Types of relationships in UML modeling
This indicates that a Person object has private fields named name and UML class diagrams include the following types of use-relationships, in order from. In database design, object-oriented programming and design has-a (has_a or has a) is a composition relationship. Association could be used on different types of UML structure diagrams: . Arity. Each association has specific arity as it could relate two or more items.
Association[ edit ] Class diagram example of association between two classes An association represents a family of links. A binary association with two ends is normally represented as a line. An association can link any number of classes.
UML Association vs Aggregation vs Composition
An association with three links is called a ternary association. An association can be named, and the ends of an association can be adorned with role names, ownership indicators, multiplicity, visibility, and other properties. There are four different types of association: Bi-directional and uni-directional associations are the most common ones.
For instance, a flight class is associated with a plane class bi-directionally.
Association represents the static relationship shared among the objects of two classes. Aggregation[ edit ] Class diagram showing Aggregation between two classes.
Here, a Professor 'has a' class to teach. Aggregation is a variant of the "has a" association relationship; aggregation is more specific than association.
- Class diagram
- UML Association vs Aggregation vs Composition
- UML Association
It is an association that represents a part-whole or part-of relationship. As shown in the image, a Professor 'has a' class to teach. As a type of association, an aggregation can be named and have the same adornments that an association can. However, an aggregation may not involve more than two classes; it must be a binary association. Furthermore, there is hardly a difference between aggregations and associations during implementation, and the diagram may skip aggregation relations altogether.
The contents of the container still exist when the container is destroyed.
UML - Basic Notations
In UMLit is graphically represented as a hollow diamond shape on the containing class with a single line that connects it to the contained class. 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.
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.Entity Relationship Diagram (ERD) Tutorial - Part 1
We can indicate the multiplicity of an association by adding multiplicity adornments to the line denoting the association. 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.
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.