title: Simple XML and HashSet
body: <p>I'm discovering Simple XML and try to serialize this simple class :</p>
  
  <pre><code>public class Div
  {
    private Set&lt; String &gt; _classes = new HashSet&lt; String &gt;() {{
      add( "a" );
      add( "b" );
      add( "c" );
    }};
  
    // some methods and attributes...
  }
  </code></pre>
  
  <p>To : </p>
  
  <pre><code>&lt;div class="a b c"&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>There is the @Attribute annotation but this can't convert the set to a string. Simple XML provides some "transformers" for doing this job but I can't find any example.</p>
  
  <p>Thanks</p>