Cannot refer to a non-final variable inside an inner class defined in a different method

Java doesn't support true closures, even though using an anonymous class like you are using here (new TimerTask() { ... }) looks like a kind of closure.

edit - See the comments below - the following is not a correct explanation, as KeeperOfTheSoul points out.

This is why it doesn't work:

The variables lastPrice and price are local variables in the main() method. The object that you create with the anonymous class might last until after the main() method returns.

When the main() method returns, local variables (such as lastPrice and price) will be cleaned up from the stack, so they won't exist anymore after main() returns.

But the anonymous class object references these variables. Things would go horribly wrong if the anonymous class object tries to access the variables after they have been cleaned up.

By making lastPrice and price final, they are not really variables anymore, but constants. The compiler can then just replace the use of lastPrice and price in the anonymous class with the values of the constants (at compile time, of course), and you won't have the problem with accessing non-existent variables anymore.

Other programming languages that do support closures do it by treating those variables specially - by making sure they don't get destroyed when the method ends, so that the closure can still access the variables.

@Ankur: You could do this:

public static void main(String args[]) {
    int period = 2000;
    int delay = 2000;

    Timer timer = new Timer();

    timer.scheduleAtFixedRate(new TimerTask() {
        // Variables as member variables instead of local variables in main()
        private double lastPrice = 0;
        private Price priceObject = new Price();
        private double price = 0;

        public void run() {
            price = priceObject.getNextPrice(lastPrice);
            System.out.println();
            lastPrice = price;
        }
    }, delay, period);      
}

To avoid strange side-effects with closures in java variables referenced by an anonymous delegate must be marked as final, so to refer to lastPrice and price within the timer task they need to be marked as final.

This obviously won't work for you because you wish to change them, in this case you should look at encapsulating them within a class.

public class Foo {
    private PriceObject priceObject;
    private double lastPrice;
    private double price;

    public Foo(PriceObject priceObject) {
        this.priceObject = priceObject;
    }

    public void tick() {
        price = priceObject.getNextPrice(lastPrice);
        lastPrice = price;
    }
}

now just create a new Foo as final and call .tick from the timer.

public static void main(String args[]){
    int period = 2000;
    int delay = 2000;

    Price priceObject = new Price();
    final Foo foo = new Foo(priceObject);

    Timer timer = new Timer();
    timer.scheduleAtFixedRate(new TimerTask() {
        public void run() {
            foo.tick();
        }
    }, delay, period);
}

You can only access final variables from the containing class when using an anonymous class. Therefore you need to declare the variables being used final (which is not an option for you since you are changing lastPrice and price), or don't use an anonymous class.

So your options are to create an actual inner class, in which you can pass in the variables and use them in a normal fashion

or:

There is a quick (and in my opinion ugly) hack for your lastPrice and price variable which is to declare it like so

final double lastPrice[1];
final double price[1];

and in your anonymous class you can set the value like this

price[0] = priceObject.getNextPrice(lastPrice[0]);
System.out.println();
lastPrice[0] = price[0];