Tutorial Q&E : Implementing Nested Classes, Question 2

Question

//1.3
Taken from http://java.sun.com/docs/books/tutorial/java/javaOO/QandE/nested-answers.html
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. ");
Question 2: The program Problem.java (above) doesn't compile. What do you need to do to make it compile? Why?
Answer 2: Add final in front of the declaration of the timer variable. A nested class declared within a method has access to any final, local variables in scope.
Could somebody please elaborate on this answer for me? I don't really feel it gives enough information for me to understand fully the "why?" part.
Thanks
Mark

Answer 1

Hi,
the problem is that the instace of the nested class is still existing after the execution of the method has been finished. But how should it have access to a variable which doesn't exists anymore?
The only way to solve this problem is to declare the variable as final. Because then the value of the variable is known when the instance of the nested class is created an can be copied. Then at each place in the nested class where the variable is used, the copy is used instead. This is only valid because the value of the variable cannot be changed after the creation of the nested class' instance.
Andre