Serbian mystery fruit/nut

Hi you plant whizzes

Can someone identify this tree, which I saw on a walk along the Danube in Belgrade, Serbia?



The fruit was smooth and hard to the touch, and about five centimeters in length.

Laura 3:)

 

Prunus domestica subsp. italica

 

Probably to be used for making slivovitz (plum brandy, Serbia's national drink).

 

Since some leaves purplish probably a purple-leaved plum. Some of these are hybrids rather than forms of pure species.