Java Sudoku Recursive Solver

Question

I am very confused why I am still getting these errors. I have commented in the code what is giving me the errors. Any advice would be great.

import java.io.*;
import java.util.*;

public class SudokuSolver  {

    private static int[][] grid;
    int r, c;
    public static void main (String [] args) throws FileNotFoundException {


        try {
            System.out.println("Enter File name: ");
            Scanner fin = new Scanner (System.in);
            String fname = fin.next();
            fin = new Scanner(new FileInputStream(fname));
            int [][] board = new int [9][9];
            for (int r = 0; r<9; r++) {
                for (int c=0; c<9; c++) {
                    board [r][c] = fin.nextInt();
                }
            }
            for (int r = 0; r < 9; r++) {
                for(int c = 0; c < 9; c++) {
                    System.out.print(board[r][c]);
                }
            }
            if(solveSudoku(grid == true)) //giving an error "Incompatible Operand Types" 
                printGrid(grid);
            else 
                System.out.printf("No solution exists");

        }
        catch (FileNotFoundException e){
            System.out.println("File could not be found. Exiting...");
            System.exit(0);

        }
    }
    public boolean solveSudoku(int[][] grid) {
        if(!findUnassignedLocation(grid)) {
            return true;
        for (int num = 1; num <= 9; num++) {
            if(isValid(grid,r,c,digit)) //Throwing error "digit can not be resolved as a variable"
            {
                grid[r][c] = num;
                if(solveSudoku(grid))
                    return true;

                grid[r][c] = 0;
            }
            return false; 
        }

        }
    }
    public boolean findUnassignedLocation(int[][] grid) {
        for(r = 0; r < 9; r++) {
            for(c= 0; c < 9; c++) {
                if(grid[r][c]==0)
                    return false;
            }
        }
        return false;
    }
    public boolean inRow(int[][] grid, int r, int digit) {
        for (int c = 0; c < 9; c++) {
            if(grid[r][c] == digit)
                return true;
        }
        return false;
    }
    public boolean inCol(int[][] grid, int c, int digit) {
        for(int r = 0; r < 9; r++ ) {
            if(grid[r][c] == digit)
                return true;
        }
        return false;
    }
    public boolean inBox(int[][] grid, int startR, int startC, int digit) {
        for (int r = 0; r <3; r++) 
            for (int c = 0; c < 3; c++)
                if(grid[r+startR][c+startC] == digit)
                    return true;
        return false; 

    }
    public boolean isValid(int[][] grid, int r, int c, int digit) {
        return !inRow(grid,r,digit) && !inCol(grid,c,digit) && !inBox(grid,r-r%3,c-c%3,digit);
    }

    static void printGrid(int[][] grid) {
        for (int row = 0; row < 9; row++)
        {
           for (int col = 0; col < 9; col++)
                 System.out.printf("%2d", grid[row][col]);
            System.out.printf("\n");
        }
    }
}

Answer 1

Change:

if(solveSudoku(grid == true)) //giving an error "Incompatible Operand Types"

if(isValid(grid,r,c,digit)) //Throwing error "digit can not be resolved as a variable"

To:

if(solveSudoku(grid))

if(isValid(grid,r,c,num))