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POHow should I design this code (parallel class hierarchies)?
 

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  1. POHow should I design this code (parallel class hierarchies)?
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    <p>I have a set of apps that all need to synchronize with a webservice:</p> <ol> <li>download some XML from a webservice.</li> <li>parse that XML.</li> <li>then update the database to match the parsed XML.</li> </ol> <p>I'd like to keep as much of the relevant code as possible in a common library to avoid duplication, and I'd like to have the different apps all slot their parsing and updating code into a fairly simple common framework.</p> <p>So there's a common sync() method:</p> <pre><code>public void sync(URI updateUrl, XMLParser parser, Updater animalUpdater) { String raw = getXML(); List&lt;ParsedAnimal&gt; parsed = parser.parse(raw); // try: // begin transaction for (ParsedAnimal pi : parsed) { animalUpdater.updateItem(pi); } // commit transaction // catch: rollback transaction, rethrow // finally: close database connection } </code></pre> <p>The parser returns either a ParsedCat, or ParsedDog, or whatever, all of which inherit from a common ParsedAnimal class:</p> <pre><code>public abstract class ParsedAnimal {...} public class ParsedCat extends ParsedAnimal {...} public class ParsedDog extends ParsedAnimal {...} </code></pre> <p>Then I have an Updater which needs to take the parsed item and inject the contents into the database:</p> <pre><code>public abstract class Updater { public abstract void updateItem(ParsedAnimal parsed); } public class CatUpdater extends Updater { @Override public void updateItem(ParsedCat parsed) {} } public class DogUpdater extends Updater {...} </code></pre> <p>This doesn't work &mdash; the contract for Updater specifies that updateItem() accepts a ParsedAnimal, and CatUpdater and DogUpdater both break that contract by only accepting a specific type of animal.</p> <p>What I have is a parallel class hierarchy &mdash; ParsedX matches up one to one with XUpdater. The <a href="http://www.codinghorror.com/blog/2006/05/code-smells.html" rel="nofollow" title="Code Smells">Code Smells</a> page on Coding Horror suggests merging the two class hierarchies into a single hierarchy, but I feel like the "thing being worked on" and the "thing doing the working" are different enough that they should be separate classes.</p> <p>Is there a neat way of structuring this, or some design pattern that would come in handy?</p>
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    1. COYes, use generics: `public abstract class Updater<T extends ParsedAnimal> {` and `public class CatUpdate extends Updater<ParsedCat> {`. And in Updater: `public abstract void updateItem(T parsed);`
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      1. POHow should I design this code (parallel class hierarchies)?
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