Error with strings

[andybeeeeee]

I've got this method that takes a location of one format and then converts it to another. i'm getting an error but cant see why:

public double convert (String data){ // example data is "5125.85111 N"

       splitData = new String[2];
       String xs;
       String ys;
       int i, length, j=12, post=0;
       double x, y, z, result;
       double ten=10, power;
       char direction;

       for(i=0; i<2; i++)
       {
           post = data.indexOf(".", j); //split the data in two at the dot
           splitData[i] = data.substring(j, post);
           j=post+1;
       }
       
       xs = splitData[0].substring(0, 4);
       ys = splitData[0].substring(4);
       x = Double.parseDouble(xs);
       y = Double.parseDouble(ys);

       length = splitData[1].length();
       direction = splitData[1].charAt(length);
       StringBuffer sb = new StringBuffer(splitData[1]);
       sb.deleteCharAt(length);  //the string needs to be put in a buffer to remove the last element
       splitData[1] = sb.toString();
       z = Double.parseDouble(splitData[1]);
       power = length - 1;
       z = z / power(ten, power); //change z to 0.z
       y = (y+z)/60;
       result = x+y;
       return result;

  }

what i'm trying to do is split the data at the . and then convert it. it all compiles ok, but at runtime i get an error saying java.lang.StringIndexOutOfBoundsException: String index out of range: -13

my minimal debugging skills have found that this is something to do with the line splitData[i] = data.substring(j, post); and that post is at -1 at this point. can anyone tell me what's going on?

thanks

[andybeeeeee]

no worries, have figured it out now!!