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copied!<p>First off <strong>good heavens do not do financial calculations in double</strong>. Do financial calculations in <strong>decimal</strong>; that is what it is for. Use double to solve <em>physics</em> problems, not <em>financial</em> problems. </p> <p>The major design flaw in your program is that <em>policy is in the wrong place</em>. Who is in charge of computing the taxes? You've put <em>the product</em> in charge of computing the taxes, but when you buy an apple or a book or a washing machine, <em>the thing you are about to buy</em> is not responsible for telling you how much tax you're going to pay on it. <em>Government policy</em> is responsible for telling you that. Your design massively violates the basic OO design principle that <strong>objects should be responsible for their own concerns</strong>, and not anyone else's. The concern of a washing machine is washing your clothes, not charging the right import duty. If the tax laws change, you don't want to change <em>the washing machine object</em>, you want to change <em>the policy object</em>.</p> <p>So, how to approach these sorts of problems in the future?</p> <p>I would have started by highlighting every important noun in the problem description:</p> <blockquote> <p><strong>Basic sales tax</strong> is applicable at a <strong>rate</strong> of 10% on all <strong>goods</strong>, except <strong>books</strong>, <strong>food</strong>, and <strong>medical products</strong> that are exempt. <strong>Import duty</strong> is an additional <strong>sales tax</strong> applicable on all <strong>imported goods</strong> at a <strong>rate</strong> of 5%, with no <strong>exemptions</strong>. When I purchase <strong>items</strong> I receive a <strong>receipt</strong> which lists the <strong>name</strong> of all the <strong>items</strong> and their <strong>price</strong> (including <strong>tax</strong>), finishing with the <strong>total cost</strong> of the items, and the total amounts of <strong>sales taxes</strong> paid. The rounding rules for <strong>sales tax</strong> are that for a tax rate of n%, a <strong>shelf price</strong> of p contains (np/100 rounded up to the nearest 0.05) amount of <strong>sales tax</strong>.</p> </blockquote> <p>Now, what are the relationships between all those nouns?</p> <ul> <li>Basic Sales Tax is a kind of Sales Tax</li> <li>Import Duty is a kind of Sales Tax</li> <li>A Sales Tax has a Rate which is a Decimal</li> <li>Books are a kind of Item</li> <li>Food is a kind of Item</li> <li>Medical Products are a kind of Item</li> <li>Items may be Imported Goods</li> <li>An Item has a Name which is a String</li> <li>An Item has a Shelf Price which is a Decimal. (Note: does an item really have a price? two identical washing machines might be for sale for different prices at different stores, or at the same store at different times. A better design might be to say that a Pricing Policy relates an Item to its Price.)</li> <li>A Sales Tax Exemption Policy describes the conditions under which a Sales Tax is inapplicable on an Item.</li> <li>A Receipt has a list of Items, their prices and their taxes.</li> <li>A Receipt has a total</li> <li>A Receipt has a total tax</li> </ul> <p>... and so on. Once you have all the relationships between all the nouns worked out, then you can start designing a class hierarchy. There is an abstract base class Item. Book inherits from it. There is an abstract class SalesTax; BasicSalesTax inherits from it. And so on.</p>

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