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Tutorial 1-UML Diagram:Use case

This tutorial provides a quick introduction to the Unified Modeling Language.
At the center of the UML are its nine kinds of modeling diagrams, which we describe here.
1.Use case diagrams
2.Class diagrams
3.Object diagrams
4.Sequence diagrams
5.Collaboration diagrams
6.Statechart diagrams
7.Activity diagrams
8.Component diagrams
9.Deployment diagrams

The UML is applicable to object-oriented problem solving. Anyone interested in learning UML must be familiar with the underlying tenet of object-oriented problem solving -- it all begins with the construction of a model.
A model is an abstraction of the underlying problem. The domain is the actual world from which the problem comes.
Models consist of objects that interact by sending each other messages. Think of an object as "alive." Objects have things they know (attributes) and things they can do (behaviors or operations). The values of an object's attributes determine its state.
Classes are the "blueprints" for objects. A class wraps attributes (data) and behaviors (methods or functions) into a single distinct entity. Objects are instances of classes.

1.Use case diagrams
Use case diagrams describe what a system does from the standpoint of an external observer. The emphasis is on what a system does rather than how.
Use case diagrams are closely connected to scenarios. A scenario is an example of what happens when someone interacts with the system. Here is a scenario for a medical clinic.

Scenario :"A patient calls the clinic to make an appointment for a yearly checkup. The receptionist finds the nearest empty time slot in the appointment book and schedules the appointment for that time slot. "
A use case is a summary of scenarios for a single task or goal. An actor is who or what initiates the events involved in that task. Actors are simply roles that people or objects play. The picture below is a Make Appointment use case for the medical clinic. The actor is a Patient. The connection between actor and use case is a communication association (or communication for short).

Actors are stick figures. Use cases are ovals. Communications are lines that link actors to use cases.

A use case diagram is a collection of actors, use cases, and their communications. We've put Make Appointment as part of a diagram with four actors and four use cases. Notice that a single use case can have multiple actors.

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