import javax.swing.JOptionPane;

/**
   This program tests a customer number to
   verify that it is in the proper format.
*/

public class CustomerNumber
{
   public static void main(String[] args)
   {
      String input;  // To hold the user's input

      // Get a customer number.
      input = JOptionPane.showInputDialog("Enter " +
         "a customer number in the form LLLNNNN\n" +
         "(LLL = letters and NNNN = numbers)");

      // Validate the input.
      if (isValid(input))
      {
         JOptionPane.showMessageDialog(null,
                 "That's a valid customer number.");
      }
      else
      {
         JOptionPane.showMessageDialog(null,
             "That is not the proper format of a " +
             "customer number.\nHere is an " +
             "example: ABC1234");
      }
      
      System.exit(0);
   }

   /**
      The isValid method determines whether a
      String is a valid customer number. If so, it
      returns true.
      @param custNumber The String to test.
      @return true if valid, otherwise false.
   */

   private static boolean isValid(String custNumber)
   {
      boolean goodSoFar = true;  // Flag
      int i = 0;                 // Control variable

      // Test the length.
      if (custNumber.length() != 7)
         goodSoFar = false;

      // Test the first three characters for letters.
      while (goodSoFar && i < 3)
      {
         if (!Character.isLetter(custNumber.charAt(i)))
            goodSoFar = false;
         i++;
      }

      // Test the last four characters for digits.
      while (goodSoFar && i < 7)
      {
         if (!Character.isDigit(custNumber.charAt(i)))
            goodSoFar = false;
         i++;
      }

      return goodSoFar;
   }
}