有人可以帮我解决这个toString方法吗

Question

I have a method which converts string to a MeterNumber. The code is as follows.

public static MeterNumber fromString(String number) {
      MeterNumber mn = new MeterNumber(false);
      int i = 0, // position in number
          n = 0, // number of digits
          len = number.length();
      boolean valid = true;
      while (valid && i < len) {
         char c = number.charAt(i);
         i = i + 1;
         if (n < 9 && Character.isDigit(c)) {
            mn.digits[n] = c - '0';
            n = n + 1;
         } else if (Character.isWhitespace(c)) { 
            // skip spaces
         } else {
            valid = false;
         }
      }
      int cs = mn.checksum();
      if (valid && i == len && n == 9 &&
          cs / 10 == mn.digits[7] && cs % 10 == mn.digits[8]) {
         return mn;
      } else { 
         return null;
      }
   }

It's suppose to read an input like this:

531 481 889 O 788.5
652 364 795 P 2442.7

It will work fine if I just provide:

531 481 889
652 364 795

However it returns null if my input is in this format:

531 481 889 O 788.5
652 364 795 P 2442.7

Can you please help me see the problem. Why is the Char and double at the end of the line interfering?

This is the code calling for the method.

 private MeterNumber meterNumber; 


public static Meter load(Scanner sc) {
      MeterNumber meterNumber = MeterNumber.fromString(sc.nextLine());
      int n = 1;
      sc.nextLine();
      Tariff[] tariffs = new Tariff[n];
      for (int i = 0; i < n; i++) {
         String kind = sc.next();
         sc.nextLine();
         if (kind.equals("P")) {
            tariffs[i] = PeakTariff.PEAK_TARIFF;
         } else if (kind.equals("O")) {
            tariffs[i] = OffPeakTariff.OFF_PEAK_TARIFF;
         } 
      }
      return new Meter(meterNumber, tariffs);
   }

public MeterNumber getMeterNumber() {
      return meterNumber;
   }

}

This is the code calling from my main method:

Scanner sc2 = new Scanner(new java.io.File("Readings.txt"));
        Meter meter = Meter.load(sc2);

        System.out.println(meter.getMeterNumber());

Thanks in advance.

Answer 1

It returns null because you said it wasn't valid.

  while (valid && i < len) {
     char c = number.charAt(i);
     i = i + 1;
     if (n < 9 && Character.isDigit(c)) {
        mn.digits[n] = c - '0';
        n = n + 1;
     } else if (Character.isWhitespace(c)) { 
        // skip spaces
     } else { //this else is the problem
        valid = false;
     }
  }

You told it to set valid to false if it is neither whitespace nor a digit. Since O and P are not digits or whitespace, it sets valid to false.

  if (valid && i == len && n == 9 &&
      cs / 10 == mn.digits[7] && cs % 10 == mn.digits[8]) {
     return mn;
  } else { 
     return null;
  }

The first thing the if statement checks is if it is valid. Since it is not, it return null.