Use Case Diagram : Introduction and Explanation

Unified Modeling Language ( UML ) covers different types of model or diagrams which represent the designing of a software system. During the designing phase of software development these diagrams represent the working flow of a software. Use Case Diagram is one of the most important diagrams used during the designing phase.

"A use case diagram is a pictorial representation of the interactions among the elements of a system"

A use case is basically used in system analysis to identify, clarify, and organize system requirements. The term "system" refers to something being developed or operated, such as any software system like a mail-order product sales or a Web site. Use case diagrams are included in UML (Unified Modeling Language), a standard notation for the modeling of real-world objects and systems.

Objectives of Use case diagram is to capture overall requirements, validating a hardware design, testing and debugging a software product under development, creating an online help reference, or performing a consumer-service-oriented task.

Example,  use cases for a product sales environment which would include item ordering, catalog updating, payment processing, and customer relations.

A use case diagram contains four components.
  1. The boundary, which defines the system of interest in relation to the world around it.
  2. The actors, usually individuals involved with the system defined according to their roles.
  3. The use cases, which are the specific roles played by the actors within and around the system.
  4. The relationships between and among the actors and the use cases.

Components of Use Case Diagram are as shown in following figure.

                                 

A detailed explanation of each component is as follow:

Actor


You can picture an actor as a user of the IT system, for example, Mr. Steel or Mrs. Smith from check-in. Because individual persons are irrelevant for the model, they are abstracted. So the actors are called "check-in employee" or "passenger":


Use Case


Use cases describe the interactions that take place between actors and IT systems during the execution of business processes:

A use case represents a part of the functionality of the IT system and enables the user (modeled as an actor) to access this functionality.

Association


An association is a connection between an actor and a use case. An association indicates that an actor can carry out a use case. Several actors at one use case mean that each actor can carry out the use case on his or her own and not that the actors carry out the use case together:

Include Relationships


An include relationship is a relationship between two use cases:

It indicates that the use case to which the arrow points is included in the use case on the other side of the arrow. This makes it possible to reuse a use case in another use case. Figure  shows an example of this relationship. In the flow of the use case, express check-in is a point at which the use case generating boarding pass is included. This means that at this point the entire process generating boarding pass is carried out. Include relationships can be viewed as a type of call to a subprogram:

Example 

A use case diagram looks something like a flowchart. Intuitive symbols represent the system elements. Here's a sample example of Library Management System.


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