Infinite While-loop!

Question

Hey there! I'm trying to do some data input validation but I haven't been able to figure it out. First I need to check if the user entered anything, that one works. Then I need to check if what the user entered was a number. But I'm getting an infinite while loop in when I try to validate if the first character entered is a letter. . . .
Thanks for your help!
public class methods
public static void main(String args[]) throws IOException
  String input ="";
  int qoh=0;
  boolean error=true;
  Scanner keyboard = new Scanner (System.in);
  //while (error)
                //   error=true;
                        while (error==true)
                            System.out.print("\nEnter Quantity on Hand: ");
                            input = keyboard.nextLine();
                            if (input.length() <1)
                                System.out.println("\n**ERROR06** - Quantity on hand must be between 0 and 500");
                                error=true;
                                System.out.println(qoh);
                                System.out.println(input);
                            else
                                error=false;
                        error = true;
                        while (error==true)
                            if (Character.isLetter(input.charAt(0)))
                                System.out.println("\n**ERROR06** - Quantity on hand must be between 0 and 500");
                                error=true;
                                System.out.println(qoh);
                                System.out.println(input);
                            else
                                qoh = Integer.parseInt(input);
                                error=false;
        }

Answer 1

1. add in breakpoints to your program.
Eg. put print statements before and after loops to confirm which loop is producing the inf loop error.
also add in print statements in your if and else blocks.
2. You should consider using keyboard.next() instead of nextLine()
Eg. how are you going to handle "500 orange crush"?
3. I have a feeling that the real problem lies at the 2nd loop where you tried to do a parseInt
What happens if you have "5lmao" charAt(0) check is passed, but it wont produce a int will it?
You might want to loop through every char in the input string to confirm they are a number.
4. I suggest
If(input.length() > 0) error = false
else print msg
OR
assume no errors before loop.
if(length() <1) error=true
else error = false
5. use the keyword "break". By that i mean set up your loop to be infinite loops, use the break operator under specified condition (error=false)
I didn't identify the error for you, but i'd say the above methods should help you with your problem