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For starters you can simplify the first part by utilizing the fact that indexOf can accept a second character indicating start point. This would lead to something like the following:

For starters you can simplify the first part by utilizing the fact that indexOf can accept a second character indicating start point. This would lead to something like the following:

Secondly, you can simplify the conversion by utilizing the substring method a little, and improve a little on performance by using StringBuilder, like in the following:

StringBuilder ipAddress = new StringBuilder();
int lenIpString = ipString.length();
//get two and two bytes from hex string and append to ipaddress 
if (lenIpString == 8) {
 long ipPart;
 for(int i = 0; i < lenIpString; i += 2)
 {
 ipPart = Long.parseLong(ipString.substring(i, i + 2), 16);
 ipAddress.append(String.format("%d.", ipPart));
 }
 
} else {
 throw new Exception("IP-string not long enough");
}

Note that this also does something if in the rare case the ipString is not 8 characters long. This also requires for the method to have an throws Exception added, and you should probably choose a better exception rather than Exception.

Finally your write to file can be enhanced to the following:

//write IP Address to file
File file = new File("SomeFilePath.txt");
FileWriter fileWriter;
try {
 fileWriter = new FileWriter(file);
 fileWriter.write(
 String.format("ip=\"%s\"",
 ipAddress.substring(0, ipAddress.length() - 1)));
 fileWriter.flush();
 fileWriter.close();
} catch (IOException exception) {
 exception.printStackTrace();
}

Finally some general notes on coding style:

• Consider splitting into three functions – These three code blocks could be extracted into (two or three) separate functions, where one serves as the output to the next. This would follow the single-responsibility principle of coding. One variant could change this function into:

   void WritePublicIPAddressToFile(String resp, String filename)
   {
   String ipAddress = findAndConvertIPAddress(resp);
   if (!ipAddress.isEmpty()) {
   writeIPAddressToFile(filename, ipAddress);
   }
   }
  

  Here I assume that the findAndConvertIPAddress() would return the empty string if the IP-address is not found as expected, and use that to bypass the file write if necessary. Now this function has a much clearer responsibility of finding and writing that IP-address to the file. No need to read lots of code to understand whats happening.

• Use StringBuilder instead of String concatenation – In general try to avoid building new strings to a minimum, and use StringBuilder when building new strings, or a variation over String.format().

• Consider having the filename as a parameter – Your code might be constructed specific for Code Review, but with a name like WritePublicIPAddressToFile I would expect that filename was one of the parameters.

• Avoid using l as variable name – Especially in combination with i or 1, it can be really confusing to use lower-case L as a variable name. Use more descriptive names instead.