Starting the Program

Here is a start to the program.

import  java.util.Scanner;

// User enters integer N.  
// The program calculates N factorial.
//
class factorial
{
  public static void main (String[] args ) 
  {
    Scanner scan = new Scanner( System.in );
    long N, fact = 1; 

    System.out.print( "Enter N: " );
    N = scan.nextLong();

    if (  )
    {
      while (   )    
      {
 
         ;

         ;
      }
      
      System.out.println( "factorial is " + fact );
    }
    
    else
    {
      System.out.println("N must be zero or greater");
    }
  }
}

Notice how the program matches the flowchart, especially how the while statement is nested in the true branch of the if statement. The indenting (and the braces {} show this structure).