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CS 535 Object-Oriented Programming
Spring Semester, 2003
Assignment 6
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Assignment Index
© 2003, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 24-Mar-03
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Assignment
6
1.
We need a Money class for financial transactions. Each Money object needs to
know its currency and a magnitude. While developing the class for this problem
you can use a string for the currency. One should be able to create money using:
Money currency: ‘USD’ amount: 12.34s2
Or
12.34s2 asMoney: ‘USD’.
Money
acts like an arithmetic object as you can add money, multiply money by numbers
and divide money by money to get a number. For problem one just support adding
money of the same currency. The following operations are valid and need to be
supported:
Operation
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Result
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Money
* Number
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Money
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Money
+ Money
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Money
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Money
- Money
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Money
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Money
/ Money
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Number
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Money
/ Number
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Money
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When
an operation results in Money return a new Money object with the result. Once a
Money object is created do not change the amount or currency of the object. One
problem in Money arithmetic operations is round off error. What is the correct
result of 0.03 USD / 2? There is no standard solution to this issue. For this
class we will just use the default behavior of FixedPoint numbers.
The
following operations are not valid and should result in an exception being
raised.
Invalid
Operations
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Money
- Number
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Number
/ Money
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Money
* Money
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Money
+ Number
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Problem
one is primarily an exercise in double-dispatching. Implement the = operator to
return true only if two Money objects are of the same currency and have the
same value. Don’t forget the hash operator. Money objects should support
the operators >, <, <=, >= on objects with the same currency.
Don’t
forget to write unit tests for your Money class.
2.
Now to add currency conversion. We need a method convertTo: to the Money class
that returns a new Money object with the amount converted to a different
currency. So for example:
| dollar euros |
dollar := Money currency: ‘USD’ amount: 1.0s2.
euros := dollar convertTo: ‘EUR’.
Conversion
of currency raises a number of issues.
First
the currency exchange rates to change frequently. The table below contains the
currencies we will support, the symbols for each currencies and the current
exchange rate (as of Friday March 21).
Second
we have to address the issue of the fractional digits supported by the
currency. For example in this country ordinary currency operations do not
handle amounts less than a cent. Converting currency can result in amounts less
than supported by the currency. In all currency exchanges round down to
smallest unit supported by the currency. (See the FixedPoint class comment.) In
the example above using the rates below the amount in euros variable should be
0.94.
Third
is number of exchange rates needed to support N different currencies. The table
below contains 9 different currencies, which leads to 9*8 = 72 different pairs
of possible currencies change rates. The number of these rates can be reduced
to 9 by converting all currencies to USD first. For example to convert 2 CAD to
Euros, first convert
2 CAD / 1.4905 = 1.34 USD
The
convert the 1.34 USD to Euros
1.34 USD * 0.948317 = 1.27 EUR
This
does allow currency traders to pocket the round off error twice.
Fourth
is that different currencies support different number of fractional digits. For
example the Japanese Yen supports no fractional digits.
You
may find it useful to use a Currency object to hold information about a
currency.
Modify
your Money code so +, - and / work with different currencies. For example:
| dollar euros |
dollar := Money currency: ‘USD’ amount: 1.0s2.
euros := Money currency: ‘EUR amount: 0.95.
dollar + euros
The
operation dollar + euros should return the sum in USD. The operation euros +
dollar should return the sum in EUR. That is the left operand determines the
currency of the result.
Provide
unit tests for your currency exchanges.
Currency
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Symbol
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Fractional
Digits
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1
USD Equals
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Canadian
Dollar
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CAD
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2
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1.4905
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Chinese
Yuan
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CNY
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2
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9.2773
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EURO
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EUR
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2
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0.948317
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Indian
Rupee
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INR
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2
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47.66
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Japanese
Yen
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JPY
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0
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121.42
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Mexican
Peso
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MXN
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2
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10.7575
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Pakistan
Rupee
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PKR
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2
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58.0958
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Thailand
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THB
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2
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43.07
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US
Dollar
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USD
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2
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1
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What
to turn in
Turn
in one copy of the source code for your tests, classes you create and methods
you add to exiting classes. The best way to do this is to put all your code in
a parcel. To insure that you have all the code, save the parcel, parcel it in
to a clean image and run your tests. If you do not turn in all your code you
will lose points and will not add them later if you complain that you just
forgot. Keeping all your code together is part of this assignment. If you print
a file out of your code do not use the XML file out. You will 30 points if you
do. Do not separate the code into code for problem one and problem two. You may
end up changing the code from problem one to meet problem two. Just turn in the
final version of the code. Please avoid using Word to edit and printout your
source code. Word changes the formatting and capitalization of words. If this
happens you will lose points.
Grading
The
assignment is worth 55 points. 5 points are for using standard Smalltalk naming
conventions and formatting. 5 points are for problem one tests. 5 points are
for problem two tests. 20 points are for meeting the problem one requirements.
20 points are for meeting problem two requirements.
Copyright ©, All rights reserved.
2003 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
OpenContent license defines the copyright on this document.
visitors since 24-Mar-03