/**
   A 3 x 3 tic-tac-toe board.
*/
public class TicTacToe
{
   private String[][] board;
   private static final int ROWS = 3;
   private static final int COLUMNS = 3;

   /**
      Constructs an empty board.
   */
   public TicTacToe()
   {
      board = new String[ROWS][COLUMNS];
      // Fill with spaces
      for (int i = 0; i < ROWS; i++)
         for (int j = 0; j < COLUMNS; j++)
            board[i][j] = " ";
   }

   /**
      Sets a field in the board. The field must be unoccupied.
      @param i the row index 
      @param j the column index 
      @param player the player ("x" or "o")
   */
   public void set(int i, int j, String player)
   {
      if (board[i][j].equals(" "))
         board[i][j] = player;
   }

   /**
      Creates a string representation of the board, such as
      |x  o|
      |  x |
      |   o|
      @return the string representation
   */
   public String toString()
   {
      String r = "";
      for (int i = 0; i < ROWS; i++)
      {
         r = r + "|";
         for (int j = 0; j < COLUMNS; j++)         
            r = r + board[i][j];
         r = r + "|\n";
      }
      return r;
   }
}