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XML Tree Structure

An XML document has a self-descriptive structure. It forms a tree structure which is referred as an XML tree. The tree structure makes easy to describe an XML document.

A tree structure contains root element (as parent), child element and so on. It is very easy to traverse all succeeding branches and sub-branches and leaf nodes starting from the root.

Example​

Example: Bookstore

bookstore.xml
<?xml version="1.0"  encoding="UTF-8"?>  
<bookstore>  
	<book category="cooking">  
		<title lang="en">Everyday Italian</title>  
		<author>Giada De Laurentiis</author>  
		<year>2005</year>  
		<price>30.00</price>  
	</book>  
	<book category="children">  
		<title lang="en">Harry Potter</title>  
		<author>J K. Rowling</author>  
		<year>2005</year>  
		<price>29.99</price>  
	</book>  
	<book category="web">  
		<title lang="en">Learning XML</title>  
		<author>Tom Nolan</author>  
		<year>2003</year>  
		<price>39.95</price>  
	</book>  
</bookstore>

Represented graphically in the following image

Example Tree Structure

XML Tree Structure​

An XML tree starts at a single root element and branches out to its child elements, which can have their own children, and so on. This creates a clear, hierarchical structure.

Core Components of the Tree​

Every part of an XML document is a node in the tree.

  • Root Element: The single, top-level element that contains all other elements. In our example, <bookstore> is the root element.
  • Element Nodes: The main building blocks of the tree, defined by start and end tags (e.g., <book>, <title>).
  • Attribute Nodes: Provide additional information about an element (e.g., category="cooking"). They are considered part of the element they belong to, not children of it.
  • Text Nodes: The actual text content inside an element (e.g., "Everyday Italian"). These are often the "leaves" of the tree, as they cannot have children.

For example:

<root type="example">  
	<child name="first">  
		<subchild>Some text content</subchild>  
	</child>  
</root>

where:

  • Root Element

    • <root> is the root element of the XML document.
    • It contains the <child> element.
    • It has one attribute, type="example".

  • Element Nodes

    • <root>, <child>, and <subchild> are element nodes.
    • Each is defined by a start tag and an end tag.

  • Attribute Nodes

    • <root> has an attribute node: type="example".
    • <child> has an attribute node: name="first".
    • Attributes belong to elements but are not child nodes of those elements.

  • Text Nodes

    • <subchild> contains the text Some text content, which is a text node.
    • Text nodes are the leaf nodes, i.e. they cannot contain other elements.

Understanding Node Relationships​

The terms parent, child, and sibling are used to describe the relationships between elements, just like in a family tree.

  • Parent

    • Definition: An element that directly contains another element. Each element has exactly one parent.
    • Example: The <bookstore> element is the parent of the two <book> elements. The first <book> element is the parent of its <title>, <author>, <year>, and <price> elements.

  • Children

    • Definition: The elements directly contained within another element.
    • Example: The two <book> elements are the children of the <bookstore> element.

  • Siblings

    • Definition: Elements that share the same parent.
    • Example: The <title>, <author>, <year>, and <price> elements inside the first <book> are all siblings. The two <book> elements are also siblings.

Self-Describing Syntax​

XML uses a much self-describing syntax.

A prolog defines the XML version and the character encoding:

<?xml version="1.0"  encoding="UTF-8"?>

The next line starts a <book> element:

<book category="cooking">

The <book> elements have 4 child elements: <title>, <author>, <year>, <price>.

<title lang="en">Everyday Italian</title>  
<author>Giada De Laurentiis</author>  
<year>2005</year>  
<price>30.00</price>

The next line ends the book element:

</book>

Conclusion​

Viewing an XML document as a tree is the key to understanding how it is processed.

This hierarchical structure is not just a visual aid: it's the fundamental model that technologies like the DOM (Document Object Model) and XPath use to parse, navigate, and query XML data.

  • Every XML document starts with a single root element.
  • The structure is defined by the parent, child, and sibling relationships between elements.
  • This tree model allows for logical and predictable traversal of the document's data.

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