What is the output of the program?

public class FlattenArray {
    public static String flatten(int[][] arr, int i, int j) {
        if (i == arr.length) return "";
        if (j == arr[i].length) return flatten(arr, i + 1, 0);
        String separator = (i == arr.length - 1 && j == arr[i].length - 1) ? "" : ",";
        return arr[i][j] + separator + flatten(arr, i, j + 1);
    }
    public static void main(String[] args) {
        int[][] matrix = { {1, 2}, {3, 4} };
        System.out.println(flatten(matrix, 0, 0));
    }
}