Defining the Root Element

Defining the Root Element

After the declaration, every XML file defines exactly one element, known as the root element. Any other elements in the file are contained within that element.

Enter the text highlighted below to define the root element for this file, slideshow:

<?xml version='1.0' encoding='utf-8'?> 

<!-- A SAMPLE set of slides --> 

<slideshow> 

</slideshow>

Adding Attributes to an Element

A slide presentation has a number of associated data items, none of which require any structure. So it is natural to define them as attributes of the slideshow element. Add the text highlighted below to set up some attributes:

... 
    <slideshow 
        title="Sample Slide Show" 
        date="Date of publication" 
        author="Yours Truly" 
        > 
    </slideshow>

When you create a name for a tag or an attribute, you can use hyphens ("-"), underscores ("_"), colons (":"), and periods (".") in addition to characters and numbers. Unlike HTML, values for XML attributes are always in quotation marks, and multiple attributes are never separated by commas.

1 Adding Nested Elements

XML allows for hierarchically structured data, which means that an element can contain other elements. Add the text highlighted below to define a slide element and a title element contained within it:

<slideshow 
    ... 
    > 

     <!-- TITLE SLIDE --> 
    <slide type="all"> 
        <title>Wake up to WonderWidgets!</title> 
    </slide> 
</slideshow>

Here you have also added a type attribute to the slide. The idea of this attribute is that slides could be earmarked for a mostly technical or mostly executive audience with type="tech" or type="exec", or identified as suitable for both with type="all".

More importantly, though, this example illustrates the difference between things that are more usefully defined as elements (the title element) and things that are more suitable as attributes (the type attribute). The visibility heuristic is primarily at work here. The title is something the audience will see. So it is an element. The type, on the other hand, is something that never gets presented, so it is an attribute. Another way to think about that distinction is that an element is a container, like a bottle. The type is a characteristic of the container (is it tall or short, wide or narrow). The title is a characteristic of the contents (water, milk, or tea). These are not hard and fast rules, of course, but they can help when you design your own XML structures.

2 Adding HTML-Style Text

Since XML lets you define any tags you want, it makes sense to define a set of tags that look like HTML. The XHTML standard does exactly that, in fact. You'll see more about that towards the end of the SAX tutorial. For now, type the text highlighted below to define a slide with a couple of list item entries that use an HTML-style <em> tag for emphasis (usually rendered as italicized text):

    ... 
    <!-- TITLE SLIDE --> 
    <slide type="all"> 
        <title>Wake up to WonderWidgets!</title> 
    </slide> 
    <!-- OVERVIEW --> 
    <slide type="all"> 
        <title>Overview</title> 
            <item>Why <em>WonderWidgets</em> are great</item> 
            <item>Who <em>buys</em> WonderWidgets</item> 
    </slide> 

</slideshow>

We'll see later that defining a title element conflicts with the XHTML element that uses the same name. We'll discuss the mechanism that produces the conflict (the DTD) and several possible solutions when we cover Parsing the Parameterized DTD (page 137).

3 Adding an Empty Element

One major difference between HTML and XML, though, is that all XML must be well-formed—which means that every tag must have an ending tag or be an empty tag. You're getting pretty comfortable with ending tags, by now. Add the text highlighted below to define an empty list item element with no contents:

    ... 
    <!-- OVERVIEW --> 
    <slide type="all"> 
        <title>Overview</title> 
        <item>Why <em>WonderWidgets</em> are great</item> 
        <item/> 
        <item>Who <em>buys</em> WonderWidgets</item> 
    </slide> 

</slideshow>

Note that any element can be empty element. All it takes is ending the tag with "/>" instead of ">." You could do the same thing by entering <item></item>, which is equivalent.

4 The Finished Product

Here is the completed version of the XML file:

<?xml version='1.0' encoding='utf-8'?> 

<!--  A SAMPLE set of slides  --> 
<slideshow 
    title="Sample Slide Show" 
    date="Date of publication" 
    author="Yours Truly" 
    > 

    <!-- TITLE SLIDE --> 
    <slide type="all"> 
        <title>Wake up to WonderWidgets!</title> 
    </slide> 

    <!-- OVERVIEW --> 
    <slide type="all"> 
        <title>Overview</title> 
        <item>Why <em>WonderWidgets</em> are great</item> 
        <item/> 
        <item>Who <em>buys</em> WonderWidgets</item> 
    </slide 
</slideshow>

Now that you've created a file to work with, you're ready to write a program to echo it using the SAX parser. You'll do that in the next section.