NullPointerException is heck of a problem!

Code:

---

        BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
        String username="";
        File file=new File("C:/aman/usernames.txt");
        for(int i=1;i>0;i++){
        System.out.println("Enter new Username");
        username=in.readLine();
        BufferedReader in1=new BufferedReader(new FileReader(file));
        String str="123";
        for( ; ; )
        {
            if(str==null)
            {
                break;
            }
            str=in1.readLine();
            if(str.equals(username))
            {
                System.out.println("This username is already taken!");
            }
        }
        }
        BufferedWriter out=new BufferedWriter(new FileWriter(file, true));
        out.newLine();
        out.write(username);
        out.close();

---

well,this is my code, and it always shows nullpointerexception at somewhere inside the for( ; ; ). please help me correct it!

:)-:

At somewhere inside the loop? You understand the stack trace tells you exactly what line the exception was from. A null pointer exception is caused generally by calling a method on something that is null.

Also, if you are reading a file in the for(;;) loop you should use a while loop like this:

Code:

---

String s;
while((s=in.readLine())!= null){
  //do stuff
}

---

Rather than testing for null and breaking out of the loop and getting input in the loop.

You should look at the order that you are doing things.
You test the variable before you change its value instead of after it is changed.

thanks both! this thing now works!