UML中的association，aggregation以及composition

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# UML

UML 我们平时经常会用到，但有一些概念一直搞不清，在这里梳理一下。

基本概念

在这里使用微软文档中的图例进行说明

Shape	Element	Description
1	Class	A definition of objects that share given structural or behavioral characteristics. For more information, see Properties of types on UML class diagrams.
1	Classifier	The general name for a class, interface, or enumeration. Components, use cases, and actors are also classifiers.
2	Collapse/ Expand control	If you cannot see the details of a classifier, click the expander at upper-left of the classifier. You might also have to click the [+] on each segment.
3	Attribute	A typed value attached to each instance of a classifier. To add an attribute, click the Attributes section and then press ENTER. Type the signature of the attribute. For more information, see Properties of attributes on UML class diagrams.
4	Operation	A method or function that can be performed by instances of a classifier. To add an operation, click the Operations section and then press ENTER. Type the signature of the operation. For more information, see Properties of operations on UML class diagrams.
5	Association	A relationship between the members of two classifiers. For more information, see Properties of associations on UML class diagrams.
5a	Aggregation	An association representing a shared ownership relationship. The Aggregation property of the owner role is set to Shared.
5b	Composition	An Association representing a whole-part relationship. The Aggregation property of the owner role is set to Composite.
6	Association Name	The name of an association. The name can be left empty.
7	Role Name	The name of a role, that is, one end of an association. Can be used to refer to the associated object. In the previous illustration, for any Order O, O.ChosenMenu is its associated Menu. Each role has its own properties, listed under the properties of the association.
8	Multiplicity	Indicates how many of the objects at this end can be linked to each object at the other. In the example, each Order must be linked to exactly one Menu. * means that there is no upper limit to the number of links that can be made.
9	Generalization	The specific classifier inherits part of its definition from the general classifier. The general classifier is at the arrow end of the connector. Attributes, associations, and operations are inherited by the specific classifier. Use the Inheritance tool to create a generalization between two classifiers.

其中重点记录一下Association，Aggregation和Composition

Association

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. 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, uni-directional, aggregation (includes composition aggregation) and reflexive. 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.

Association简单理解就是代表两个类之间的关系。

Aggregation

Aggregation is a variant of the “has a” association relationship; aggregation is more specific than 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.[7]

Aggregation can occur when a class is a collection or container of other classes, but the contained classes do not have a strong lifecycle dependency on the container. The contents of the container still exist when the container is destroyed.

In UML, it 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.

Example: Library and Students. Here the student can exist without library, the relation between student and library is aggregation.

Aggregation.是 “has a”的关系，比association更加具体，它代表了part-whole或者part-of的关系，比如一个教授“has a”一个班级来上课。

当一个类是其他类的容器时，aggregation的关系就产生了，但是被包含的类与包含的类之间没有强的生命周期依赖，容器中的东西依然存在即使容器以及不在了。

Composition

composition关系中的两个类有强生命周期依赖，也即意味着一个类被删了，另一个类也随之被删了

Differences between Composition and Aggregation[edit]

• Composition relationship

  1. When attempting to represent real-world whole-part relationships, e.g. an engine is a part of a car.

  2. When the container is destroyed, the contents are also destroyed, e.g. a university and its departments.

• Aggregation relationship

  1. When representing a software or database relationship, e.g. car model engine ENG01 is part of a car model CM01, as the engine, ENG01, may be also part of a different car model.[8]

  2. When the container is destroyed, the contents are usually not destroyed, e.g. a professor has students; when the professor dies the students do not die along with them.

Reference

1. Types of UML Diagrams
2. UML Class Diagrams: Reference
3. UML Association vs Aggregation vs Composition
4. Class diagram