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POHow do I create a class hierarchy of typed factory method constructors and access them from Scala using abstract types?
 

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  1. POHow do I create a class hierarchy of typed factory method constructors and access them from Scala using abstract types?
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    <p>(Essentially I need some kind of a synthesis of these two questions (<a href="https://stackoverflow.com/questions/1586811/is-there-a-self-type-in-scala-that-represents-the-current-type">1</a>, <a href="https://stackoverflow.com/questions/7616692/how-can-i-invoke-the-constructor-of-a-scala-abstract-type">2</a>), but I'm not smart enough to combine them myself.)</p> <p>I have a set of JAXB representations in Scala like this:</p> <pre><code>abstract class Representation { def marshalToXml(): String = { val context = JAXBContext.newInstance(this.getClass()) val writer = new StringWriter context.createMarshaller.marshal(this, writer) writer.toString() } } class Order extends Representation { @BeanProperty var name: String = _ ... } class Invoice extends Representation { ... } </code></pre> <p>The problem I have is with my unmarshalling "constructor" methods:</p> <pre><code>def unmarshalFromJson(marshalledData: String): {{My Representation Subclass}} = { val mapper = new ObjectMapper() mapper.getDeserializationConfig().withAnnotationIntrospector(new JaxbAnnotationIntrospector()) mapper.readValue(marshalledData, this.getClass()) } def unmarshalFromXml(marshalledData: String): {{My Representation Subclass}} = { val context = JAXBContext.newInstance(this.getClass()) val representation = context.createUnmarshaller().unmarshal( new StringReader(marshalledData) ).asInstanceOf[{{Type of My Representation Subclass}}] representation // Return the representation } </code></pre> <p>Specifically, I can't figure out how to attach these unmarshalling methods in a typesafe and DRY way to each of my classes, and then to call them from Scala (and hopefully sometimes by using only abstract type information). In other words, I would like to do this:</p> <pre><code>val newOrder = Order.unmarshalFromJson(someJson) </code></pre> <p>And more ambitiously:</p> <pre><code>class Resource[R &lt;: Representation] { getRepresentation(marshalledData: String): R = {{R's Singleton}}.unmarshalFromXml(marshalledData) } </code></pre> <p>In terms of my particular stumbling blocks:</p> <ul> <li>I can't figure out whether I should define my <code>unmarshalFrom*()</code> constructors once in the <code>Representation</code> class, or in a singleton <code>Representation</code> object - if the latter, I don't see how I can automatically inherit that down through the class hierarchy of <code>Order</code>, <code>Invoice</code> etc.</li> <li>I can't get <code>this.type</code> (<a href="https://stackoverflow.com/questions/1586811/is-there-a-self-type-in-scala-that-represents-the-current-type">as per this answer</a>) to work as a way of self-typing <code>unmarshalFromJson()</code> - I get a compile error <code>type mismatch; found: ?0 where type ?0 required: Representation.this.type</code> on the <code>readValue()</code> call</li> <li>I can't figure out how to use the implicit <code>Default[A]</code> pattern (<a href="https://stackoverflow.com/questions/7616692/how-can-i-invoke-the-constructor-of-a-scala-abstract-type">as per this answer</a>) to work down my Representation class hierarchy to call the singleton unmarshalling constructors using type information only</li> </ul> <p>I know this is a bit of a mammoth question touching on various different (but related) issues - any help gratefully received!</p> <p>Alex</p>
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