Department of
Computer Science

 

CS420/520: Object-Oriented Programming

 

Assignment 1: Boxes that Dance



Set: Wednesday 3rd April 2019

Due: Monday 15th April 2019 at or before midnight

 

 This assignment involves adding some functionality to the "Box" objects that I'm providing for you in the box module.

Your task is to create a "dance" with two boxes, Alice and Bob. This means that the two boxes should respond to each other's motions. For example, if Alice moves right, Bob might move left; if Alice moves towards Bob, Bob should move towards Alice. I'm deliberately leaving the details of this up to you. The movements "dancers" of one of the  could be controlled by mouse movements, or by pseudo-random numbers, or by keystrokes; the second "dancer" should move in response to the first.

You should create a new subclass of box called dancingBox.  This does not mean that all of your code should go in dancingBox; you may well discover that some basic functionality of a box is missing from the box class, in which case you should add it there.  You should place box responsibilities in box, and dance responsibilities in  dancingBox.

Start the dance by making a request like:

alice.danceWith(bob)

Pair Programming

As mentioned in class, this work can be very rewarding (and certainly easier) if done in pairs. I recommend this strongly, although it’s not strictly required. Pairs can be arranged using Piazza. If you pair, make sure that both partners get to "drive" (write code), and both partners get to "navigate" (think about where you are going).   Put a post on Piazza to "Instructors" with the names of the students in your "pair" as soon as it is set up, so that I can create a D2L group for you.

Submitting

When you are done, upload your grace modules to the D2L assignment dropbox that I will create for this purpose.  Be sure to include instructions for starting and controlling your dance as a note in D2L.  (You can put that information as a comment in your modules too, but then I'll need to know where to look!)  If you have a partner, upload the code only once; if you have published the names of your pair on Piazza,  I will have created a “group” for you in D2L.

Hints

  • Try to get familiar with using the Grace Web Environment, before next Wednesday, 10th April.  I'll dedicate some time in class to getting you ‘unstuck’.
  • There is a pseudo-random number method in the random module.
  • You will find Grace Resources on the “Resources” page of the Piazza site.
  • Keep a notebook. As you figure stuff out, write it down, so that you don’t have to figure it out more than once. Consider whether you might post your notes on Piazza — will they help other students, “giving the game away”
  • Here is what we learned from previous versions of this assignment: 
    1. Don't email code from yahoo.com!
    2. Size of Methods <= 10 lines
    3. If you have a non-trivial algorithm, explain it.
    •   Use Explaining variable names (Beck p. 108)
    •   Use Intention-revealing selectors for method names (Beck p. 49)
    •   If all else fails, use a comment! But first read Method Comment (Beck p. 40).
    4. Methods should (usually) either DO something or ANSWER a value, but not both. This is not a firm rule, but a useful guideline. The main exceptions are object mutators like add and at(_)put(_), which, by convention, have the effect of changing the object, but which also answer the object that was changed.
    5. As a consequence of the above, accessor methods usually should not have side effects!
    6. Follow the directions for the assignment
    7. Hand in on time.
    8. Start the selectors of Boolean attributes with some form of the verb to be, such as "is" or "has".  For example: bob.isReady, alice.hasPartner
    9. Use module-level methods when appropriate.
    10. Classes (methods that create objects) can have any name that is meaningful. They are not restricted to being called new; my class for creating boxes  has a factory method called named(_) that takes the name of the to-be-created object as its argument.

How to get Started

  1. Start now. You will get stuck. Allow time to get unstuck.
  2. Look at the grading rubric (pdf, rtf) so that you know what criteria we will be grading on.
  3. Start by loading the box module that I wrote.
  4. Implement a method dance (which should make the receiver do a solo dance). Try it out.
  5. Implement danceWith(anotherBox) (which makes the receiver dance with anotherBox). danceWith(_) will require that you add an instance variable to box, so that a box can remember its partner.
  6. It’s OK to start by modifying the existing classes; you can refactor the code to move the appropriate behaviour into your own dancingBox class later.  Or you can start by creating a dancingBox as a subobject of box.

Don't Panic

I'm aware that I'm throwing you in to the "deep end of the pool" with this assignment. This is partly because I don't have all the resources that I would like at hand, and partly because a good way to "get" OOP is to struggle with it. However, you are in a safe environment. There are plenty of life preservers available, in the form of books and web resources, and there are people to help: your classmates, and your instructor. I expect that you will get stuck. There will be time in the next class session (Wednesday 10th) for getting you “unstuck”. So, come to class armed with questions.  Don't expect to solve this the night before it is due.

Also, be aware that there is nothing “golden” about the box module that I wrote. It's probably not very good, and it’s certainly buggy.  It's based on someone else's code that interacts with the DOM of the web browser, which you may well know more about than I do.