Tic Tac Toe - Adding Draw System / Game Over

I have been trying to add a draw system to my code so far once all the boxes have been taken it does not tell the player its a draw game over. I have tried and deleted some code but I have commented some stuff that did'nt work at the bottom and also the X's and O's can be replaced in the same spaces?

import java.util.*;
//import for scanner
public class NoughtsAndCrosses {
public static int row,col; //integer for rows and columns
public static Scanner scan = new Scanner(System.in); //Scans the program
public static char[][] board = new char [4][4]; //Characters for the board set out 4/4

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