Weisfeld. The object-oriented thought process

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Chapter 11 Objects and Portable Data: XML

Figure 11.3 Creating the DTD in Notepad.

To enter this information into an XML document, we can use a text editor, just as we

used for the DTD. However, as we will see later, there are tools that have been created

specifically for this purpose. Figure 11.4 shows the XML document written using

Notepad.This document is called beta.xml.

Table 11.2 Adding the Values to the Table

Object Category Field Value

supplier name <companyname>

The Beta Company

address

<street> 12000 Ontario St

<city> Cleveland

<state> OH

<zip> 24388

product

type Vacuum Cleaner

price 50.00

count 20

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Integrating the DTD into the XML Document

Note that the second line ties this document to the supplier DTD that we defined earlier.

<!DOCTYPE supplier SYSTEM “supplier.dtd”>

Looking at Figure 11.4, we can see that the tag structure mimics the specification. It is

important to realize that the tags are nested and that only the end tags contain any data.

Some of the tags are basically high-level tags. In some ways it is similar to the concept of

abstract classes.You can think of the <address> tag as being “abstract” because we don’t

really define it. However, the <street> tag can be considered “concrete” given that we

actually assign a value to it. In other words, the <street> tag does contain information,

whereas the address tag does not:

<street>12000 Ontario St</street>

There is a better way to inspect the XML document.As stated previously, there are

many tools that have been written to assist in the development of XML documents. One

of these early tools is called XML Notepad, and it has a similar look and feel to Notepad,

provided in the Microsoft operating systems.

Figure 11.4 The Beta Company XML document with

the DTD.

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Chapter 11 Objects and Portable Data: XML

XML Notepad

Microsoft does not provide XML Notepad at this time. You can still find XML Notepad by do-

ing a simple Internet search for “XML Notepad.” You can download Microsoft’s XML Validator

at http://www.microsoft.com/downloads/details.aspx?FamilyID=d23c1d2c-1571-4d61-

bda8-adf9f6849df9&displaylang=en.

XML Notepad can help us understand the structure of an XML document.After you in-

stall XML Notepad, you can open the beta.xml file. Figure 11.5 shows what happens

when you open the beta.xml file with XML Notepad.When the document opens, ex-

pand all of the plus signs to look at all the elements.

XML Notepad lists each level of the document, starting with the supplier tag. Note that as

we have said before, only the end elements contain any information.

The obvious advantage to developing the DTD is that it can be used for more than

one document —in this case, for more than one supplier. Let’s say we have a company

that makes skates called Gamma Company, which wants to supply Alpha Company.What

Gamma Company needs to do is create an XML document that conforms to the supplier

DTD. Opening up this document with XML Notepad presents the picture seen in

Figure 11.6.

Figure 11.5 Opening the beta.xml file with XML

Notepad.

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Integrating the DTD into the XML Document

Note that beta.xml and gamma.xml conform to the supplier DTD.The question is, what

happens when the XML document does not conform to the DTD? It is at this point

where we see the power of the DTD. Let’s purposely create an error in the gamma.xml

file by taking out all the information pertaining to the name.

<name>

<companyname>The Gamma Company</companyname>

</name>

Basically, we are creating an invalid document—invalid per the supplier DTD.The in-

valid document is found in Figure 11.7. Be aware that Notepad will not indicate that the

document is invalid because Notepad does not check for validity.You need to use an

XML validator to check for validity.

We now have an invalid document based on the supplier DTD. How do we verify that

it is invalid? We can open the invalid gamma.xml document with XML Notepad. Notice

the result, as indicated in Figure 11.8. Here XML Notepad provides a dialog box indicat-

ing that an invalid document was detected.

Because the supplier DTD was expecting a document to conform to its definition, an

error was generated. In fact, the error message is quite specific as to what the problem is.

The DTD was expecting the name information.Thus, to create a proper XML document

for this system, all the appropriate information must be supplied and supplied in the

proper format.The <address> tag must be provided for the document to be valid.

Figure 11.6 Opening the gamma.xml file with XML

Notepad.

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Chapter 11 Objects and Portable Data: XML

Figure 11.7 An invalid document (no name information).

Figure 11.8 The invalid document error in XML

Notepad.

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Integrating the DTD into the XML Document

One of the primary points to recognize here is that this error checking would not have

happened in HTML. In fact, you can open up the XML file with a browser, as seen in

Figure 11.9.

Now let’s see what happens when we open up the invalid

gamma.xml document with a

browser. Figure 11.10 shows the gamma.xml file when it’s opened in Internet Explorer.

Note that even though the document is invalid, the browser opens it and even displays

it.This is because the browser is not checking to make sure the document conforms to

the DTD, whereas XML Notepad does perform this check. In theory, this is one of the

major advantages that XML provides when working with data.Although HTML is used

to display the data, XML is used to format the data.This is a very important distinction.

