How can i connect imagine a Qt interface(C++) with a logic in java. I feel like I've should know this already but I don't, I know there are API but I don't want to do it,

More context: They told me to write an aplication, game such as 4 connect, battleship or that kind of games, so I've thought to make the "snakes and ladders", i knew how i could do the 2 thinks separately but I don't know how to connect the frontend with backend, even tho I knew the games should works with threads. So I don't know if I could do it with the interface in Qt, because the main porpouse of the practice is to work with threads