title: JAXB -field based on two sibling XML elements
body: <p>for example  i have this xml:</p>
  
  <pre><code>&lt;xml&gt;
    &lt;a&gt;1&lt;/a&gt;
    &lt;b&gt;2&lt;/b&gt;
  &lt;/xml&gt;
  </code></pre>
  
  <p>and this class:</p>
  
  <pre><code>@XmlRootElement(name = "xml")
  public class xml{
  
  int aPlusb;
  ....
  }
  </code></pre>
  
  <p>i know how to create an XmlAdapter but i would like it to set the filed aPlusb to be the plus between the value of element a and the value of filed b.</p>
  
  <p>is there a way to do this in JAXB without making an XmlTransient field and calculate it separately? </p>