Design issue: Client <- CustomeData -> Service

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http://stackoverflow.com – I'm designing a client / service software. client / service are both on some android. client and service communicate using messenger and data format is class CustomeData. CustomeData also has some inner classes which define detail structure. Do I need to make CustomeData as thin as possible? I intent to use it as data format only recognized by both client and service. Service has additional operations on CustomeData, and may store other info to it too, for example _ID in DB. But client don't care what _ID is. I'm not sure whether I need to attach more operations on CustomeData or create new (HowTos)