printing out usernames

[bobx33]

I am trying to print out a list of usernames for my chat program. It currently works except for the username portion. The Server sends a specific line that tells the client to print it as a username. That line is
name//name. For example, if the name of the client was Eric, then the server would send Eric//Eric to the client. The client will then receive the line through a buffered reader. However, my current code keeps getting an error at the i=input.substring... line. Its an IndexOutOfBoundsException: String index out of range: -1.
Here's the part of my code that needs help.

class  ChatThread extends Thread {
	ChatServer cServer;
	String[] sentence;
	int i;
        public void run() {
            String input = "Eric";
            try {
                while(true) {
                    input = br.readLine();
                    i = input.substring(0,input.indexOf("//")).length();
                    String s = input.substring(i,i+2);
                    if(input.substring(i, i+2).equals("//")){
                        sentence = txtUser.getText().split("\n");
                        for(String word: sentence){
                            if(word.equals(input.substring(i+2))){}
                            else{
                                txtUser.append(input.substring(i+2) + "\n");
                            }
                        }
                    }
 
                    else{
                        txtMes.append(input + "\n");
                    }
                }
             }
             catch(Exception e){
                 e.printStackTrace();
             }

txtMes and txtUser are JTextAreas

[GregBrannon]

Welcome to the Forum! Please read this topic to learn how to post code correctly and other useful tips for newcomers.

Add some print statements to determine the actual values of the index and the cause of the problem. Apparently, your code isn't providing the results you'd anticipated.