question: public static String relativePath ( File file , File dir ) { checkArgument ( file . isFile ( ) || file . isDirectory ( ) , "%s is not a file nor a directory." , file . getPath ( ) ) ; checkArgument ( dir . isDirectory ( ) , "%s is not a directory." , dir . getPath ( ) ) ; return relativePossiblyNonExistingPath ( file , dir ) ; }
target:  Computes the relative of a file or directory with respect to a directory.