You might ask, what benefit does a tool like XML Notepad provide in the overall sup-

plier example, and what is it used for? To answer the first part of the question, XML

Notepad, or some editor like it, allows us to verify that the document is valid early on in

the process.To answer the second part of the question, XML Notepad or a similar editor

can be used to actually construct the document.

Figure 11.9 The beta.xml document opened in

Internet Explorer.

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Chapter 11 Objects and Portable Data: XML

Figure 11.10 The invalid gamma.xml document

opened in Internet Explorer.

Using Cascading Style Sheets

From a technical perspective, the concept of portable data often focuses on the movement

of data between two points. However, getting the data from point A to point B provides

no real value unless the data is presented in an appropriate way.Thus, we must consider

how the data transported in an XML system is presented to a user.

Remember that although XML is primarily used to define data, HTML is basically a

presentation mechanism. However, XML and HTML can be used in tandem to present

data via a browser.

Although XML is not generally used for presentation purposes, there are ways to for-

mat XML. One of these is to use cascading style sheets (CSS). CSS are used heavily in the

HTML world to format content.To a certain degree, CSS can be used to format XML.

Recall that the supplier XML document contains definitions for <companyname>,

<street>, <city>, <state>, and <zip>. Suppose that we want to format each of these

definitions, as seen in Table 11.3, to provide a specification that formats the elements of

the XML document.

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Using Cascading Style Sheets

We can represent this in a CSS with the following style sheet.

companyname{font-family:Arial, sans-serif;

font-size:24;

color:blue;

display:block;}

street {font-family:”Times New Roman”, serif;

font-size:12;

color:red;

display:block;}

city {font-family:”Courier New”, serif;

font-size:18;

color:black;

display:block;}

state {font-family:”Tahoma”; serif;

font-size:16;

color:gray;

display:block;}

zip {font-family:”Arial Black”, sans-serif;

font-size:6;

color:green;

display:block;}

This style sheet is implemented by adding a line of code in our XML document:

<?xml-stylesheet href=”supplier.css” type=”text/css” ?>

For example, in the case of the ZIP Code, the simple text displayed earlier is now for-

matted with a font of Arial Black, the color green, and a font size of 6.The attribute

display:block in this case will bring each attribute to a new line.

This code is inserted in the following manner:

<?xml version=”1.0” standalone=”no”?>

<?xml-stylesheet href=”supplier.css” type=”text/css” ?>

<!DOCTYPE supplier SYSTEM “supplier.dtd”>

<!— The XML data —>

Table 11.3 Cascading Style Sheet Specification

Tag Font Family Size Color Display

<companyname> Arial, sans serif 24 Blue Block

<street> Times New Roman, serif 12 Red Block

<city> Courier New, serif 18 Black Block

<state> Tahoma; serif 16 Gray Block

<zip> Arial Black, sans serif 6 Green Block

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Chapter 11 Objects and Portable Data: XML

<name>

<companyname>The Beta Company</companyname>

</name>

<street>12000 Ontario St</street>

<city>Cleveland</city>

</address>

</supplier>

With the CSS in the XML document, we can now open the document with a

browser. Figure 11.11 illustrates how this looks.

Take a look at Figure 11.10 again to see how this document was presented without the

CSS.

Figure 11.11 The XML document using a cascading

style sheet.

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References

Conclusion

In this chapter, we discussed many aspects of XML and why it is a very important tech-

nology within the IT community. It is rare when many major players in the IT market

buy into the same standard, but this has happened in the case of XML.

From the object-oriented perspective, you should come away from this chapter with

the understanding that object-oriented development goes far beyond OO languages and

encompasses the data as well. Because data is the fundamental part of information sys-

tems, it is important to design object-oriented systems that focus on the data. In today’s

business environment, moving data from one point to another is of paramount impor-

tance.

There are many levels of investigation you can visit when it comes to XML.This book

is about concepts, and by the end of this chapter, you should have a good general idea of

what XML is used for, as well as some of the tools that are used. Another level that was

mentioned briefly in this chapter was that of the style-sheets. By using cascading style

sheets and other technologies, you can better format your XML documents.

References

Hughes, Cheryl. The Web Wizard’s Guide to XML. Addison-Wesley, 2003. Boston, MA.

Watt,Andrew H. Sams Teach Yourself XML in 10 Minutes. Sams Publishing, 2003.

Indianapolis, IN.

McKinnon,Al & Linda. XML:Web Warrior Series. Course Technology, 2003. Boston, MA.

Holtzer, Steven. Real World XML. New Riders, 2003. Indianapolis, IN.

Deitel, et al. XML How to Program. Prentice Hall, 2001. Upper Saddle River, New Jersey.

WC3: http://www.w3.org/XML/

W3Schools: http://www.W3Schools.com/