ITEC 120 2008spring ibarland, jdymacek

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lect07a
solidifying our knowledge
lect07a

We will review make sure we understand:

• The notion of state, for objects. (How do they differ from local variables?)
• Reviewing the purpose of constructors, along with the syntax for (a) writing and (b) calling constructors.
• Functions which take a class as an argument
• brainstorming alternate approaches/possibilities for designing a Date class (see below)

Practice: A VideoGame has a title, a web page, and a suggested price.
Write a class with appropriate fields and constructor. Write a getter for the price.

brainstorming Dates

In lab, we have followed The Design Recipe for modeling Dates.

Brainstorm: what further info a Date might have?

• day-of-week (field, or method?)
• isHoliday (field, or method?)
• famousPeopleBornOn -- Why not?
• Appointments -- how to represent? What info is involved w/ an appointment?

model vs. view

Let's look at a different language than Java -- fire up a spreadsheet (like Excel). Spreadsheet programming is also programming (although it's not object-oriented).

• We can call a function to create a date: =date(2007,4,3).
• Separately, we control how that date is displayed. (Note how you can display it as year-month-day, or as month-day-year, but the arguments to the function date(int, int, int) can't be changed.)
• How do you think the date is stored internally? As an object with three fields? Let's made a second cell which just refers to the contents of the first. But this time we won't tell Excel to format the output in any particular way -- we'll change the formatting just to “general”.
• Aha! The function date(int, int, int) returns an int! More fairly “Excel uses a single int as its representation of the date”. No problem there; they use different toPrettyString methods which take a date and display it differently.
• Strengths and weaknesses:
  • Some operations become easy -- comparing if one date comes before another, or computing n days after a given date.
  • Some operations become more difficult: getMonth, etc.. Although once those are written, then it's not hard to each of that list's of display functions;
  • Some operations become impossible: isValid. If a user enters Feb.30th, the computer can't catch their error, once they've converted it to an int; the computer will blissfully compute away, compounding a possible error at a billion operations per second.

    how to catch?: Perhaps the *constructor* could catch this problem: It takes the inputs 2007,2,29 and it constructs an internal date object (int). But before returning it can ask getMonth() and getDay(); if these give different answers than 2 and 29 then it can ask the user “2007.Feb.29 is not a valid day; did you want to use 2007.Mar.01?”

  • Excel's approach violates the Zero'th Law of Programming (the one which underpins all the others):

    The Zero'th Law of Programming: The structure of your data and of your program should reflect how you (as a human) conceive of the problem at hand.

    (To be fair, Excel does do a good job of separating the abstract data type (Date) from its representation (int). Though if it did a perfect job, we'd have no way of figuring out their internal representation.¹) This topic is discussed more in ITEC220 (CS2). We'll also say that it separates the model (the date) from the view (the many ways of formatting the Date).)
  Note that there's even a view menu, which controls how the spreadsheet (the model) is displayed. Changing the view doesn't actually change the underlying spreadsheet at all.

• Task (optional): play around with Excels Time constructor² and see if you can figure out Excel's representation of time-of-day.

  In fact, Excel just has one way to represent moments in time, which is what it uses for both dates and for time-of-day -- clever clever! Perhaps their representation is not so bad after all -- it is reflecting how physicists view time, and thus arguably is not actually a lapse of the zero'th law.

Objects containing objects

What if we decided that a VideoGame also contains a release-date?

• What type should we use, to model a date?
• Declare a field of that type.
• Modify the constructor.
• How to call that constructor? (examples of the input)

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©2008, Ian Barland, Radford University Last modified 2008.Jan.24 (Thu)

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