The Use Case Diagram

To ensure that everyone can immediately understand a use case diagram, standardized building blocks are used for the illustration. There are three essential elements to this:

• Actor: Actors, both persons and systems, are depicted as stick figures.
• System: The system being addressed by the use case diagram is depicted as a rectangle.
• Use case: A use case is depicted as an oval shape which usually contains a short phrase describing the process.

The relationships between these elements are described using lines to link them. These are called associations. A solid line between an actor and a use case indicates that the actor and the use case described in the oval are related. Dashed lines are used for relationships between different use cases. Since there are two different types of associations that can exist between use cases, the lines are expanded upon using a keyword referred to as a stereotype in UML which is enclosed in double chevrons. An arrow is also used to indicate the dependency between the two use cases. There are two distinct stereotypes:

• Include associations: This type of association means that a base use case includes a second use case. This is indicated using a dashed line connecting the two use cases with an arrow pointing toward the second use case.
• Extend associations: This type of association has a dashed line with an arrow pointing in the opposite direction towards the base use case. This means that the second use case extends the base use case under certain conditions, but it does not always happen.

So, while the associations require that both use cases be executed, the second use case in the extend association is only executed under certain conditions. These conditions are specified in the UML use case diagram as extension points. This is illustrated in two ways:

• Extending the use case oval: Under the name of the use case, the possible extension points are named and briefly described.
• Adding a note: The extended stereotype is connected via a dashed line to a stylized note (i.e. a rectangle with a bent corner) which is labeled with “condition” and “extension point”. Next to “condition” enclosed in curly brackets, you will find the condition which must be met for the second use case to be executed. Next to “extension point”, you will find the name of the use case oval so that the extension can be clearly identified.

In this example, the ATM is the system and the bank customer is the actor intending to execute the “withdraw money” use case. This use case is connected through dependencies to two other additional use cases: “authentication” and “PIN and account verification”. If the authentication is not successful, the customer’s request cannot be executed. To prevent the customer from making an infinite number of attempts, the card should be blocked after the PIN has been incorrectly entered three times. Therefore, an extension point is added to the “authentication” use case with the condition “incorrectly enter PIN three times”. If this condition is met, the “block card” use case is executed which is connected to the “authentication” use case via an extend relationship. Here is what the use case diagram for this example looks like: