Java Reference
In-Depth Information
The rules for creating trees are simple. You start at the top of the tree with the document and the ele-
ment that contains all other elements in the page. The document is the
root node
. A
node
is just a point
on the tree representing a particular element or attribute of an element, or even the text that an element
contains. The root node contains all other nodes, such as the DTD declaration, the XML declaration if
applicable, and the root element (the HTML or XML element that contains all other elements). The root
element should always be the <html/> element in an HTML document. Underneath the root element
are the HTML elements that the root element contains. Typically an HTML page will have <head/> and
<body/> elements inside the <html/> element. These elements are represented as nodes underneath
the root element's node, which itself is underneath the root node at the top of the tree (see Figure 12-1).
<HTML>
<HEAD>
<BODY>
Figure 12-1
The two nodes representing the
<head/> and <body/> elements are examples of
child nodes
, and the
<html/> element's node above them is a
parent node
. Since the <head/> and <body/> elements are both
child nodes of the
<html/> element, they both go on the same level underneath the parent node <html/>
element. The
<head/> and <body/> elements in turn contain other child nodes/HTML elements, which
will appear at a level underneath their nodes. So child nodes can also be parent nodes. Each time you
encounter a set of HTML elements within another element, they each form a separate node at the same
level on the tree. The easiest way of explaining this clearly is with an example.
An Example HTML Page
Let's consider a basic HTML page such as this:
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN”
“http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>
<html xmlns=”http://www.w3.org/1999/xhtml”>
<head>
</head>
<body>
<h1>My Heading</h1>
<p>This is some text in a paragraph.</p>
</body>
</html>
The <html/> element contains <head/> and <body/> elements. Only the <body/> element actually
contains anything. It contains an
<h1/> element and a <p/> element. The <h1/> element contains the
text
My Heading. When you reach an item, such as text, an image, or an element, that contains no others,
the tree structure will terminate at that node. Such a node is termed a
leaf node
. You then continue to the
<p/> node, which contains some text, which is also a node in the document. You can depict this with
the tree structure shown in Figure 12-2.
