UML Association vs Aggregation vs Composition
UML realization is an abstraction relationship between two sets of elements, one representing a specification and the other represents an implementation of the. Implementation: describes how classes will implement their interfaces The UML symbol for this relationship is an arrow with an unfilled. UML Class diagram relationships explained with examples and images. This article explains how to correctly determine and implement the.
The figure below shows a generalization.
We will talk about it later on in this UML guide. Free Download Association If two classes in a model need to communicate with each other, there must be link between them, and that can be represented by an association connector. Association can be represented by a line between these classes with an arrow indicating the navigation direction. In case arrow is on the both sides, association has bidirectional association. We can indicate the multiplicity of an association by adding multiplicity adornments to the line denoting the association.UML - UML Relationships
The example indicates that a Student has one or more Instructors: A single student can associate with multiple teachers: The example indicates that every Instructor has one or more Students: We can also indicate the behavior of an object in an association i.
Association vs Aggregation vs Composition The question "What is the difference between association, aggregation and composition" has been frequently asked lately. Actually, Aggregation and Composition are subsets of association meaning they are specific cases of association.
In both aggregation and composition object of one class "owns" object of another class. But there is a subtle difference: Aggregation implies a relationship where the child can exist independently of the parent. Class parent and Student child. Delete the Class and the Students still exist. Composition implies a relationship where the child cannot exist independent of the parent.
House parent and Room child. Here the student can exist without library, the relation between student and library is aggregation.
Composition[ edit ] Two class diagrams. The diagram on top shows Composition between two classes: A Car has exactly one Carburetor, and a Carburetor has at most one Car Carburetors may exist as separate parts, detached from a specific car.
The diagram on bottom shows Aggregation between two classes: A Pond has zero or more Ducks, and a Duck has at most one Pond at a time. The UML representation of a composition relationship shows composition as a filled diamond shape on the containing class end of the lines that connect contained class es to the containing class.
Differences between Composition and Aggregation[ edit ] Composition relationship 1. When attempting to represent real-world whole-part relationships, e.
When the container is destroyed, the contents are also destroyed, e. When representing a software or database relationship, e. When the container is destroyed, the contents are usually not destroyed, e.
Thus the aggregation relationship is often "catalog" containment to distinguish it from composition's "physical" containment. In practice, means that any instance of the subtype is also an instance of the superclass. An exemplary tree of generalizations of this form is found in biological classification: The relationship is most easily understood by the phrase 'an A is a B' a human is a mammal, a mammal is an animal.
The UML graphical representation of a Generalization is a hollow triangle shape on the superclass end of the line or tree of lines that connects it to one or more subtypes. The generalization relationship is also known as the inheritance or "is a" relationship. The superclass base class in the generalization relationship is also known as the "parent", superclass, base class, or base type. The subtype in the specialization relationship is also known as the "child", subclass, derived class, derived type, inheriting class, or inheriting type.
Note that this relationship bears no resemblance to the biological parent—child relationship: A is a type of B For example, "an oak is a type of tree", "an automobile is a type of vehicle" Generalization can only be shown on class diagrams and on use case diagrams.
Dependency in UML
The UML graphical representation of a Realization is a hollow triangle shape on the interface end of the dashed line or tree of lines that connects it to one or more implementers.
A plain arrow head is used on the interface end of the dashed line that connects it to its users. In component diagrams, the ball-and-socket graphic convention is used implementors expose a ball or lollipop, whereas users show a socket. Realizations can only be shown on class or component diagrams.
UML Class Relationship Diagrams
A realization is a relationship between classes, interfaces, components and packages that connects a client element with a supplier element. One class depends on another if the independent class is a parameter variable or local variable of a method of the dependent class. This is different from an association, where an attribute of the dependent class is an instance of the independent class.
Sometimes the relationship between two classes is very weak. They are not implemented with member variables at all. Rather they might be implemented as member function arguments.