XStream deserialize xml having list of hierarchical object

Question

I have below xml which i want to deserialize in XStream.

<comments>
  <B>
    <id>1</id>
    <name>Name 1</name>
  </B>
  <C>
    <id>2</id>
    <name>name 2</name>
    <desc>Desc 2</desc>
  </C>
  <B>
    <id>3</id>
    <name>name 3</name>
  </B>
</comments>

The object hierarchy is as below

@XStreamAlias("comments")
class Comments {
  @XStreamImplicit
  List<A> a = new ArrayList<A>();
}

@XStreamAlias("A")
class A {

}

@XStreamAlias("B")
class B extends A {
  long id ;
  String name;
}

@XStreamAlias("C")
class C extends A {
  long id;
  String desc;
  String name;
}

The Deserialize code that i have is

XStream xstream = new XStream();
xstream.autodetectAnnotations(Boolean.TRUE);
xstream.alias("comments", Comments.class);
String comments= "path to comments xml";
Comments comments = (Comments)xstream.fromXML(new File(path));

While running the above, i get the below exception,

com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter$UnknownFieldException: No such field test.Comments.B
---- Debugging information ----
field               : B
class               : test.Comments
required-type       : test.Comments
converter-type      : com.thoughtworks.xstream.converters.reflection.ReflectionConverter
path                : /comments/B
line number         : 2
version             : not available
-------------------------------
at     com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.handleUnknownField(AbstractReflectionConverter.java:495)
at     com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter.doUnmarshal(AbstractReflectionConverter.java:351)

Could anyone pls help me deserialize the above.

It would also be great help if anyone could point out the best place to learn XStream, I have been searching throughout net for any reference material but apart the XStreams homepage, i dont see anyone talk abt advanced topics.

Thanks

Answer 1

Annotations are meant for serialization, not for de-serialization. You can leave the annotations out and use the following:

XStream xstream = new XStream();
xstream.alias("comments", ArrayList.class);
xstream.alias("B", B.class);
xstream.alias("C", C.class);
xstream.alias("A", A.class);
Object o = xstream.fromXML(in);

The result will be a list with A, B or C objects depending on the element name.