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CLASSES A Class in the software system is represented by a rectangle with the name of the class written inside it. Classes may also be used to represent domain or other non-software elements. An optional compartment below the class name can show the class's ''attributes'' (i.e., its ''properties''). Each ''attribute'' is shown with at least its name, and optionally with its type, initial value, and other properties. The class's ''operations'' (indicating the Methods ) can appear in another compartment. Each ''operation'' is shown with at least its name, and optionally also with its parameters and return type. Other compartments may be defined, e.g. to capture responsibilities, requirements, constraints. Attributes and operations may have their visibility marked as follows:
A Class diagram shows the details about how the different classes are linked together, what are the relationships between them, what are the attributes and methods of those classes, etc. 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 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. Aggregation '' Aggregation '' is a variant of the "has a" or association relationship; composition is more specific than aggregation. 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. ''Composition'' has a strong ''life cycle dependency'' between instances of the container class and instances of the contained class(es): If the container is destroyed, every instance that it contains is destroyed as well. 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. 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 whole 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, CM02. This is often called a "catalog" relationship. 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.
:A is a type of B :E.g. "an oak is a type of tree", "a sedan is a type of vehicle" Generalizations can only be shown on class diagrams and on Use Case Diagram s. 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 an unfilled arrowhead towards the supplier. Realizations can only be shown on class diagrams. 1 General Relationship Dependency (UML) 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 NAG Language]] --> 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: SEE ALSO EXTERNAL LINKS
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