Java: Why we can declare arraylist as final and what is impact on this?

Question

public class ServletClassName extends HttpServlet {
//declare instance variables for the servlet
int  instanceVar ; // not thread−safe
// overriding the Generic Servlet.init ( )
@Override
public void init ( ) throws ServletException {
// initialize instance variables, etc.
instanceVar = 0 ;
}
. . .
}

Per each page request, in the service() method (which eventually will be either doGet(), doPost(), or whatever),
you can manipulate instance variables.

Write a Servlet that counts the total number of visits (how many times it's being visited from all clients). The Servlet should display the following information

• Total Page Visits:
• Client Remote Address:
• Client Host Address: <client_fully_qualied_url__or__client_IP_address>

The last two can be obtained from the HttpServletRequest, Refer to the Java EE API

then how can re-do these using the Java API for HttpSession.

Answer 1

When you make an arraylist as final you cannot rebind the variable to point to a different collection instance: for example -

final List<Integer> list = new ArrayList<Integer>();
list = new ArrayList<Integer>(); // Since `list' is final, this won't compile

Answer 2

final as you know means that you cannot re-assign that object reference after it is assigned. But you can call any method of the object, so you can perform insert, search and any other operation to the object.