The Elements of UML(TM) 2.0 Style

Category: Programming
Author: Scott W. Ambler
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by anonymous   2019-01-13

First:

There is no competition between any of the UML diagrams to be the "first one". Sometimes it is better to work on some diagrams simultaneously and iteratively.

Second:

Each diagram can be used in different contexts and for different purposes.

Use Case Diagram vs. Activity Diagram

"Use Cases" are scenarios which show how the user will use the system to achieve their goals.

So:

Instead of showing this "scenario" with written use cases, you can visualize its' steps with an activity diagram.

But in order to find use cases, you should discover the system requirements to some degree, (e.g. the scope, broad feature set, priority, cost, etc.).

In some business domains, such as for an automation project, in order to discover requirements/use cases, you may have to investigate current business flow. Sometimes this business flow can be complex, so you may want to investigate it with an activity diagram.

So:

An activity diagram can be used to investigate a business process to understand and discover the flow, to better discover requirements.

So:

An Activity Diagram can be used at different levels of software development stages for different purposes.

Just like other diagrams, you can use the Activity Diagram at any time, anywhere, as soon as it can help you to ask the right question, to understand and explore any issue related to your purpose.

Here is a summary purpose of Activity Diagrams:

The purpose of the activity diagram is to model the procedural flow of actions that are part of a larger activity. In projects in which use cases are present, activity diagrams can model a specific use case at a more detailed level. However, activity diagrams can be used independently of use cases for modeling a business-level function, such as buying a concert ticket or registering for a college class. Activity diagrams can also be used to model system-level functions, such as how a ticket reservation data-mart populates a corporate sales system's data warehouse. UML Basics : The activity diagram by by Donald Bell

To get a quick grasp of which diagrams can be used for which purposes, I advise you to check out Scott W. Ambler's mini book: The Elements of UML(TM) 2.0 Style