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XML for the absolute beginner

A guided tour from HTML to processing XML with Java


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Page 3 of 10

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An XML conceptual example
All this talk of "inventing your own tags" is pretty foggy: What kind of tags would a developer want to invent and how would the resulting XML be used? In this section, we'll go over an example that compares and contrasts information representation in HTML and XML. In a later section ("XSL: I like your style") we'll go over XML display.

First, we'll take an example of a recipe, and display it as one possible HTML document. Then, we'll redo the example in XML and discuss what that buys us.

HTML example
Take a look at the little chunk of HTML in Listing 1:


<!-- The original html recipe -->
<HTML>
<HEAD>
<TITLE>Lime Jello Marshmallow Cottage Cheese Surprise</TITLE>
</HEAD>
<BODY>
<H3>Lime Jello Marshmallow Cottage Cheese Surprise</H3>
My grandma's favorite (may she rest in peace).
<H4>Ingredients</H4>
<TABLE BORDER="1">
<TR BGCOLOR="#308030"><TH>Qty</TH><TH>Units</TH><TH>Item</TH></TR>
<TR><TD>1</TD><TD>box</TD><TD>lime gelatin</TD></TR>
<TR><TD>500</TD><TD>g</TD><TD>multicolored tiny marshmallows</TD></TR>
<TR><TD>500</TD><TD>ml</TD><TD>cottage cheese</TD></TR>
<TR><TD></TD><TD>dash</TD><TD>Tabasco sauce (optional)</TD></TR>
</TABLE>
<P>
<H4>Instructions</H4>
<OL>
<LI>Prepare lime gelatin according to package instructions...</LI>
<!-- and so on -->
</BODY>
</HTML>

Listing 1. Some HTML

(A printable version of this listing can be found at example.html.)

Looking at the HTML code in Listing 1, it's probably clear to just about anyone that this is a recipe for something (something awful, but a recipe nonetheless). In a browser, our HTML produces something like this:

Lime Jello Marshmallow Cottage Cheese Surprise

My grandma's favorite (may she rest in peace).

Ingredients

Qty Units Item
1 box lime gelatin
500 g multicolored tiny marshmallows
500 ml Cottage cheese
dash Tabasco sauce (optional)

Instructions

  1. Prepare lime gelatin according to package instructions...

Listing 2. What the HTML in Listing 1 looks like in a browser

Now, there are a number of advantages to representing this recipe in HTML, as follows:

  • It's fairly readable. The markup may be a little cryptic, but if it's laid out properly it's pretty easy to follow.

  • The HTML can be displayed by just about any HTML browser, even one without graphics capability. That's an important point: The display is browser-independent. If there were a photo of the results of making this recipe (and one certainly hopes there isn't), it would show up in a graphical browser but not in a text browser.

  • You could use a cascading style sheet (CSS -- we'll talk a bit about those below) for general control over formatting.

There's one major problem with HTML as a data format, however. The meaning of the various pieces of data in the document is lost. It's really hard to take general HTML and figure out what the data in the HTML mean. The fact that there's an <Ingredient> of this recipe with a <Qty> (quantity) of 500 ml (<Units>) of <Item> cottage cheese would be very hard to extract from this document in a way that's generally meaningful.

Now, the idea of data in an HTML document meaning something may be a bit hard to grasp. Web pages are fine for the human reader, but if a program is going to process a document, it requires unambiguous definitions of what the tags mean. For instance, the <TITLE> tag in an HTML document encloses the title of the document. That's what the tag means, and it doesn't mean anything else. Similarly, an HTML <TR> tag means "table row," but that's of little use if your program is trying to read recipes in order to, say, create a shopping list. How could a program find a list of ingredients from a Web page formatted in HTML?

Sure, you could write a program that grabs the headers out of the document, reads the table column headers, figures out the quantities and units of each ingredient, and so on. The problem is, everyone formats recipes differently. What if you're trying to get this information from, say, the Julia Childs Web site, and she keeps messing around with the formatting? If Julia changes the order of the columns or stops using tables, she'll break your program! (Though it has to be said: If Julia starts publishing recipes like this, she may want to think about changing careers.)

