Here is a recursive program that gives wrong output, please help me

I saw a program like this in a book:

Here is a program: print out integers. Here you have a positive integer n and want to print it out. The name of the program is printOut(n). Suppose there is an existing routine that will only process a single number and output it to the terminal .The routine that prints a single digit is named printDigit.

The program requires the use of recursion.

The sample code given in the book is:

    public static void printOut(int n)
    {
        if (n >= 10)
        {
            printOut(n / 10);
        }
        else
        {
            printDigit(n % 10);
        }
    }

The code of the routine printDigit is not given in the book. After thinking, I think the program should be like this:

public class PrintOutTest
{
    public static void main(String[] args)
    {
        Print.printOut(76234);
    }
}

class Print
{
    public static void printOut(int n)
    {
        if (n >= 10)
        {
            printOut(n / 10);
        }
        else
        {
            printDigit(n % 10);
        }
    }

    static void printDigit(int n)
    {
        System.out.print(n); //7
    }
}

But when I run it, the terminal only outputs the number 7.

I took a closer look and found that the printDigit method only entered once, which should be the problem. But I don’t know how to modify it. Please ask if you can help me. Thanks

You need to emit the digits from left to right (most significant to least significant).

It is easier to get the rightmost digit (this is n % 10) than the leftmost one (this is n divided by... some power of 10). So the recursion will take this shape:

To print out a number:
  if longer than one digit
    recurse on the number without its rightmost digit
  print out the rightmost digit

and more precisely,

To print_number n:
  if n > 9
    print_number n / 10
  print_digit n % 10

PrintDigit is only called when n is less than 10, so that happens only once...

You need to also call PrintDigit when you make the recursive call. So:

    public static void printOut(int n)
    {
        if (n >= 10)
        {
            printOut(n / 10);
        }
        printDigit(n % 10);
    }

In each iteration, the number goes diving by 10
for your example 76234 > 7623 > 762 > 76 > 7 and now 7 is smaller than 10 so the recursive function called the print method and 7 gets printed.

below is an example to fix the problem.

class Print
{
    public static void printOut(int n)
    {
        if (n == 0) return ;
        printOut(n / 10);
        printDigit(n % 10);
    }

    static void printDigit(int n)
    {
        System.out.print(n); //7
    }
}

Your printDigit function is OK. Just dont wrap it with else block:

    public static void printOut(int n)
    {
        if (n >= 10)
        {
            printOut(n / 10);
        }
        printDigit(n % 10);
    }

And also, I personally don't prefer to learn any programmin language from books cause they gets outdated and writers often suck at algorythms. And once they make a mistake there is no chance to republish a book but in Github there is issues, pull requests, comments, and code is editable also forkable.