Question

I am really stuck on this exception

private static void getUserComment(String s) {
    while(s.contains("author'>")){
        System.out.println(s.substring(s.indexOf("author'>"),
                                       s.indexOf("<div id='")));
        s = s.substring(0, s.indexOf("author'>")) +
                           s.substring(s.indexOf("<div id='"+9));

    } 
}

Answer 1

You should use a proper parser or at least do some regular expression pattern matching (which is already “bad enough” for HTML or XML).

That said, your “offset” of 9 is likely the indirect cause of the exception:

s.indexOf("<div id='"+9)

This will make a literal string <div id='9 which is not found; indexOf then returns -1 and this causes the exception in the substring method. Maybe you wanted to actually add 9 to the index like this? s.indexOf("<div id='")+9

Note that the function is useless anyways, changing s will only change the local variable and not the original variable (parameters are by value in Java).