Now, imagine that this recipe page came from data in a database and you'd like to be able to ship this data around. Maybe you'd like to add it to your huge recipe database at home, where you can search and use it however you like. Unfortunately, your input is HTML, so you'll need a program that can read this HTML, figure out what all the "Ingredients," "Instructions," "Units," and so forth are, and then import them to your database. That's a lot of work. Especially since all of that semantic information -- again, the meaning of the data -- existed in that original database but were obscured in the process of being transformed into HTML.

Now, imagine you could invent your own custom language for describing recipes. Instead of describing how the recipe was to be displayed, you'd describe the information structure in the recipe: how each piece of information would relate to the other pieces.

XML example
Let's just make up a markup language for describing recipes, and rewrite our recipe in that language, as in Listing 3.


<?xml version="1.0"?>
<Recipe>
   <Name>Lime Jello Marshmallow Cottage Cheese Surprise</Name>
   <Description>
     My grandma's favorite (may she rest in peace).
   </Description>
   <Ingredients>
      <Ingredient>
         <Qty unit="box">1</Qty>
         <Item>lime gelatin</Item>
      </Ingredient>
      <Ingredient>
         <Qty unit="g">500</Qty>
         <Item>multicolored tiny marshmallows</Item>
      </Ingredient>
      <Ingredient>
         <Qty unit="ml">500</Qty>
         <Item>Cottage cheese</Item>
      </Ingredient>
      <Ingredient>
         <Qty unit="dash"/>
         <Item optional="1">Tabasco sauce</Item>
      </Ingredient>
   </Ingredients>
   <Instructions>
      <Step>
Prepare lime gelatin according to package instructions
      </Step>
      <!-- And so on... -->
   </Instructions>
</Recipe>

Listing 3. A custom markup language for recipes

It will come as little surprise to you, being the astute reader you are, that this recipe in its new format is actually an XML document. Maybe the fact that the file started with the odd header


<?xml version="1.0"?>

gave it away; in fact, every XML file should begin with this header. We've simply invented markup tags that have a particular meaning; for example, "An <Ingredient> is a <Qty> (quantity in specified units) of a single <Item>, which is possibly optional." Our XML document describes the information in the recipe in terms of recipes, instead of in terms of how to display the recipe (as in HTML). The semantics, or meaning of the information, is maintained in XML because that's what the tag set was designed to do.

Notes on notation
It's important to get some nomenclature straight. In Figure 1, you see a start tag, which begins an enclosed area of text, known as an Item, according to the tag name. As in HTML, XML tags may include a list of attributes (consisting of an attribute name and an attribute value.) The Item defined by the tag ends with the end tag.


Figure 1. An XML start tag and its corresponding end tag

Not every tag encloses text. In HTML, the <BR> tag means "line break" and contains no text. In XML, such elements aren't allowed. Instead, XML has empty tags, denoted by a slash before the final right-angle bracket in the tag. Figure 2 shows an empty tag from our XML recipe. Note that empty tags may have attributes. This empty tag example is standard XML shorthand for <Qty units="g"></Qty>.


Figure 2. An empty tag

In addition to these notational differences from HTML, the structural rules of XML are more strict. Every XML document must be well-formed. What does that mean? Read on!

Ooh-la-la! Well-formed XML
The concept of well-formedness comes from mathematics: It's possible to write mathematical expressions that don't mean anything. For example, the expression

2 ( + + 5 (=) 9 > 7

looks (sort of) like math, but it isn't math because it doesn't follow the notational and structural rules for a mathematical expression (not on this planet, at least). In other words, the "expression" above isn't well-formed. Mathematical expressions must be well-formed before you can do anything useful with them, because expressions that aren't well-formed are meaningless.

