Computer Science 15-110 (Lecture 4), Spring 2010
Homework 2b
Due:  Thu 28-Jan-2009 at 10pm (email copy to koz) (no late submissions accepted).
Hw2 Submission Coordinator:  koz

Follow the same instructions as hw1, only (of course) replace "hw1" with "hw2" throughout.

  1. Writing Static Methods
    1. nearestBusStop
    2. lineIntersection
  2. Writing Graphics Helper Methods
    1. Central African Republic
    2. Netherlands Antilles
  3. Bonus/Optional:  Modern Marvels
  4. Bonus/Optional:  Scratch

  1. Writing Static Methods

    Note:  In this section, you will adapt your solutions to some hw1 problems so that they now use appropriate helper methods and test methods.
    1. nearestBusStop
      In a program named, write a static method named nearestBusStop that takes one parameter, an int value representing a street number (which you may assume to be positive), and returns the street number of the nearest bus stop, where buses stop on streets that are multiples of 8 (8th, 16th, 24th, etc), and where ties are broken in favor of the lower bus stop. So we see:
      * nearestBusStop(11) returns 8
      * nearestBusStop(12) returns 8
      * nearestBusStop(13) returns 16
      You can assume there is a bus stop on 0th street.

      Then, write a static test method, testNearestBusStop, that follows the pattern in the class notes for test methods, and which includes a reasonable suite of test cases to be sure that your nearestBusStop method works properly.

      That test method is robotic.  What if we want to test the program manually, where we run the program like last week and enter different values each time?  For that, you will write yet another static method named manuallyTestNearestBusStop.  That method will take no parameters and return nothing (that is, it is void).  It will operate exactly like the interactive program from hw1.  That is, it will be interactive, and it must match the following output exactly (where the user input is underlined, and therefore can change each time you run the program):

      Enter the street number: 12
      The nearest bus stop to street 12 is at street 8.

      Finally, your main method should first call the robotic test method and then the manual test method, like so:

      public static void main(String[] args) {

    2. lineIntersection
      Using the same techniques as the previous problem, convert the line intersection problem into methods.  Now, the lineIntersection clearly takes 4 parameters (m1, b1, m2, and b2, all int values), but what does it return?  A method can only return one value, and yet it seems this has to return two values -- the x and y values of the intersection.  Later on, we'll learn how to combine multiple values into a single object, and then return that object.  We don't know how to do that yet.  So what to do?  Answer:  just return the x value of the point of intersection.  After all, once we have that, computing the y value is really easy (right?).  So...

      In a program named, write the methods lineIntersection that takes 4 int values -- m1, b1, m2, and b2 -- and returns an int value, the x value of the point of intersection of the lines y=m1x+b1 and y=m2x+b2.

      Also, write the methods testLineIntersection and manuallyTestLineIntersection, and have your main method call each of these (the robotic method first).  Your manual method must match the output as specified in last week's assignment..

      Note:  as with the previous problem (and all future problems!), you will be graded on the quality of your robotic test method.  It should be carefully considered so that it contains the right combination of test cases so that it is likely to catch most common errors in the actual method being tested.
  2. Writing Graphics Helper Methods

    Note: As we are still limited to just rectangles and ovals this week, you should replace any stars in flags with ovals (of the same size, location, and color).

     You must deal with the "thin white stripe" that occurs between stripes when the screen size is not an even multiple of the stripe size.

    a. Central African Republic
    (file: and
    (larger image with details)
    Write the following graphics helper method:
       public void paintFlagOfTheCAR(Graphics page, int left, int top, int width, int height)
    This method takes a page and a bounding box (a rectangular area describing an outer boundary) and it paints the flag of the Central African Republic so that it completely fills the given bounding box.

    In the file, include a paint method that uses this graphics helper method to fill the entire window with the flag of the CAR.  Your paint method should look like this:
       public void paint(Graphics page) {
         paintFlagOfTheCAR(page, 0, 0, getWidth(), getHeight());

    Great!  But it gets better!  In another file named, include a paint method that calls that graphics helper method 4 separate times -- once each to fill each quadrant of the window with its own copy of the flag of the CAR, with a 3-pixel margin around all sides of each flag.  Note:  you will have to include a copy of your paintFlagOfTheCAR graphics helper method in each file that uses it!

    b.  Netherlands Antilles
    (file: and
    (larger image with details)

    In the file, paint the flag of the Netherlands Antilles again, only this time you must use a well-chosen helper method to paint each star.  The method should be called paintStar, and it should take the page, the center x and y positions, and the radius of the star.  Of course, for now it will only paint a circle in place of the star.

    And now we're going all out:  in the file, paint a flag of the Netherlands Antilles, only replace all the stars with same-sized little flags of the CAR!  To make this work, replace the body of your paintStar method so that instead of painting a circle it makes a suitable call to your paintFlagOfTheCAR helper method.  You may need to make the window rather large for this to look right (ok, it will look strange even when it is "right"!).  Remember:  you will have to include a copy of your paintFlagOfTheCAR graphics helper method in each file that uses it!
  3. Bonus/Optional:   Modern Marvels (up to 2.5 pts)
    Modern Marvels:  Great Inventions -- Computers
    1. Watch this 4-minute video:
    2. Write a Java program,, that prints a timeline of all the events listed in the video.  For example,
         System.out.println("1645:  Blaise Pascal invents 50 computing machines....");
      Include every event listed in the show, in order, with enough detail to make it obvious that you paid attention!
  4. Bonus/Optional:   Scratch (up to 5 pts)
    Scratch ( is an interesting and increasingly popular drag-and-drop programming language designed mainly for pre-collegiate (perhaps even pre-high-school) programmers.  Go to the Scratch page, download it, try it out.  Write a few programs in a subdirectory hw2-scratch (in your hw2 directory).  Include some programs you wrote along with a brief but well-considered write-up comparing the programming experience in Scratch versus that in Java (well, such as you know Java to this point).

Carpe diem!