How to modify Java code while running in debug mode?

How can I enable this "Debugging in Runtime" Notch is talking about in this video in Eclipse?

As a test, I'd like to be able to edit the output of the following code and change it to "Hello Runtime Debugging" while it's running.

public class HelloWorld {
    public static void main(String[] args) throws InterruptedException {
        doIt();     
    }

    private static void doIt() throws InterruptedException {
        for (int i = 0; i < 1000; ++i) {
            System.out.println("Hello World " + i);
            Thread.currentThread().sleep(100);
        }
    }
}

EDIT: I modified the code, now I get the results I was looking for. Suraj Chandran's answer below explains it.

private static void doIt() throws InterruptedException {
    for (int i = 0; i < 1000; ++i) {
        print(i);
        Thread.currentThread().sleep(100);
    }
}

private static void print(int i) {
    System.out.println("Hello Sir " + i);
}

Eclipse supports hot swapping code during debugging, out of the box.

While debugging, just change any code and save it, Eclipse will automatically transfer the modified code to the target VM.

Note that you can't make structural changes to the code, like adding new methods, changing method signature or adding new fields. But you can change the code within a method.

EDIT: Note that changing the code during debugging will make that method re-execute from the beginning, resetting the local variables in that method.