Can I have one itemStateChanged method to monitor changes triggered by user for many Displayable objects in a MIDlet (e.g. for a form and a list), using if clauses to decide what was changed, or do I have to create a different method for each Displayable I want to monitor?
If you are talking about instantiating several Form and one ItemStateListener and calling Form.setItemStateListener() several times with the same parameter, then yes, you can do that.
Theoretically, you should write thread-safe code in your implementation of itemStateChanged but given the way existing java-me implementations handle events and the very small number of MIDP-compliant phones using 2 physical screens, I don't think it's very important in practice.