How can a piece of hardware be an actor when designing a use case diagram?
I got confused because I've read on Wikipedia this:
A use case should not include detail regarding user interfaces and screens. This is done in user-interface design, which references the use case and its business rules.
If you give me an example about hardware being an actor, I'd be grateful.
How about a third party system - for example a warehouse management system that produces a feed of stock level changes for different products, which is consumed by your retail application.
That would be an actor. It will not have a UI or screen, but communicate with your system, cause different events to occur and have its own business rules.
I'd suggest the important part here is the definition of an actor.
An actor specifies a role played by a person or thing when interacting with the system
In the system of traffic intersection, there are many 'hardware' actors, including Car and Traffic Light. The system under consideration is the rules around what do to (yield, merge, stop) and when.
The following can all be "actors" on a system you are describing, if these components are outside the scope of the system being described:
If the source of a request for your system to do something is outside the scope of the system, it is usually not necessary to separate the human component from any external tool or hardware they are using to facilitate the requests on your system. In such cases, the actors could very well be automatons.