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