/**
   A 3 x 3 tic-tac-toe board.
*/
public class TicTacToe
{
   /**
      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;
   }

   public String getWinner()
   {
       int a = 0;
       int b = 0;
       String win = "";

       for (int i = 0; i < 3; i++)
       {
            for (int j = 0; j < 3; j++)
            {
                if (board[i][j].equals(" "))
                    return null;
                else if (board[i][j].equals("x"))
                    a++;
                else if (board[i][j].equals("o"))
                    b++;
            }
       }

       if (a == 3)
        {
            win = "Player x is the winner.";
        }
       else if (b == 3)
       {
            win = "Player o is the winner.";
        }

       return win;
   }

   private String[][] board;
   private static final int ROWS = 3;
   private static final int COLUMNS = 3;
}