When LongRef.create will generated in class file compiled from scala code

I compiled my spark code using maven in idea Intellij with scala.version 2.11.8 and scala.binary.version 2.11. One line of code looks like

var currentMaxId = -1L

And I found below line in the generated class file

currentMaxId = LongRef.create(-1L); 

But when I try to test with a new project with the same line of code, I found only

long currentMaxId = -1L;

in generated class file.

So, how and when "currentMaxId = LongRef.create(-1L)" will produced in class file?