Go4Expert - Java Warning Message

hello all, I have a problem with my program in java netBeans...
I created an IP monitoring program and there will be a "warning message" when there is a change of IP. This is a program that I created:

Code:

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         

        // TODO add your handling code here:

        try{

InetAddress minta= InetAddress.getLocalHost();

jTextField1.setText(minta.getHostAddress());

jTextField2.setText(minta.getHostName());

FileWriter fstream = new FileWriter("List_IP.txt");

PrintWriter out = new PrintWriter(fstream);

out.write(minta.getHostAddress());

out.close();

                }catch (IOException e){//Catch exception if any

                        System.err.println("Error: " + e.getMessage());

program detects the IP and then save the file "txt".
program runs automatically, and when the IP changes, the program will give "warning message". I was wondering how to make it .. anyone can help me? pleaseee ......