Why I should declare a final object!

Question

import java.util.*;
public class Problem {
public static void main(String[] args) {
Timer timer = new Timer();
timer.schedule(new TimerTask() {
public void run() {
System.out.println("Exiting.");
timer.cancel();
5000);
System.out.println("In 5 seconds this
application will exit. ");
When I complie the program there is one error as follow:
Problem.java:11: local variable timer is accessed from within inner class; needs
to be declared final
timer.cancel();
1 error
I don't kown why I must declare the timer as "final"!
Help me!

Answer 1

What you have is an anonymous inner class. For this class to be able to access local variables in the containing class they must be final. When the compiler compiles the code it will create the inner class with a reference to the variable. If you were able to change the variable the inner class would lose the reference.