Decimal numbers are sometimes difficult to work with because they can't always be stored with exact precision. This also leads to difficulty in displaying the value of a decimal number because you need to deal with getting the precision right. Fred G. found this bit of ingenuity:

  // File format
  //
  // ABBBBBCDDDDDEEEEEEE...
  //
  // A - bytes 0-0   integer portion of field x (range: 0-9)
  //     (done this way because we can not represent a decimal number exactly)
  //
  // B - bytes 1-5   decimal portion of field x (range: 00000-99999 to represent 0.00000-9.99999)
  //     (done this way because we can not represent a decimal number exactly)
  //
  // C - bytes 6-6   integer portion of field y (range: 0-9)
  //     (done this way because we can not represent a decimal number exactly)
  //
  // D - bytes 7-11  decimal portion of field y (range: 00000-99999 to represent 0.00000-9.99999)
  //
  // E - bytes 12-18 discount rate (range: 0.00000-9.99999)
  // ...
  public class Xyz {

    public class Data {
       private double ab;
       private double cd;
       private double e;
       // ...

       public Data(double ab, double cd, double e /* ... */ ) {
         this.ab = ab;
         this.cd = cd;
         this.e  = e;
         // ...
       }

       public double getAB() { return ab; }
       public double getCD() { return cd; }
       public double getE()  { return e;  }
       // ...
    }

    public String createDataToWriteToFile(List<Data> list) {
      DecimalFormat df1 = new DecimalFormat("0.00000");
      DecimalFormat df2 = new DecimalFormat("00000");
      DecimalFormat df3 = new DecimalFormat("0");

      StringBuilder sb = new StringBuilder();

      for (Data d : list) {
          String s = df3.format(Math.floor(d.getAB())) + 
                     df2.format((d.getAB() - Math.floor(d.getAB()))*100000) +
                     df3.format(Math.floor(d.getCD())) + 
                     df2.format((d.getCD() - Math.floor(d.getCD()))*100000) +
                     df1.format(d.getE());
          sb.append(s);
          sb.append("\r\n");
      }

      return sb.toString();
    }
  }

If only there were a reliable existing way to turn decimal numbers into a string for easy printing and display...