Tic Tac Toe in Java

tictactoe.java

import java.io.*;
import java.lang.*;

class InvalidMoveException extends Exception {}
class InvalidBoardStateException extends RuntimeException {}

class Board {
    private int n, n2;
    private char[] a;
    private static final char pchar[] = {'-', 'X', 'O'};

    public static char getPChar(int i) {
        return pchar[i];
    }

    public Board(int size) {
        n = size;
        n2 = n*n;
        a = new char[n2];
        for(int i=0; i<n2; ++i) {
            a[i] = pchar[0];
        }
    }

    public int getSize() {return n;}

    public String toString() {
        return new String(a, 0, 3) + "\n" + new String(a, 3, 3) + "\n" + new String(a, 6, 3);
    }

    public char getCell(int i) {
        return a[i];
    }
    public char getCell(int i, int j) {
        return a[i*n + j];
    }

    public void setCell(int i, char ch) throws InvalidMoveException {
        if(i >= n2 || a[i] != pchar[0] || (ch != pchar[1] && ch != pchar[2])) {
            throw new InvalidMoveException();
        }
        else {
            a[i] = ch;
        }
    }

    /**
    Returns 0 if player 1 wins, 1 if player 2 wins, -1 if no one wins.
    Throws InvalidBoardStateException if there are 2 winners.
    */
    public int getWinner() {
        char ch = getWinnerChar();
        if(ch == pchar[0]) {
            return -1;
        }
        else if(ch == pchar[1]) {
            return 0;
        }
        else {
            return 1;
        }
    }

    /**
    Returns pchar[1] if player 1 is winner, pchar[2] if player 2 is winner and pchar[0] if tie.
    Throws InvalidBoardStateException if there are 2 winners.
    */
    public char getWinnerChar() {
        char result = pchar[0];

        // check rows
        for(int i=0; i<n; ++i) {
            char ch = getCell(i, 0);
            if(ch != pchar[0]) {
                boolean winningRow = true;
                for(int j=1; j<n; ++j) {
                    if(getCell(i, j) != ch) {
                        winningRow = false;
                    }
                }
                if(winningRow) {
                    if(result != pchar[0] && result != ch) {
                        throw new InvalidBoardStateException();
                    }
                    result = ch;
                }
            }
        }

        // check columns
        for(int j=0; j<n; ++j) {
            char ch = getCell(0, j);
            if(ch != pchar[0]) {
                boolean winningCol = true;
                for(int i=1; i<n; ++i) {
                    if(getCell(i, j) != ch) {
                        winningCol = false;
                    }
                }
                if(winningCol) {
                    if(result != pchar[0] && result != ch) {
                        throw new InvalidBoardStateException();
                    }
                    result = ch;
                }
            }
        }

        // check diagonals
        char ch = getCell(0, 0);
        if(ch != pchar[0]) {
            boolean winningDiag = true;
            for(int i=1; i<n; ++i) {
                if(ch != getCell(i, i)) {
                    winningDiag = false;
                }
            }
            if(winningDiag) {
                if(result != pchar[0] && result != ch) {
                    throw new InvalidBoardStateException();
                }
                result = ch;
            }
        }
        ch = getCell(n-1, 0);
        if(ch != pchar[0]) {
            boolean winningDiag = true;
            for(int i=1; i<n; ++i) {
                if(ch != getCell(n-1-i, i)) {
                    winningDiag = false;
                }
            }
            if(winningDiag) {
                if(result != pchar[0] && result != ch) {
                    throw new InvalidBoardStateException();
                }
                result = ch;
            }
        }

        return result;
    }
}

class Main
{
    private static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    public static int readInt() throws IOException {
        return Integer.parseInt(br.readLine());
    }

    public static void main(String[] args) throws IOException
    {
        final int n = 3;
        int activePlayer = 0;
        Board board = new Board(n);
        int turni=0;
        for(turni=0; turni<n*n; ++turni) {
            System.out.println();
            System.out.println(board.toString());

            while(true) {
                System.out.print("" + board.getPChar(activePlayer+1) + "> ");
                int index = readInt();
                try {
                    board.setCell(index, board.getPChar(activePlayer+1));
                    break;
                }
                catch(InvalidMoveException e) {
                    System.out.println("Invalid move.");
                }
            }

            int winner = board.getWinner();
            if(winner >= 0) {
                System.out.println("Player " + board.getPChar(activePlayer+1) + " wins.");
                break;
            }

            activePlayer = 1 - activePlayer;
        }
        if(turni == n*n) {
            System.out.println("Tie");
        }
    }
}