include maker(notice that, not all sudoku can be solved)

sudoku.java

import java.util.ArrayList;
import java.util.Random;

public class sudoku {
    public int emptycnt(char[][] board) {
        int reuslt = 0;
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (board[i][j] == '.')
                {
                    reuslt++;
                }
            }
        }
        return  reuslt;
    }
    public void solveSudoku(char[][] board) {
        ArrayList<Integer> empty = new ArrayList<Integer>();
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (board[i][j] == '.')
                {
                    empty.add(i * 9 + j);
                }
            }
        }
        int len = empty.size();
        dfs(board, 0, len, empty);
    }

    char[][] result;

    public boolean dfs(char[][] board, int cur, int len, ArrayList<Integer> empty) {
        if (cur == len)
            return true;
        int index = empty.get(cur);
        int row = index / 9;
        int col = index % 9;
        for (int v = 1; v <= 9; v++) {
            if (isValid(board, v, row, col)) {
                board[row][col] = (char) (v + '0');
                if (dfs(board, cur + 1, len, empty)) {
                    result = board;
                    return true;
                }
                board[row][col] = '.';
            }
        }
        return false;
    }

    public boolean isValid(char[][] board, int v, int row, int col) {
        for (int i = 0; i < 9; i++) {
            if (board[row][i] == v + '0')
                return false;
            if (board[i][col] == v + '0')
                return false;
            int rows = 3 * (row / 3) + (i / 3);
            int cols = 3 * (col / 3) + (i % 3);
            if (board[rows][cols] == v + '0')
                return false;
        }
        return true;
    }
    public void maker(char[][] board, int hardness) {
        Random ran = new Random();
        for (int i = 0; i < hardness; i++) {
            int row = ran.nextInt(8);
            int col = ran.nextInt(8);
            int v = 1+ran.nextInt(8);
            if (board[row][col] != '.')
            {
                i--;
                continue;
            }
            while (!isValid(board, v, row, col)){
                v = 1+ran.nextInt(8);
                board[row][col] = '.';
            }
            board[row][col] = (char) (v + '0') ;
        }
    }
    public static void main(String[] args) {
        char[][] board = new char[9][9];
        sudoku sudoku = new sudoku();
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                board[i][j] = '.';
            }
        }
        sudoku.maker(board, 31);
        sudoku.solveSudoku(board);
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                System.out.print(board[i][j] + " ");
            }
            System.out.println("");
        }

    }
}