TicTacToe.java

public class TicTacToe{
    public static final int X = 1, O = -1;
    public static final int EMPTY = 0;
    private int board[][] = new int[3][3];
    private int player;

    public TicTacToe(){clearBoard();}

    public void clearBoard(){
        for (int i = 0; i <3; i++) {
            for (int j = 0; j < 3; j++) {
                board[i][j]=EMPTY;
            }
        }
        player = X;
    }

    public void putMark(int i, int j) throws IllegalArgumentException {
        if ((i<0) || (i>2) || (j<0) || (j>2)) {
            throw new IllegalArgumentException("Invalid boardposition.");
        }
        if (board[i][j] != EMPTY) {
            throw new IllegalArgumentException("Board position occupied.");
        }
        board[i][j]=player;
        player= -player;
    }
 public boolean isWin(int mark) {
    return ((board[0][0] + board[0][1] + board[0][2] == mark*3) // row 0
        || (board[1][0] + board[1][1] + board[1][2] == mark*3) // row 1
        || (board[2][0] + board[2][1] + board[2][2] == mark*3) // row 2
        || (board[0][0] + board[1][0] + board[2][0] == mark*3) // column 0
        || (board[0][1] + board[1][1] + board[2][1] == mark*3) // column 1
        || (board[0][2] + board[1][2] + board[2][2] == mark*3) // column 2
        || (board[0][0] + board[1][1] + board[2][2] == mark*3) // diagonal
        || (board[2][0] + board[1][1] + board[0][2] == mark*3)); // rev diag
}
    public int winner(){
        if (isWin(X)) {
            return(X);
        } 
        else if (isWin(O)) {
            return(O);
        }
        else{
            return 0;
        }
    }
    public String toString(){
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                switch (board[i][j]){
                    case X:{
                        sb.append("X");
                        break;
                    }
                    case O:{
                        sb.append("O");
                        break;
                    }
                    case EMPTY:{
                        sb.append(" ");
                        break;
                    }
                }
                if (j<2) {
                    sb.append("|");
                }
            }
            if (i<2) {
                sb.append("\n_____\n");
            }
        }
        return sb.toString();
    }

    public static void main(String[] args){
        TicTacToe game = new TicTacToe();
        // O wins
        // game.putMark(0,0); game.putMark(0,2);
        // game.putMark(1,1); game.putMark(0,1);
        // game.putMark(2,0); game.putMark(2,2);
        // game.putMark(2,1); game.putMark(1,2);
        // X Wins
        // game.putMark(1,1); game.putMark(0,2);
        // game.putMark(2,1); game.putMark(0,0);
        // game.putMark(0,1); game.putMark(2,2);
        // game.putMark(1,2); game.putMark(1,0);
        // game.putMark(2,0);
        // Tie
        game.putMark(1,1); game.putMark(0,2);
        game.putMark(2,2); game.putMark(0,0);
        game.putMark(0,1); game.putMark(2,1);
        game.putMark(1,2); game.putMark(1,0);
        game.putMark(2,0);
        System.out.println(game);
        int winningPlayer = game.winner();
        String[] outcome = {"O Wins","Tie", "X Wins"};
        System.out.println(outcome[1+winningPlayer]);
    }
}