UML Sequence Diagrams

2. Concepts and Classes in Conceptual Modelling

Conceptual Modelling (sometimes called Domain Modelling) is the activity of finding out which concepts are important to system. This process helps us to understand the problem further, and develop a better awareness of our customer's business. A concept is an idea, thing, or object.  More formally, a concept may be considered in terms of its symbol, intension, and extension:

• Symbol – words or images representing a concept. It can be referred to when we talk about the concept.
• Intension – the definition of a concept.
• Extension – the set of examples or instances to which the concept applies.
• The intension to "represent a course offered as part of a degree in that university"; and 
• The extension of all other modules offered by that university.

Note that the terms class and type are used more by UML and not concept. As long as we are in requirement analysis stage, the two terms class and concept may be used interchangeably.. Each instance of a class is called an object of the class. For example a class called "Student". JahnSmith and JaneBrown are instances of a class Student. Therefore, a class defines a set of objects. 

The notions of class and object are interlinked as one cannot exist without the other, and any object belongs to a class. The differences are: 

• An object is a concrete entity – exists in space and time (persistence property of objects); 
• A class is an abstraction of a set of objects

The UML defines the term class as "a description of a set of objects that share the same attributes, operations, methods, relationships, and semantics".