# Use Case Diagrams

## Overview

Use case diagrams describe a set of actions (called use cases) that a system should or can perform in collaboration with one or more external users of the system (called actors). Each use case should provide some observable and valuable result to the actors.

1. Use Case Diagrams describe the high-level **functional behavior** of the system.
2. It answers **what system does** from the user point of view.
3. Use case answers **‘What will the system do?’** and at the same time tells us **‘What will the system NOT do?’**.

## Example

![](https://2407442552-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Lpv9LvHzpublmUWisvz%2F-Ls5uOGIDPpBy4B0MRX2%2F-Ls5uPmUO4nu-xO450LW%2Fuse_case_diagram.png?generation=1572074008690504\&alt=media)

## Componets

* **System boundary**: A system boundary defines the scope and limits of the system. It is shown as a rectangle that spans all use cases of the system.
* **Actors**: An actor is an entity who performs specific actions. These roles are the actual business roles of the users in a given system. An actor interacts with a use case of the system. For example, in a banking system, the customer is one of the actors.
* **Use Case**: Every business functionality is a potential use case. The use case should list the discrete business functionality specified in the problem statement.
* **Include**: Include relationship represents an invocation of one use case by another use case. From a coding perspective, it is like one function being called by another function.
* **Extend**: This relationship signifies that the extended use case will work exactly like the base use case, except that some new steps will be inserted in the extended use case.


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