xstream cannot serialize subclasses of HashMap

this test fails:

public class XStreamTest extends TestCase
{
public static class MyMap extends HashMap implements Map
{

}
public void testCanExportSubclassesOfMap()

{ MyMap myMap = new MyMap(); myMap.put("hehe", "hoho"); XStream xstream = new XStream(); String xml = xstream.toXML(myMap); MyMap myOtherMap = (MyMap) xstream.fromXML(xml); assertEquals(myMap, myOtherMap); }

}