Cannot serialize inner sets of HashMap

The sets of a HashMap are not properly serialized. They do not have any elements in the XML representation and are additionally not properly unmarshalled, since the returned collection throws a NPE accessing the size method. Test cases below.

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final public void testKeySetOfHashMapCanBeSerialized()

{ final Map map = new HashMap(); map.put("JUnit", null); final XStream xstream = new XStream(); final String xml = xstream.toXML(map.keySet()); final Collection collection = (Collection) xstream.fromXML(xml); assertNotNull(collection); assertEquals(1, collection.size()); assertEquals("JUnit", collection.iterator().next()); }

final public void testValueSetOfHashMapCanBeSerialized()

{ final Map map = new HashMap(); map.put(Boolean.TRUE, "JUnit"); final XStream xstream = new XStream(); final String xml = xstream.toXML(map.values()); final Collection collection = (Collection) xstream.fromXML(xml); assertNotNull(collection); assertEquals(1, collection.size()); assertEquals("JUnit", collection.iterator().next()); }

final public void testEntrySetOfHashMapCanBeSerialized()

{ final Map map = new HashMap(); map.put(Boolean.TRUE, "JUnit"); final XStream xstream = new XStream(); final String xml = xstream.toXML(map.entrySet()); final Collection collection = (Collection) xstream.fromXML(xml); assertNotNull(collection); assertEquals(1, collection.size()); final Map.Entry entry = (Map.Entry) collection.iterator().next(); assertEquals(Boolean.TRUE, entry.getKey()); assertEquals("JUnit", entry.getValue()); }