A few more specifications and their implementations are still missing. We should invoke the drop() method whenever our TicTacToe class is instantiated. We should also make sure that RuntimeException is thrown when drop() returns false. We'll leave those specifications and their implementations as an exercise for you.
The source code can be found in the 03-mongo branch of the tdd-java-ch06-tic-tac-toe-mongo Git repository (https://bitbucket.org/vfarcic/tdd-java-ch06-tic-tac-toe-mongo/branch/03-mongo). The classes in particular are TicTacToeSpec and TicTacToe.