CSC/ECE 517 Fall 2011/ch7 7a kr

CSC/ECE 517 Fall 2011/ch7 7a kr

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

A common task in real world application programs is conducting operations on and manipulating money values. The base data type used to represent money can be one of several possible implementations. Object oriented languages have the ability to represent this information using classes. Primitive data types such as integers, floating point numbers can also be used to represent money. There are certain inherent advantages and disadvantages with each of these methods of representation. This wiki article explores these different implementations and analyzes their relative merits and demerits. We also focus on the various operations that can be performed with money values and analyze how different languages and schemes help while working with money values.

Money Gem (Ruby)

The Ruby Money gem borrows quite a bit from the Skrien text methodologies while using the dynamic properties of Ruby to allow for more flexibility. However, it should be noted that this implementation does not support an object containing mixed currencies as mentioned in the Skrien text and some behavior implemented goes against it's principles.

The Money gem consists of a main Money class which houses several sub-classes and modules which allow for everything to be done from the main Money class directly without the users knowledge. This implementation also mostly sticks to the idea that a Money object is immutable and thus any conversions and operations result in a new Money object. The only exception to this is that the currency identifier for a Money object can be changed once it has been created.

Internally, a Money object stores money as an integer in the form of cents. It should be noted that while the variable is called cents, it can apply to all currencies which have a subunit of denomination. The Money object also stores a bank and currency as well as default bank and currency types at the class level. Similar to the way the Skrien text uses the Currency and CurrencyConverter classes, the Money class uses internal classes for currency information and currency conversion as well, called Currency and Bank respectively.

The Currency class contains information such as the ISO4217 three letter and number codes for common currencies as well as number of subunits in a denomination (ie, cents in a dollar) and delimiters for units. Through the use of the currency class, Money objects can be initialized with a Currency value and have specific output formats based on currency. The currency class also internally extends the CurrencyLoader class which allows for specifying different locations for loading currency information to allow for flexible currency management.

The Bank class is also an internal class to the Money class but it resides at the class level. This means that the Bank object follows the Singleton pattern in that only one Bank object exists across all of the currency. The purpose of the Bank is to maintain information related to currency values such that currency conversion can take place. Since the Bank class inherits from an interface, this allows for extensible money conversion schemes such as being able to 'scrape' data from the internet and use it to populate conversion ratios.

The Money class also has multiple modules internally that expand the operations that can be performed on Money objects, including converting to and from numbers, strings and symbols. Through the use of Ruby's dynamic nature, to_money() methods are added to the Numeric, String and Symbol types.

Advantages

1. The internal CurrencyLoader class that is extended allows for currency information to be loaded on-the-fly
2. Due to the Bank implementation, additional classes can be loaded and changed out at runtime to allow for more complex, an potentially real-time, currency conversion information.
3. The fact that the Currency and Bank classes are internal to the Money class, the user doesn't have to know about their implementation