Code that gets a valid double value within a specified range
*******************************************************


Scanner sc = new Scanner(System.in);
double subtotal = 0.0;
boolean isValid = false;
while (isValid == false)
{
    // get a valid double value
    System.out.print("Enter subtotal:   ");
    if (sc.hasNextDouble())
    {
        subtotal = sc.nextDouble();
        isValid  = true;
    }
    else
    {
        System.out.println("Error! Invalid number. Try again.\n");

    }
    sc.nextLine();    discard any other data entered on the line
    
    // check the range of the double value
    if (isValid == true &&  subtotal <= 0)
    {
        System.out.println("Error! Number must be greater than 0.");
        isValid = false;
    }
    else if (isValid == true && subtotal >= 10000)
    {
        System.out.println("Error! Number must be less than 10000.");
        isValid = false;

    }
    
}


*******************************************************

Note:


1. A user enter data, to make sure a data validation.

2. When an entry is invalid, display an error, give a
   chance to enter again by using a while loop.
3. Two types of checking a numberic entry are
   1. to make sure that a valid numberic format
   2. to make sure that a valid range (range checking)