Hello,
developing a demo app makes a lot of sense to me, but the best way to demonstrate skills to most potential employers in Europa is to get certified. Get 2 or 3 developer certificates and the
SCEA 5.0 after you made great progress in the tutorial. Potential employers often don't take the time to review a demo app to see if it is good.
I recommend the Sun Java EE tutorial.
It's "Java EE" now, not "J2EE".
Kai