How can i make function with a different implementation on windows than in linux. Is this the use case for expect/atual?

can either be an interface, implemented differently on windows and linux. Or can be expect/actual. It really depends with the details. However, almost always favour interfaces, if you encounter limitations, go for expect/actuals

but do i make a module for windows code and one for linux code? How do I know which one to implement during build?

You can use

System.getProperty("os.name")

thanks a lot that's what i was looking for.