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Homework 4

Assigned Feb 18, due Feb 25

For all the programming assignments, be sure to follow the design recipe. Each problem asks you either to create a new class with some methods, or to add a new method to a class you've already written. For classes, you need to identify the names and types of the instance variables, then write each of the methods. For each method, go through the usual contract, examples, skeleton, inventory, body, test steps. I recommend writing the examples by using the tester library, as discussed in class on Feb. 11, so you can easily run all your tests and confirm that they all pass.

Be sure to choose meaningful names for methods and parameters, and watch for opportunities to re-use methods you, I, or the textbook have already written.

Also turn in a log of time spent and errors encountered in the assignment, with brief comments describing each error ("mismatched parentheses" is self-explanatory, but more complex errors might need more description). You may do this using the PSP forms, or simply by keeping track in a text file or on paper and turning it in.

• In a Dummy class, write the following method:
  • smallestOf3, which takes in three numbers and returns the smallest of them.
• Starting with the Posn class I gave you, add several more methods:
  • aboveDiagonal, which given a Posn, tells whether it is above the main diagonal (the line y=x) of a square graphics window. Note that in computer graphics, larger x coordinates are to the right as usual, but larger y coordinates are down, not up as in high school algebra. So the diagonal is as shown in this picture; the green dot is above it and the red dot is below it.
  
  
  • same, which given two Posns, tells whether they have the same coordinates.
  • Imagine that the Posn represents a mouse-click on a video-game window that's 300 pixels wide, and you want to do different things depending on whether the mouse click was in the left third, the middle third, or the right third of the window. Develop a method region, which given a Posn, returns one of the strings "left", "middle", or "right" depending on whether the Posn is in the left 100 pixels, the middle 100 pixels, or the right 100 pixels of the 300-pixel-wide window.
• Develop a class Voting which has no instance variables, but the following (static) methods:
  • count4votes, which takes in five strings: the first is a candidate name, and the other four are votes cast. It returns how many of the votes were for that candidate.
    Here are some examples to clarify what I mean:

    t.checkExpect (Voting.count4votes("Anne","Bob","Charlie","Bob","Hortense"), 0);
    t.checkExpect (Voting.count4votes("Anne","Bob","Anne","Phil","Charlie"), 1);
    t.checkExpect (Voting.count4votes("Anne","Anne","Bob","Anne","Mary"), 2);
    t.checkExpect (Voting.count4votes("Bob","Anne","Bob","Charlie","Bob"), 2);

  • whoWon, which takes in three integers: how many votes Anne got, how many Bob got, and how many Charlie got. It returns the name of the winner, or "tie" if two or more candidates tied for first place. (This method should not assume there are only four voters; it should work correctly with hundreds or thousands.)
  • whoWon4votes, which takes in four strings, representing the votes cast by four voters; it returns the name of the winner or "tie". You may assume for now that the only candidates in the race are Anne, Bob, and Charlie (we'll drop this assumption later). (Hint: look for opportunities to re-use methods you've written before.)
• Back in the Dummy class, write the method
  • cardRefund, which takes in how much you charged on your credit card, and tells how much refund you get at the end of the year, according to the following rules:
    • 0.25% of the first $500 you charge,
    • 0.50% of the next $1000 you charge (i.e. anything between $500 and $1500),
    • 0.75% of the next $1000 you charge (i.e. anything between $1500 and $2500),
    • 1% of anything you charge over $2500.
    So for example, a customer who charged $400 would get back $1.00, which is 0.25% of $400. A customer who charged $1400 would get back 0.25% of the first $500 (making $1.25), plus 0.50% of the next $900 (i.e. $4.50), for a total refund of $5.75.
• Starting with the Date class from homework 3,
  • add a legal method that, given a Date, tells whether it's "legal": the month and day numbers are both positive, and day number is no more than 30, and the month number is no more than 12.
  • add a same method that, given two Dates, tells whether they're the same. (You may assume that both Dates are legal.)
  • add a monthName method that takes in a month number and returns the name of that month. (Hint: At present, you'll need to use if; later on we'll see a more elegant way.)
  • modify the toString method so that it uses month names properly.
  • Extra credit: modify the dayInYear method so it knows that January is 31 days, February 28, March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, and December 31. Don't worry about leap years for now. (Hint: You might want to write a "helper method" to make this easier.)
  • More extra credit: handle leap years correctly.
  • More extra credit: modify the daysLater method from homework 3 (or add it if you didn't write it before) so it handles the lengths of months correctly.

What to Turn In

You should end up with several classes: Dummy, Posn, and Date, as well as a test class for each, with names like PosnTest, DateTest, etc. I recommend keeping all of these in one BlueJ project (with a name like "Homework4"); zip up the whole folder and attach it to an e-mail to me. If you've entered your time and error logs using the on-line forms, I've already got them; if not, e-mail me your log files.

Grading standards

Programming:

Note: I won't actually grade every one of these methods, but a representative sample; the rest are for practice. If I were to grade them all, the points would look something like this:

General skills:

Total:         depends on how many methods and classes I actually grade