Java Spring developer for ICal integration

We need to build the iCal( apple Cal) services which can integarte and Sync with Google Cal/Outlook Calanders. And also should be able to sync with the user Calander, basically we require create, edit, delete a appointment, update/reschedule .We have a portal anyone can register During the registration, they can select what kind of calendar they have, they can, they can update on that. User can choose Ical when they select the calendar, it means he is trying to give away his slots available on his calendar .