Tic-Tac-Toe in Java using 2-D arrays

Question

I've run into some trouble while trying to code a simple tic-tac-toe program. My issue lies in the checkForWinner method, which was provided to us as part of the exercise. When I try to compile the program gives me the following error message:

incomparable types: java.lang.String and char

The following is the code:

import java.util.*;
    public class tic
    {
       public static void main(String[] args) 
       {
        Scanner input = new Scanner(System.in);
        String[][] board = 
        {
            {" ", " ", " "},
            {" ", " ", " "},
            {" ", " ", " "}
        };
        boolean done = false;
        int player = 1;
        int row = 0;
        int col = 0;
        while (done != true) 
        {
            System.out.println("Enter the row and column for your next move");
            row = input.nextInt();
            col = input.nextInt();
            if (player == 1) 
            {
                board[row][col] = "X";
                player = 2;
            } 
            else 
            {
                board[row][col] = "O";
                player = 1;
            }
            printBoard(board);

        }
        checkForWinner(board);

      }

      public static void printBoard(String[][] board) 
      {
        for (int row = 0; row < 3; row++) 
        {
            for (int col = 0; col < 3; col++) 
            {
                System.out.print("|" + board[row][col] + "|");
            }
            System.out.println();
            System.out.println("-------");
        }
      }
      public static boolean checkForWinner(String[][] board)
       {
        // checkForWinner() method determines if a pattern of data stored
        // in the 2 D char array indicates the a player has won the game.

            boolean flag = false;
            boolean flag1 = false;
            boolean flag2 = false;
            boolean flag3 = false;
            boolean flag4 = false;

            // checks the contents of each row for matching data   
            for (int i = 0; i <= 2; i++)
            {
                if ((board[i][0] == board[i][1] && board[i][1] == board[i][2]) && board[i][2] != ' ') 
                    flag1 = true;
            }

             // checks the contents of each column for matching data
            for (int j = 0; j <= 2; j++)
            {
                if ((board[0][j] == board[1][j] && board[1][j] == board[2][j]) && board[2][j] != ' ') 
                    flag2 = true;
            }

            // checks the contents of one diagonal for matching data
            if ((board[0][0] == board[1][1] && board[1][1] == board[2][2]) && board[2][2] != ' ') 
                    flag3 = true;

            // checks the contents of the other diagonal for matching data
            if ((board[0][2] == board[1][1] && board[1][1] == board[2][0]) && board[2][0] != ' ') 
                    flag4 = true;

            // checks if any of the previous conditions evaluated to true        
            if (flag1 == true || flag2 == true || flag3 == true || flag4 == true)
                flag = true;

           // returns true if a winner was found; returns false is no winner     
           return flag;
       } // end of checkForWinner method
    }

The error tells me that I should be using type char, but when I've tried changing the arrays to be of type char it tells me that the following bits have incompatible types:

char[][] board = 
    {
        {" ", " ", " "},
        {" ", " ", " "},
        {" ", " ", " "}
    };

And:

            board[row][col] = "X";

            board[row][col] = "O";

How can I change the code to be compatible with char data and why am I getting these errors?

*I'm not trying to change the checkForWinner method to accept String data as it was provided and no changes can be made to it. *

Answer 1

The problem is - board[i][2] != ' '. It should be board[i][2] != " " instead.

' ' is a char while board[i][2] is a String.

Also, board[i][0] == board[i][1] is bug, do board[i][0].equals(board[i][1]) instead. And for the not-equals do !board[i][2].equals(" ").

Answer 2

The " " is interpreted as a string literal. If you want char values, use the single quotes ' '

Answer 3

I see two problems here (and in similar lines):

if ((board[i][0] == board[i][1] && board[i][1] == board[i][2]) && board[i][2] != ' ') 
    flag1 = true;

Here board[i][2] != ' ' you are comparing a String (double quotes) with a char (single quotes). Use double quotes for String literals.

Also, when comparing Strings, use .equals(). The == operator will only return true if the two Strings are the same object, i.e. they share the same space in memory.

So, that if statement should look like:

if ((board[i][0].equals(board[i][1]) && board[i][1].equals(board[i][2])) && !board[i][2].equals(" "))