Thread: printing out usernames

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

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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.