A well-formed XML document is simply one that follows all of the notational and structural rules for XML. Programs that intend to process XML should reject any input XML that doesn't follow the rules for being well-formed. The most important of these rules are as follows:

  • No unclosed tags
    You can get away with all kinds of wacko stuff in HTML. For example, in most HTML browsers, you can "open" a list item with <LI> and never "close" it with </LI>. The browser just figures out where the </LI> would be and automatically inserts it for you. XML doesn't allow this kind of sloppiness. Every start tag must have a corresponding end tag. This is because part of the information in an XML file has to do with how different elements of information relate to one another, and if the structure is ambiguous, so is the information. So, XML simply doesn't allow ambiguous structure. This nonambiguous structure also allows XML documents to be processed as data structures (trees), as I'll explain shortly in the discussion of the Document Object Model.

  • No overlapping tags
    A tag that opens inside another tag must close before the containing tag closes. For example, the sequence

    <Tomato>
    Let's call <Potato>the whole thing off</Tomato>
    </Potato>

    isn't well-formed because <Potato> opens inside of <Tomato> but doesn't close inside of <Tomato>. The correct sequence must be

    <Tomato>
    Let's call <Potato>the whole thing off</Potato>
    </Tomato>

    In other words, the structure of the document must be strictly hierarchical.

  • Attribute values must be enclosed in quotes
    Unlike HTML, XML doesn't allow "naked" attribute values (i.e., HTML tags like <TABLE BORDER=1>, where there are no quotes around the attribute value). Every attribute value must have quotes (<TABLE BORDER="1">).

  • The text characters (<), (>), and (") must always be represented by 'character entities'
    To represent these three characters (left-angle bracket, right-angle bracket, and double quotes) in the text part of the XML (not in the markup), you must use the special character entities (&lt;), (&gt;), and (&quot;), respectively. These characters are special characters for XML. An XML file using, say, the double quote character in the text enclosed in tags in an XML file isn't well-formed, and correctly designed XML parsers will produce an error for such input.

'Well-formed' means 'parsable'
A generic XML parser is a program or class that can read any well-formed XML at its input. Many vendors now offer XML parsers in Java for free; (you'll find links to these packages in Resources at the bottom of this article). XML parsers recognize well-formed documents and produce error messages (much like a compiler would) when they receive input that isn't well-formed. As we'll see, this functionality is very handy for the programmer: You simply call the parser you've selected and it takes care of the error detection and so on. While all XML parsers check the well-formedness of documents (meaning, as we've seen, that all the tags make sense, are nested properly, and so on), validating XML parsers go one step further. Validating parsers also confirm whether the document is valid; that is, that the structure and number of tags make sense.

For example, most browsers will display a document that (nonsensically) has two <TITLE> elements, but how can this be? Only one title or no title makes sense.

For another example, imagine that in Listing 3 the "cottage cheese" ingredient looked like this:


      <Ingredient>
         <Qty unit="ml">500</Qty>
         <Qty unit="g">9</Qty>
         <Item>Cottage cheese</Item>
      </Ingredient>

This XML document is certainly well-formed, but it doesn't make sense. It isn't structurally valid. It is nonsense for a <Qty> to contain a <Qty>. What's the <Qty> of this <Ingredient>?

The problem is, we have a document that's well-formed, but it isn't very useful because the XML doesn't make sense. We need a way to specify what makes an XML document valid. For example, how can we specify that a <Qty> tag may contain only text (and not any other elements) and report as errors any other case?

The answer to this question lies in something called the document type definition, which we'll look at next.


Next page >
Page 1 XML for the absolute beginner
Page 2 HTML: All form and no substance
Page 3 An XML conceptual example
Page 4 Make up a markup
Page 5 So, what good is made-up markup?
Page 6 Cascading Style Sheets: not just for HTML anymore
Page 7 XSL: I like your style
Page 8 Modeling information structure in XML
Page 9 XML and Java
Page 10 Become a tree surgeon!

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