Java Program runs infinitely with no cause- No StackOverflow but program never terminates - Sudoku generator

Question

I am working on trying to build a basic Sudoku generator in java 9+.

The Code works seemingly fine until a random point wherein it stops returning a new number, however the function generating the random number does still produce the numbers as I tested with a log. But it reaches a certain digit and just stops printing. I cannot figure out why. I am very new to algorithms and backtracking so any insight you can provide is appreciated.

Code below

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

class Scratch_5
{

    static boolean safe = true;
    static int[][] grid = new int[9][9];
    public static void main(String[] args)
    {
        for(int i = 0; i < grid.length; i++)
        {
            for(int j = 0; j < grid[i].length; j++)
            {
                int temp = 0;
                do
                {
                    temp = getNumber();
                }while (!noConflict(grid, i, j, temp));
                grid[i][j] = temp;
                System.out.print(grid[i][j] + " ");
            }
            System.out.println();
        }

    }

    static int getNumber()
    {
        Random r = new Random();

        int temp = 0;
        temp = r.nextInt(10-0);
        return temp;
    }
    public static boolean noConflict(int[][] array, int row, int col, int num) {

        for (int i = 0; i < 9; i++) {
            if (array[row][i] == num) {
                return false;
            }
            if (array[i][col] == num) {
                return false;
            }
        }

        int gridRow = row - (row % 3);
        int gridColumn = col - (col % 3);
        for (int p = gridRow; p < gridRow + 3; p++) {
            for (int q = gridColumn; q < gridColumn + 3; q++) {
                if (array[p][q] == num) {
                    return false;
                }
            }
        }
        return true;
    }
}

Thank you.

Answer 1

Your logic is flawed. You will create configurations that cannot be completed, so you will be stuck in the do while loop. Let me demonstrate this with a simple example:

1 2 3 4 5 6 7 8 9
4 5 6 1 2 3 . . .

Any of the positions with a dot cannot be filled, because there is already a 7, 8 or 9 in the box.

You could write a sudoku solver, then you place random numbers in the grid and try to solve it. As soon as there is a unique solution (this can be checked by solving from the bottom, try 1 then 2 then 3 and so on and from the top try 9 first then 8 then 7 and so on, if the 2 solutions are matching it is unique), you have a sudoku (not a solved one as the one you are trying to generate).