UML Sequence Diagrams

During the requirements analysis phase, the system can be treated as a single "black box", which means that we can look at the system's behavior (what it does) without explaining how it does it. Read this section to see an example of a simplified trace diagram that shows only system input events. This is called a system sequence diagram.

3. Identifying Concepts

The requirement document which is up to now including: the overview, goal, functional requirements, use cases, use case diagram and use case descriptions is the good source of information in identifying concepts. Two activities that are fundamental to the useful application of object oriented in OOA are:

  • To identify as many candidate objects from the problem domain, and 
  • Later select those candidate objects that are significant to develop and be specified in an object model (conceptual model)

Liu suggests the following candidate concepts from the requirement document as shown in Table 3.10.

Table 3.10: Guide to Identify Concepts

Concept Category Examples
physical or tangible objects (or things POST (i.e. Point-of-Sale Unit), House, Car, Sheep, People, Airplane
places Store, Office, Airport, Police Station
documents, specifications, designs, or descriptions of things Product Specification, Module Description, Flight Description
transactions Sale, Payment, Reservation
roles of people Cashier, Student, Doctor, Pilot
containers of other things Store, Bin, Library, Airplane
things in a container Item, Book, Passenger
other computers or electromechanical systems external to our system Credit Card Authorization System, Air-Traffic Control
abstract noun concepts Hunger, Acrophobia
organizations Sales Department, Club, Object Airline
historic events, incidents Sale, Robbery, Meeting, Flight, Crash, Landing
Processes (often not represented as a concept, but may be) Selling A Product, Booking A Seat
rules and policies Refund Policy, Cancellation Policy

Catalogs Product Catalog, Parts Catalog
records of finance, work, contracts, legal matters Receipt, Ledger, Employment Contract, Maintenance Log
financial instruments and services Line Of Credit, Stock
manuals, books Employee Manual, Repair Manual

Another useful and simple technique for the identification of concepts is to identify or extract noun and noun phrases from the requirement document and consider them as candidate concepts or attributes.


Care must be applied when these methods are used; mechanical noun-to-concept mapping is not possible, words in natural languages are ambiguous, and concept categories may include concepts which are about either attributes, events or operations which should not be modeled as classes. We should concentrate on the objects/classes involved in the realization of the use cases.

Consider the POST system, from the use case Buy Items with Cash. We can identify some noun phrases as shown in Figure 3.8. Some of the noun phrases in the use case are candidate concepts; some may be attributes of concepts.

Figure 3.8: Selected Possible Concepts for the POST System

The following guidelines are useful when identifying concepts:

  • It is better to over specify a conceptual model with lots of fine-grained concepts, than to under specify it. 
  • Do not exclude a concept simply because the requirements do not indicate an obvious need to remember information about it. 
  • It is common to miss concepts during the initial identification phase, and to discover them later during the consideration of attributes or associations, or during the design phase. When found, they are added to the conceptual model. 
  • Put a concept down as a candidate in the conceptual model when you are not sure it must not be included.