Working with Exception Classes Solution

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This assignment will give you a chance to try out exceptions, while also learning about several interesting Java classes that can be used for financial calculations. In your 2019SPCS5004SV/assignment-5 GitHub repository is a class Money that demonstrates their use.

This class assumes decimal currency, without unusual divisions like 1/5. Money objects are immutable. The class uses BigDecimal for the amount, and Currency to indicate the currency of the amount. Storing units with a quantity makes it convenient to work with quantities that have multiple units. Other examples include distance in either English units (e.g. inches, feet, yards, miles), metric (e.g. milimeters, centimeters, meters, kilometers) or specialized units like angstroms (10-10 meters). This assignment will acquaint you with this technique

Currency represents a currency. Currencies are identified by their ISO 4217 currency codes. The class is designed so that there is never more than one Currency instance for any given currency. Therefore, there is no public constructor. You obtain a Currency instance using the getInstance() methods, including one that accepts a Locale (e.g. Locale.US) and one that accepts a country code (e.g. “en-US”).

BigDecimal allows specifying amounts as double, long, String, or as an unscaled value and an int scale that can be negative or positive (e.g. 12000 or {12, -3}, 1234.56 or {123456, 2}). BigDecimal can round to a number of decimal places. Money rounds to the number of decimal places for the Currency using “banker’s rounding.”

This version of Money creates instances that use the Currency for the default Locale. The constructors that include a Currency are included but have only package visibility to facilitate testing. Your assignent is to full enable Money operations in multiple currencies. There are several parts to this assignment.

  1. Make the constructors that enable creating Money in multiple currencies public.

  2. Operations between currencies are not supported. You should create a new nested static class MismatchedCurrencyException that can be thrown in every Money operation on two or more currencies. You will be surprised to learn that the base for this exception will be RuntimeException. This is one of those rare “exceptions” to the general rule that all developer-created exceptions should be checked exceptions. Money is a lot like Integer, Double, or BigDecimal where it is also considered burdensome to declare and catch every ArithmeticException or NumberFormatException that can be thrown. So enjoy this rare opportunity to work with an unchecked exception! (Do add @throws to methods that can throw the exception, however.)

  3. Add a new method Money.asCurrency(Currency
    aCurrency, double exchangeRate)
    that converts an instance to one for the specified Currency by multiplying the instance amount by the given exchange rate, and returns the new instance with that amount and currency.

  4. Enhance the unit tests in Money_test to create and work with Money in multiple currencies, and to ensure thatMismatchedCurrencyException is thrown where appropriate. Be sure to add a unit test for the new asCurrency() currency conversion method.

Note: The unit tests on lines 44 and 55 work correctly under Java 8. However, under Java 10, the currency symbol for the U.S. Dollar in the UK locale has changed from “USD” to “US$”. If these tests fail on your computer, you should change the expected value to “US$”.