[Albert]

First of all, you'll need a board.

Code:

private char[][] board = new char[3][3];

I would recommend making final chars to represent what is in each sqaure.

Code:

private final char EMPTY = ' ';
 private final char PLAYER1 = 'X';
 private final char PLAYER2 = 'O';

you need to initialize the board.

Code:

 public void init() {
     for (int x = 0; x < 3; x++) {
         for (int y = 0; y < 3; y++) {
            board[x][y] = EMPTY;
         }
     }
 }

to check the winner, this gets a bit tedious.

Code:

public char winner() {
     for (int i = 0; i < board.length; i++) {
         /* check horizontally */
         if (board[i][0] == board[i][1] &&
                 board[i][0] == board[i][2]) {
             if (board[i][0] != EMPTY) {
                 return board[i][0];
             }
         }
         /* check vertically */
         if (board[0][i] == board[1][i] &&
                 board[0][i] == board[2][i]) {
             if (board[0][i] != EMPTY) {
                 return board[0][i];
             }
         }
     }
      
     /* check diagonally */
     if (board[0][0] == board[1][1] &&
             board[0][0] == board[2][2]) {
         if (board[0][0] != EMPTY) {
             return board[0][0];
         }
     }
     if (board[0][2] == board[1][1] &&
              board[0][2] == board[2][0]) {
         if (board[0][2] != EMPTY)) {
             return board[0][2];
         }
     }
     return EMPTY;
 }

Greetings

Albert