Background:- Use case diagram is a platform that can provide a common understanding for the end
users, developers and the domain experts. It is used to capture the basic functionality i.e. use cases,
and the users of those available functionality, i.e. actors, from a given problem statement.
In this experiment, we will learn how use cases and actors can be captured and how different use
cases are related in a system.
Use case diagrams
Use case diagrams belong to the category of behavioural diagram of UML diagrams. Use case
diagrams aim to present a graphical overview of the functionality provided by the system. It consists
of a set of actions (referred to as use cases) that the concerned system can perform one or more actors,
and dependencies among them.
Actor
An actor can be defined as an object or set of objects, external to the system, which interacts with the
system to get some meaningful work done. Actors could be human, devices, or even other systems.
For example, consider the case where a customer withdraws cash from an ATM. Here, customer is a
human actor.
Actors can be classified as below:
• Primary actor: They are principal users of the system, who fulfill their goal by availing some
service from the system. For example, a customer uses an ATM to withdraw cash when he
needs it. A customer is the primary actor here.
• Supporting actor: They render some kind of service to the system. "Bank representatives",
who replenishes the stock of cash, is such an example. It may be noted that replenishing stock
of cash in an ATM is not the prime functionality of an ATM.
In a use case diagram primary actors are usually drawn on the top left side of the diagram.
Use Case
A use case is simply a functionality provided by a system.
Continuing with the example of the ATM, withdraw cash is a functionality that the ATM provides.
Therefore, this is a use case. Other possible use cases include check balance, change PIN, and so on.
Use cases include both successful and unsuccessful scenarios of user interactions with the system. For
example, authentication of a customer by the ATM would fail if he enters wrong PIN. In such case, an
error message is displayed on the screen of the ATM.
Subject
Subject is simply the system under consideration. Use cases apply to a subject. For example, an ATM
is a subject, having multiple use cases, and multiple actors interact with it. However, one should be
careful of external systems interacting with the subject as actors.
Graphical Representation
An actor is represented by a stick figure and name of the actor is written below it. A use case is
depicted by an ellipse and name of the use case is written inside it. The subject is shown by drawing a
rectangle.
Label for the system could be put inside it. Use cases are drawn inside the rectangle, and actors are
drawn outside the rectangle.
Association between Actors and Use Cases
A use case is triggered by an actor. Actors and use cases are connected through binary associations
indicating that the two communicates through message passing.
An actor must be associated with at least one use case. Similarly, a given use case must be associated
with at least one actor. Association among the actors are usually not shown. However, one can depict
the class hierarchy among actors.
