Question

I’m not sure how to model sub-methods in UML sequence diagram. When in the execution of one method another method is called (from the same class). I tried to give an example below:

How would you guys model this in UML (in a sequence diagram)?

..
car1.drive();
..

…
in Car class:

..
drive(){
    this.startEngine();
}
startEngine(){
    this.getKey();
    this.insertKey();
}
..

a small begin:

objx     car1
----     ----
|         |
| drive() |
|-------->| startEngine()
|         |------------.
|         |            |
|         |<-----------.
|         |

But where comes the getKey() method?
Must this be communicated via another sequence diagram? Or is there a way to include sub procedures?

Answer 1

You use a self-message, as in the following:

Note the “stacked” lifelines? Those are activation levels. It shows that the drive method calls startEngine, which then calls getKey followed by insertKey. I’ve left the returns implicit.