They are not initialised at compile  time. They are initialised at
runtime, when the class  is loaded. The compiler  only adds the expression
to initialize the static  variable, in the .class file. For a primitive,
this can be a literal, like in 'int i=5;', so then there would be 4
bytes in the .class file that hold the integer value 5, and that will be
assigned to i when the class is loaded (or just after the class is loaded).