In a class diagram, an aggregation represents a special relationship between two classes that indicates that an object of one class (the part class) can be part of another object of another class (the whole or container class). This relationship expresses that the part class can exist independently of the container and may also belong to other containers.
Aggregation is often depicted using a diamond-shaped symbol that points towards the container class. This notation indicates that the part class is connected to the container but is not necessarily "owned" by it. This means that the part class can continue to exist even if the container no longer exists. Here are some key characteristics of an aggregation relationship:
Part-Whole Relationship: Aggregation signifies that the part class is a part of the container class but is not necessarily tightly bound to it.
Independence: The part class can be created, used, or deleted independently of the container class. The existence of the part class is not dependent on the container class.
Navigation: Through aggregation, it is possible to access the part class from the container class, but not necessarily the other way around. This means that the container class "contains" the part class, but the part class can also be used elsewhere.
A common example of an aggregation relationship is the relationship between a car (container class) and its wheels (part class). The wheels are part of the car, but they can also exist independently and be used for other purposes.
It's important to note that aggregation is a weaker form of relationship compared to "composition," where the part class is tightly bound to the container class and typically exists only in the context of the container class. Distinguishing between aggregation and composition is important in UML diagrams as it allows for more precise representation of relationships between classes and objects.
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