CSC/ECE 517 Fall 2011/ch7 7a or

Wiki Chapter: CSC/ECE 517 Fall 2011/ch7 7a or

7a. Representing money. Skrien Chapter 6 gives an example of a class that can be used to represent money. But how is it done in real programs? Investigate, and report on the advantages and disadvantages of other approaches vs. Skrien's.

Introduction

Money is very important to people worldwide and is something that often must be represented in computer programs. This raises the question of how money should be represented, which can vary from program to program. There is much to be considered such as string formatting, the way money is used, and the variety of different currencies in the world. In Chapter 6 of Skrien's Object-Oriented Design Using Java, he suggests an implementation that represents money as a class, but there are other approaches that can be taken, each with their respective advantages and disadvantages.

Properties of Money

Money has certain properties and uses that must be considered in implementation.

Responsibilities/Using Money

The following are ways in which a representation of money should be able to be used: [1]

• To represent a positive or negative amount of money.
• Possibly representing multiple components of a currency - i.e. dollars and cents.
• Possibly representing amounts of multiple currencies - i.e. a mix of US Dollars, euros, and yen.'
• To have a string formatting potentially including currency symbols like the dollar sign ($) and/or commas and/or decimal points.
• Addition, subtraction, multiplication, and division.

The degree to which of the above must be met will depend on the depth of the implementing project.

Different Currencies

Some applications may only handle only one currency such as the US Dollar, but more generally applied applications, such as banking, must be able to handle different kinds of currencies.

There are currently over 180 currencies in use around the world.[2]

Implementation must be aware of how to specifically represent each currency used. Some currencies use decimal points, such as how the US has 100 cents to a dollar, while others have no decimal, such as yen. Each currency also has a different sign symbol used such as $, €, or ¥. Different currencies also have different rules about how rounding values is handled.

Special consideration must be taken when using different currencies in conjunction with each other. Foreign exchange rates can be used to convert between different currencies, but the exchange rate often change, so attention must be paid to that. In some cases, it is also important to keep track of how much money the user has of more than one currency at a time, so that must be handled somehow. [3][4]

Immutable vs. Mutable

Money values can be either mutable, meaning changeable, or immutable, meaning unchanging. Mutable money values are useful in that a new value does not need to be created for every change. However, mutable values cause problems for use in hash maps or with concurrency. The issue with concurrency is a read and write issue in a system with multiple users. With changing values, money not be what the user expects and lead to unpredictable behavior. Skrien feels this is a significant enough disadvantage to prefer an immutable representation for money.[5]

Resources

• Need to go through these for information
  • PDF from class lecture
  • Java's Currency class docs
  • A standalone class to tackle the problem (in Java)
  • Representing money (Java)
  • Working with money in Java
  • Time and Money Java project - doesn't seem to have much documentation, though
  • Joda Money class library for Java
  • BigDecimal and Your Money (a criticism of using BigDecimal format)
  • Tutorial on using BigDecimal
  • JUnit Test Infected: Programmers Love Writing Tests (has a Money class example
  • Currency Internationalization (i18n), Multiple Currencies and Foreign Exchange (FX) -nothing programming-wise, but a good overview of the considerations for currency
  • Yet Another C++ Money Class
  • Representing currencies (txt document with example and details)