Class diagram

In the Unified Modeling Language (UML), a class diagram is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, and the relationships between the classes.

Relationships

A relationship is a general term covering the specific types of logical connections found on class and object diagrams. UML shows the following relationships:

Instance Level Relationships

Link

A Link is the basic relationship among objects. It is represented as a line connecting two or more object boxes. It can be shown on an object diagram or class diagram. A link is an instance of an association.

Association

Class diagram example of association between two classes

An Association represents a family of links. Binary associations (with two ends) are normally represented as a line, with each end connected to a class box. Higher order associations can be drawn with more than two ends. In such cases, the ends are connected to a central diamond.

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 five 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. Associations can only be shown on class diagrams.

Example: "department offers courses", is an association relationship.

Aggregation

Class diagram showing Aggregation between two classes

Aggregation is a variant of the "has a" or association relationship; aggregation is more specific than association. It is an association that represents a part-whole relationship. 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.

Aggregation can occur when a class is a collection or container of other classes, but where the contained classes do not have a strong life cycle dependency on the container--essentially, if the container is destroyed, its contents are not.

In UML, it is graphically represented as a clear diamond shape on the containing class end of the tree of lines that connect contained class(es) to the containing class.

Composition

Composition is a stronger variant of the "has a" or association relationship; composition is more specific than aggregation. It is represented with a solid diamond shape.

Composition usually has a strong life cycle dependency between instances of the container class and instances of the contained class(es): If the container is destroyed, normally every instance that it contains is destroyed as well. Note that a part can (where allowed) be removed from a composite before the composite is deleted, and thus not be deleted as part of the composite.

The UML graphical representation of a composition relationship is a filled diamond shape on the containing class end of the tree of lines that connect contained class(es) to the containing class. kabhi kabhi

Differences between Composition and Aggregation

The whole of a composition must have a multiplicity of 0..1 or 1, indicating that a part must be for only one whole. The part of an aggregation may have any multiplicity.

When attempting to represent real-world whole-part relationships, e.g., an engine is part of a car, the composition relationship is most appropriate. However, when representing a software or database relationship, e.g., car model engine ENG01 is part of a car model CM01, an aggregation relationship is best, as the engine, ENG01 may be also part of a different car model. Thus the aggregation relationship is often called "catalog" containment to distinguish it from composition's "physical" containment.

Class Level Relationships

Generalization

The Generalization relationship indicates that one of the two related classes (the subtype) is considered to be a specialized form of the other (the supertype) and supertype is considered as Generalization of subtype. In practice, this means that any instance of the subtype is also an instance of the supertype. An exemplary tree of generalizations of this form is found in binomial nomenclature: human beings are a subtype of simian, which are a subtype of mammal, and so on. The relationship is most easily understood by the phrase '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 supertype 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 supertype in the generalization relationship is also known as the "parent", superclass, base class, or base type.

The subtype in the generalization 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: the use of these terms is extremely common, but can be misleading.

Generalization-Specialization relationship

A is a type of B

E. g. "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.

''''''==== Realization ===='''''' [[In UML modeling, a realization relationship is a relationship between two model elements, in which one model element (the client) realizes the behavior that the other model element (the supplier) specifies. A realization is displayed in the diagram editor as a dashed line with a unfilled arrowhead towards the supplier.

Realizations can only be shown on class diagrams.

A realization is a relationship between classes, interfaces, components, and packages that connects a client element with a supplier element. A realization relationship between classes and interfaces and between components and interfaces shows that the class realizes the operations offered by the interface.]]

General Relationship

Dependency

A dependency exists between two defined elements if a change to the definition of one would result in a change to the other. This is indicated by a dashed arrow pointing from the dependent to the independent element. Several named varieties exist. A dependency can be between instances, classes, or both

Multiplicity

The association relationship indicates that (at least) one of the two related classes makes reference to the other. In contrast with the generalization relationship, this is most easily understood through the phrase 'A has a B' (a mother cat has kittens, kittens have a mother cat).

The UML representation of an association is a line with an optional arrowhead indicating the role of the object(s) in the relationship, and an optional notation at each end indicating the multiplicity of instances of that entity (the number of objects that participate in the association). Common multiplicities are:...

0..1	No instances, or one instance (optional, may)
1	Exactly one instance
0..* or *	Zero or more instances
1..*	One or more instances (at least one)

External links