CS115 lecture notes, Spring 1999
Week 4, Friday

Reading:  Chapter 4

Hand back exams.

Flow of execution
-----------------

public class Buzz {

  public static void baffle (String blimp) {
    System.out.println (blimp);
    zippo ("ping", -5);
  }

  public static void zippo (String quince, int flag) {
    if (flag < 0) {
      System.out.println (quince + " zoop");
    } else {
      System.out.println ("ik");
      baffle (quince);
      System.out.println ("boo-wa-ha-ha");
    } 
  }

  public static void main (String[] args) {
    zippo ("rattle", 13);
  }
}


Interesting thing here is that

main calls zippo

zippo calls baffle

baffle calls zippo again

That would seem to cause a circle, which would continue
forever IF zippo called baffle ALL THE TIME.

But it doesn't.  The second time zippo runs, it just
prints something and returns.  That allows baffle to
return, which allows the first version of zippo to
return.

Using state diagrams to keep track of method invocations.


Homework 3
----------

A note on the homeworks: the lab time is intended to get
you started; you have the rest of the week to finish.

If you get off to a good start and then hit a wall, stop.

-----

What is a bounding box?  It's four parameters that specify
the location and size of a rectangle.

The rectangle is invisible; it is only used as a guideline.

drawOval and fillOval take  bounding boxes as arguments
and use them to determine the boundaries of the oval.

drawRectangle and fillRectangle take  bounding boxes as arguments
and use them to determine the boundaries of the Rectangle.

In the case of rectangles, the shape that is drawn is the same
as the bounding box, but in general that is not true.


Where do the bounding boxes come from?
-------------------------------------

When paint invokes paintBox, it passes a BB that is the size of
the whole window.  In effect, it gives paintBox permission to
draw anywhere on the screen.

When paintBox invokes paintSnowperson, it passes a BB that is
the same height as the window, one third the width, and centered.

Strictly, paintSnowperson should not draw outside the box, although
there is actually nothing to prevent it.

paintSnowperson then calculates bounding boxes for the head,
thorax and abdomen.  It uses each BB twice, as an argument to
drawOval, and as arguments to paintFace, paintShirt, and,
recursively, paintSnowperson.


Tricky things
-------------

Algebra in coordinate space is a little tricky, so don't
worry if it takes a while to sink in.

You have to choose some numbers to determine the proportions
of the snowman, but you should minimize the number of constants
that appear in the code.

Instead of making the programmer do computations at programming
time, make the computer do it at run-time.

That way, if you change any of the proportions, everything
gets updated.

Resizing the screen is the best way to check your program.
If the proportions of the snowpeople change or the ovals
stop touching, or they start overlapping, something is wrong.


Next week -- recursion.  Read 4.15!  We'll probably have a quiz.