ISBN-10 to ISBN-13 Conversion in Java

public class ISBNConvertor {

  private static String CheckDigits = new String("0123456789X0");

  ///////////////////////////////// Main /////////////////////
  public static void main(String[] args) {
    String ISBN = new String();
    if (args.length==1) {
      ISBN = args[0];
      if (ISBN.length()==10) 
                System.out.println(ISBN1013(ISBN));
      else if (ISBN.length()==13) 
                 System.out.println(ISBN1310(ISBN));
      else ISBN = "Invalid ISBN";
    }
  }

  /////////////// Change a character to its integer value ///////
  static int CharToInt(char a) {
    switch (a) {
      case '0':   return 0;
      case '1':   return 1;  
      case '2':   return 2;
      case '3':   return 3;  
      case '4':   return 4;
      case '5':   return 5;
      case '6':   return 6;
      case '7':   return 7;
      case '8':   return 8;
      case '9':   return 9;
      default:    return -1;
    }
  }

  ////////////////////// Convert ISBN-13 to ISBN-10 //////
  public static String ISBN1310(String ISBN) {
    String s9;
    int i, n, v;
    boolean ErrorOccurred;
    ErrorOccurred = false;
    s9 = ISBN.substring(3, 12);
    n = 0;
    for (i=0; i<9; i++) {
      if (!ErrorOccurred) {
        v = CharToInt(s9.charAt(i));
        if (v==-1) ErrorOccurred = true;
        else n = n + (10 - i) * v; 
      }
    }
    if (ErrorOccurred) return "ERROR";
    else {
      n = 11 - (n % 11);
      return s9 + CheckDigits.substring(n, n+1); 
    }
  }

  ////////////////////// Convert ISBN-10 to ISBN-13 //////
  public static String ISBN1013(String ISBN) {
    String s12;
    int i, n, v;
    boolean ErrorOccurred;
    ErrorOccurred = false;
    s12 = "978" + ISBN.substring(0, 9);
    n = 0;
    for (i=0; i<12; i++) {
      if (!ErrorOccurred) {
        v = CharToInt(s12.charAt(i));
        if (v==-1) ErrorOccurred = true;
        else {
          if ((i % 2)==0) n = n + v;
          else n = n + 3*v;
        }
      }
    }
    if (ErrorOccurred) return "ERROR";
    else {
      n = n % 10;
      if (n!=0) n = 10 - n;
      return s12 + CheckDigits.substring(n, n+1);
    }
  }
}