Class Diagram

The class diagram is the main building block of object oriented modelling. It is used both for general conceptual modelling of the systematics of the application, and for detailed modelling translating the models into programming code. Class diagrams can also be used for data modeling. The classes in a class diagram represent both the main objects, interactions in the application and the classes to be programmed.

A class diagram is an illustration of the relationships and source code dependencies among classes in the Unified Modeling Language (UML). In this context, a class defines the methods and variables in an object, which is a specific entity in a program or the unit of code representing that entity. Class diagrams are useful in all forms of object-oriented programming (OOP). The concept is several years old but has been refined as OOP modeling paradigms have evolved.

In a class diagram, the classes are arranged in groups that share common characteristics. A class diagram resembles a flowchart in which classes are portrayed as boxes, each box having three rectangles inside. The top rectangle contains the name of the class; the middle rectangle contains the attributes of the class; the lower rectangle contains the methods, also called operations, of the class. Lines, which may have arrows at one or both ends, connect the boxes. These lines define the relationships, also called associations, between the classes.

Introduction

A class with three sections.

In the diagram, classes are represented with boxes which contain three parts:

• The top part contains the name of the class
• The middle part contains the attributes of the class
• The bottom part gives the methods or operations the class can take or undertake

In the design of a system, a number of classes are identified and grouped together in a class diagram which helps to determine the static relations between those objects. With detailed modelling, the classes of the conceptual design are often split into a number of subclasses.

In order to further describe the behaviour of systems, these class diagrams can be complemented by state diagram or UML state machine.

Scope

The UML specifies two types of scope for members: instance and classifier.

• Classifier members are commonly recognized as “static” in many programming languages. The scope is the class itself.
• Attribute values are equal for all instances
• Method invocation does not affect the instance’s state
• Instance members are scoped to a specific instance.
• Attribute values may vary between instances
• Method invocation may affect the instance’s state (i.e., change instance’s attributes)

To indicate a classifier scope for a member, its name must be underlined. Otherwise, instance scope is assumed by default.

 

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

Links

A Link is the basic relationship among objects.

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

Aggregation

Class diagram showing Aggregation between two classes

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 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 hollow diamond shape on the containing class end of the tree with a single line that connects the contained class to the containing 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.

 

Composition

Class diagram showing Composition between two classes at top and Aggregation between two classes at bottom

Composition is a stronger variant of the "owns a" association relationship; composition is more specific than aggregation.

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, where allowed, a part can 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.

Differences between composition and aggregation

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

As you create your class diagram, you can set the visibility of each attribute and operation in the properties window, or by clicking to open a pop-up menu in the diagram. Convenient fast editing buttons and a specialized class diagram toolbar are also provided to help you easily add elements. UModel identifies the visibility property of each attribute and operation with a unique pictorial icon. The icons are keyed to indicate whether an attribute is public, protected, or private.

sumber : http://en.wikipedia.org/wiki/Class_diagram