This will be a text based implementation but would be easy to port into a graphic based version.

TicTacToe2.java

1. Complete the constructor so the board can be any size, not just 3 by 3. Read the comments for details
2. complete the changePlayer method.
3. complete the mark method, that puts the replaces the grid location with the letter stored in player, and then calls the changePlayer method (Don't reduplicate code inside the mark method).
4. complete the getPlayer method
5. make a method called movesRemaining that returns the number of spaces in grid
6. write a main method that makes an instance of the TicTacToe2 class and the calls the print method:

public static void main(String[] args)

Next we can work on the game, and the testing the winner() method of the TicTacToe2 class.

TicTacToeTexGame.java

WinnerTester.java