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    <p>The key insight here is that the constructor arguments names <em>are</em> available, as they are the names of the fields created by the constructor. So <strong>provided that the constructor does nothing with its arguments but assign them to fields</strong>, then we can ignore it and work with the fields directly.</p> <p>We can use:</p> <pre><code>def setFields[A](o : A, values: Map[String, Any]): A = { for ((name, value) &lt;- values) setField(o, name, value) o } def setField(o: Any, fieldName: String, fieldValue: Any) { // TODO - look up the class hierarchy for superclass fields o.getClass.getDeclaredFields.find( _.getName == fieldName) match { case Some(field) =&gt; { field.setAccessible(true) field.set(o, fieldValue) } case None =&gt; throw new IllegalArgumentException("No field named " + fieldName) } </code></pre> <p>Which we can call on a blank person:</p> <pre><code>test("test setFields") { val p = setFields(new Person(null, null, -1), Map("firstname" -&gt; "Duncan", "lastname" -&gt; "McGregor", "age" -&gt; 44)) p.firstname should be ("Duncan") p.lastname should be ("McGregor") p.age should be (44) } </code></pre> <p>Of course we can do better with a little pimping:</p> <pre><code>implicit def any2WithFields[A](o: A) = new AnyRef { def withFields(values: Map[String, Any]): A = setFields(o, values) def withFields(values: Pair[String, Any]*): A = withFields(Map(values :_*)) } </code></pre> <p>so that you can call:</p> <pre><code>new Person(null, null, -1).withFields("firstname" -&gt; "Duncan", "lastname" -&gt; "McGregor", "age" -&gt; 44) </code></pre> <hr> <p>If having to call the constructor is annoying, Objenesis lets you ignore the lack of a no-arg constructor:</p> <pre><code>val objensis = new ObjenesisStd def create[A](implicit m: scala.reflect.Manifest[A]): A = objensis.newInstance(m.erasure).asInstanceOf[A] </code></pre> <p>Now we can combine the two to write </p> <pre><code>create[Person].withFields("firstname" -&gt; "Duncan", "lastname" -&gt; "McGregor", "age" -&gt; 44) </code></pre>
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    1. USDuncan McGregor
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    1. COThis is spot on what I am looking for, especially the final solution using [Objenesis](http://objenesis.googlecode.com/svn/docs/index.html) to avoid the call to the default constructor. The only thing that makes me a bit uneasy is relying on the java.lang.reflection API since I am not sure the Scala compiled byte code is 100% compatible with Java's byte code. But if it works... why not? Thanks a lot!
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    2. CO@Christoffer Soop: It isn't "compatible with" Java's byte code, it's exactly the same. It has to be in order to run on JVM.
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      1. USAlexey Romanov
    3. CO@Alexey Yes the byte code is 100% JVM compatible, but you cannot decompile Scala compiled byte code to Java with 100% success rate. How do I know that reflection works? Reflection, to my knowledge, infer Java constructs from byte code but I am not convinced that all scala produced byte code has a Java equivalent.
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