Rose apple trees will only grow outside in zone ten or higher, but they are not large trees. You could probably get one to fruit in a pot, but it will take many years to get one to fruit for you. Here's some photos of mine.
FLOWERS

I find rose apple tastes like scented cardboard. lol
At least in here the fruit just rottens in the ground. Perhaps there are several cultivars.

They're best picked from the tree. If you have fruitflies in your area, they can infest the tree.
Why would you be eating scented cardboard? lol.
kevin

Scented cardboard is the best the description ever!! lol
I want to add that it is a bit juicier than a cardboard.

Cardboard is good for the digestive system. Lots of fiber!! lol

Leona

I forgot to state that I was told it tastes like scented cardboard!!!!! >=:
LOL

My dh grew up with rose apple and loves them (YUCK). I have yet to find a spot for a tree in my already crowded yard but hearing they do well in a pot has me excited Cagary. Do you grow them in pots? Are they easy to maintain that way?

I think many of the Syzygium species have this problem. Most can be sweet(still not very sweet compared to other fruits but sweet to the point where its quite enjoyable) and others that are just tasteless. Wax jambu has this same problem. Lots of variability in seedlings when it comes to tastes I guess.