I have made a Java game that is a mix between Pacman and Snake. 

It works fine on my computer. I have tried sending it to my friend, he has a Mac and I have a Windows. His computer says

> 
> can't be opened because it is from an unidentified developer 
> 
> 
> 

How would I fix this? I have tried looking it up, but I can not find a fix. (If more information is needed let me know I will add it.)