How can I make implicit to node which is not collection?

I want to convert this class in xml file of this type. What should I do to remove the node <с>? How can I make implicit to node which is not collection?

Java:

public class root {
    int a;
    int b;
    int d;
}

XML:

<root>
  <a>123</a>
  <b>345</b>
  <c>
     <d>567</d>
  </c>
</root>