UML Class Relationship Diagrams
UML Class Diagram: Association, Aggregation and Composition .. In your example of strong association, you showed its multiplicity Quick answer to a UML FAQ: Association vs Aggregation vs Composition? Consider the differences and similarities between the classes of the following Visual Paradigm Community Edition is a UML software that supports all UML diagram types. We can indicate the multiplicity of an association by adding multiplicity. Association; Directed Association; Reflexive Association; Multiplicity; Aggregation Reflexive Association Relationship in UML Class diagrams.
The aggregate is semantically an extended object that is treated as a unit in many operations, although physically it is made of several lesser objects. 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.
Aggregation multiplicity UML - Stack Overflow
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. 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. 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.
UML Class Relationship Diagrams
They are not implemented with member variables at all. 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.
UML Association vs Aggregation vs Composition
House parent and Room child. Rooms don't exist separate to a House. We should be more specific and use the composition link in cases where in addition to the part-of relationship between Class A and Class B - there's a strong lifecycle dependency between the two, meaning that when Class A is deleted then Class B is also deleted as a result Aggregation Example: It's important to note that the aggregation link doesn't state in any way that Class A owns Class B nor that there's a parent-child relationship when parent deleted all its child's are being deleted as a result between the two.
Actually, quite the opposite!
Summing it up - To sum it up association is a very generic term used to represent when on class used the functionalities provided by another class. We say it's a composition if one parent class object owns another child class object and that child class object cannot meaningfully exist without the parent class object. If it can then it is called Aggregation. Generalization vs Specialization Generalization is a mechanism for combining similar classes of objects into a single, more general class.
Generalization identifies commonalities among a set of entities. The commonality may be of attributes, behavior, or both. In other words, a superclass has the most general attributes, operations, and relationships that may be shared with subclasses.
A subclass may have more specialized attributes and operations. Specialization is the reverse process of Generalization means creating new sub classes from an existing class.