Question

“A complete and consistent class diagram is all that is required to implement and to understand an application”. Discuss the correctness or otherwise of this statement. In your answer, among other things, consider how class diagrams evolve and who has a stake in the understanding and implementation of an application.

Answer 1

Yes, The given statement is also correct but it is in a simple way.

As per my knowledge

The class diagram is a static diagram. It represents the static view of an application. The class diagram is not only used for visualizing, describing, and documenting different aspects of a system but also for constructing executable code of the software application.

The class diagram describes the attributes and operations of a class and also the constraints imposed on the system. The class diagrams are widely used in the modeling of object-oriented systems because they are the only UML diagrams, which can be mapped directly with object-oriented languages.

The class diagram shows a collection of classes, interfaces, associations, collaborations, and constraints. It is also known as a structural diagram.

Purpose of Class Diagrams:
The purpose of the class diagram is to model the static view of an application. Class diagrams are the only diagrams that can be directly mapped with object-oriented languages and thus widely used at the time of construction.

UML diagrams like activity diagram, sequence diagram can only give the sequence flow of the application, however, the class diagram is a bit different. It is the most popular UML diagram in the coder community.

The purpose of the class diagram can be summarized as −

• Analysis and design of the static view of an application.
• Describe the responsibilities of a system.
• The base for component and deployment diagrams.
• Forward and reverse engineering.

Essential elements of A UML class diagram:

1. Class Name
2. Attributes
3. Operations

Class Name:

The name of the class is only needed in the graphical representation of the class. It appears in the topmost compartment. A class is the blueprint of an object which can share the same relationships, attributes, operations, & semantics. The class is rendered as a rectangle, including its name, attributes, and operations in sperate compartments.

Following rules must be taken care of while representing a class:

• A class name should always start with a capital letter.
• A class name should always be in the center of the first compartment.
• A class name should always be written in bold format.
• An abstract class name should be written in italics format.

Attributes:
An attribute is named property of a class that describes the object being modeled. In the class diagram, this component is placed just below the name-compartment.

A derived attribute is computed from other attributes. For example, an age of the student can be easily computed from his/her birth date.

Attributes characteristics

• The attributes are generally written along with the visibility factor.
• Public, private, protected and package are the four visibilities which are denoted by +, -, #, or ~ signs respectively.
• Visibility describes the accessibility of an attribute of a class.
• Attributes must have a meaningful name that describes the use of it in a class.

Relationships
There are mainly three kinds of relationships in UML:

1. Dependencies
2. Generalizations
3. Associations

Dependency

A dependency means the relation between two or more classes in which a change in one may force changes in the other. However, it will always create a weaker relationship. Dependency indicates that one class depends on another.

In the following example, Student has a dependency on College

Generalization:

A generalization helps to connect a subclass to its superclass. A sub-class is inherited from its superclass. Generalization relationship can't be used to model interface implementation. Class diagram allows inheriting from multiple superclasses.

In this example, the class Student is generalized from Person Class.

Association:

This kind of relationship represents static relationships between classes A and B. For example; an employee works for an organization.

Here are some rules for Association:

• Association is mostly verb or a verb phrase or noun or noun phrase.
• It should be named to indicate the role played by the class attached at the end of the association path.
• Mandatory for reflexive associations
• In this example, the relationship between student and college is shown which is studies.

Multiplicity:

A multiplicity is a factor associated with an attribute. It specifies how many instances of attributes are created when a class is initialized. If a multiplicity is not specified, by default one is considered as a default multiplicity.

Let's say that that there are 100 students in one college. The college can have multiple students.

Aggregation

Aggregation is a special type of association that models a whole- part relationship between aggregate and its parts.

Composition:

The composition is a special type of aggregation which denotes strong ownership between two classes when one class is a part of another class.

For example, if college is composed of classes student. The college could contain many students, while each student belongs to only one college. So, if college is not functioning all the students also removed.

The class diagram is evolved for gving overview of the system or project how going to develop it is.

The stakeholders in the understanding and implementation of an application is the developers because they have to implement the project or system using the class diagram they can easily understand everything about the project how to develop it.And also Admins they are the ones who are going to use the system so they have to have this understandnig for hosting the system and to understand how the functionalities are working in the system .