Chapter: Animation
...Section: Sequences of pictures in rapid succession

We're going to need a way to pause our program so that we can control how long each picture displays. Here's a handy method:

    void pause(int time) {
        try { Thread.sleep(time); }
        catch (InterruptedException e) { }
    }

Here's code to show two images alternately, each showing for 150 milliseconds:

	currentpic = scratch1;
	repaint();
	pause(150);
	currentpic = scratch2;
	repaint();
	pause(150);

This works because the paint(Graphics g) method is now:

    public void paint(Graphics g) {
	if (currentpic != null)
	    g.drawImage(currentpic, xpos, ypos, this);
    }

To make the alternation between picture happen numtimes times, we put the altenating pictures code in a method:

    public void scratch(int numtimes) {
	currentpic = scratch1;
	repaint();
	pause(150);
	currentpic = scratch2;
	repaint();
	pause(150);
	if (numtimes == 0); // we are done -- do nothing
	else scratch(numtimes-1);
    }

Q. 3

So what happens if you call scratch(3)

So the simple answer is that it displays the alternating pictures and then makes a call to scratch(2)

Q. 4

So what happens if you call scratch(2)

Q. 5

So what happens if you call scratch(1)

Q. 6

So what happens if you call scratch(0)

Thus scratch(3) displays the alternating pictures 4 times, and then stops.

Q. 7

So what happens if you call scratch(347)

Q. 8

Now let's not get into a sequence of questions and answers. Tell me simply what does scratch(347) do?

Here's the whole program.

Exercise 4

Exercise 5