This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The answer is what it would be for a real world assignment - enough detail that your solution is clearly understood.
I'm aware that's a rather indistinct answer, and that's how life goes sometimes. You need to be able to explain the assumptions you made and give a clear picture of what you were thinking and what your intentions with the design were, but anything beyond that is extraneous. That's not to say more than the required amount is harmful, mind you, but just that you need to ensure that someone other than yourself will understand if they read it.
Hope that helps!
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!