's architecture.
• Improved code reusability by making accurate distinctions between dependent and independent objects.
• Flexibility in adjusting software components based on whether they are loosely or tightly coupled.

Software Engineering Concepts

When delving into software engineering, understanding the principles of aggregation and composition is key for creating robust and efficient systems. These concepts are fundamental in object-oriented design as they help to build systems with clear and predictable object interactions.


By applying aggregation and composition judiciously, developers address design challenges by modeling real-world phenomena accurately. Considering these principles during the software development process leads to:

• The creation of systems that mirror real-world logic and relationships.
• Enhanced maintainability and scalability of the software due to clear object dependencies.
• Reduced risk of errors as objects are correctly organized and managed in terms of their lifecycles and relationships.