Both WorkBook and WritableWorkbook classes are extended by Object (the Parent/Root of all classes).
WorkBook contains a method createWorkBook() which accepts a File type parameter and returns a WritableWorkBook after processing. createWorkBook() is only defined in WorkBook method and as it returns WritableWorkBook so
WritableWorkBook obj = WorkBook.createWorkBook(File fileObj);
This will pass fileObj to createWorkBook of WorkBook and get a WritableWorkBook object which will be assigned to WritableWorkBook object.
I hope it seems clear now.