How to save an n-ary tree of nodes to and from an xml file

Question

Hello,
I am trying to represent a n-ary tree of nodes in xml.
More accurately, I am trying to save/instantiate a
tree of nodes to-from an xml file.
How do I represent the parent-child relationships?
Java: (simplified)
class Node {
Arraylist childNodes;
Node parent;
XML:
From what I have been learning about XML,
DTD:
<?xml version="1.0" ?>
<!DOCTYPE acd
     <!ELEMENT Node (childNodes, parent)>
     <!ATTLIST Node id ID #REQUIRED>
     <!ELEMENT childNodes (Node*)>
     <!ELEMENT parent (Node?)>
]>
Qs:
1) How do I represent the relationships? What would
normally be a reference in Java, how do I represent in
XML?
2) Do I use ID, IDREF? I have been trying to find some
examples to learn how. i.e. Does the IDREF, ID
automatically become an in-memory reference (or pointer)?
3) Is it preferable to use XML schema?
thanks,
Anil Philip
Olathe, KS
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Answer 1

I downloaded XML Spy and used it to correct my earlier schema.
Qs:
In the instance document xml file;
1) How would one display the parent node?
2) If a child has a reference to a parent node as in the schema below, how can one know that the references are the same?
i.e. when the parent node is first declared and when it is referred to by the child.
Perhaps this raises a larger question - how does XML handle recursive references?
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://juwo.com/acd" xmlns="http://juwo.com/acd" elementFormDefault="qualified" attributeFormDefault="unqualified">
     <xs:annotation>
          <xs:documentation>ACD nodes. juwo LLC 2005</xs:documentation>
     </xs:annotation>
     <xs:complexType name="Node">
          <xs:sequence>
               <xs:element name="parent" type="Node" minOccurs="0"/>
               <!-- Node[] childNodes -->
               <xs:element name="childNodes" type="ListOfNodes"/>
               <!-- String data -->
               <xs:element name="data" type="xs:string"/>
          </xs:sequence>
          <!-- Node parent -->
     </xs:complexType>
     <xs:complexType name="ListOfNodes">
          <xs:sequence>
               <xs:element name="i" type="Node" minOccurs="0" maxOccurs="unbounded"/>
          </xs:sequence>
     </xs:complexType>
</xs:schema>