Getting Null Pointer Exception when using isEmpty() method

Question

I'm trying to implement a login feature for my program but it's returning a null pointer exception. I understand that this happens when you refer to a place in memory that has nothing in it, but as far as I can see I have instantiated all my objects properly - please correct me if I am wrong!

I am trying to implement an add user feature: I have a list of usernames and passwords and I have an existing method which reads that file in and stores it in an array. I want to write a new login to the list so I wrote a new method which converts this array to an ArrayList and will eventually write a new login to it and then write the Login file out again. The problem is I am getting the Null Pointer Exception.

Method 1:

public String[] readFile(){
    ArrayList<String> dataList = new ArrayList<String>();
    String Line;
    try {
        String line = br.readLine();
        do {                
            dataList.add(Line);
            line = br.readLine();
        }
        while (!line.isEmpty());
        br.close ();
    }
    catch (Exception e) {
        e.printStackTrace();
    }
    String[] dataArr = new String[dataList.size()];
    dataArr = dataList.toArray(dataArr);

    return dataArr; // Returns an array containing the separate lines of the file
}

Method 2:

public void addNewUser (String username, String password){
    String[] dataArr = readFile();  // Read in the list of profiles and store it in an array
    ArrayList<String> dataAL = new ArrayList<String>(Arrays.asList(dataArr));   // store array of profiles as ArrayList
    dataAL.add(username + "\t" + password);

}

Answer 1

You are probably getting null pointer here

while (!line.isEmpty());

change it to

while(line!=null && !line.isEmpty())

If you paid attention to the exception's stack trace you should see the exact line where the exception is being raised