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Learn XML by Parimal Sudas, Assistant Professor, Dronacharya College of Engineering, Gurgaon

Learn XML

By : Parimal Sudas, Assistant Professor, Dronacharya College of Engineering, Gurgaon
Industry : Teaching/Education Functional Area : Open Source
Keywords :

Future

Learn

XML

Web Development

XML And HTML

Activity:  5 comments  622 views  last activity : 07 06 2010 20:18:04 +0000
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XML
 

What is XML?

  • XML stands for EXtensible Markup Language
  • XML is a markup language much like HTML
  • XML was designed to describe data
  • XML tags are not predefined. You must define your own tags
  • XML uses a Document Type Definition (DTD) or an XML Schema to describe the data
  • XML with a DTD or XML Schema is designed to be self-descriptive
  • XML is a W3C Recommendation

The Main Difference between XML and HTML

XML was designed to carry data.XML is not a replacement for HTML.
XML and HTML were designed with different goals:XML was designed to describe data and to focus on what data is.
HTML was designed to display data and to focus on how data looks.HTML is about displaying information, while XML is about describing information.

XML Does not DO Anything

XML was not designed to DO anything.Maybe it is a little hard to understand, but XML does not DO anything. XML was created to structure, store and to send information.The following example is a note to Tove from Jani, stored as XML: 
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
The note has a header and a message body. It also has sender and receiver information. But still, this XML document does not DO anything. It is just pure information wrapped in XML tags. Someone must write a piece of software to send, receive or display it.

XML is Free and Extensible

XML tags are not predefined. You must "invent" your own tags.The tags used to mark up HTML documents and the structure of HTML documents are predefined. The author of HTML documents can only use tags that are defined in the HTML standard (like <p>, <h1>, etc.).XML allows the author to define his own tags and his own document structure.The tags in the example above (like <to> and <from>) are not defined in any XML standard. These tags are "invented" by the author of the XML document.

XML is a Complement to HTML

XML is not a replacement for HTML.It is important to understand that XML is not a replacement for HTML. In future Web development it is most likely that XML will be used to describe the data, while HTML will be used to format and display the same data.My best description of XML is this: XML is a cross-platform, software and hardware independent tool for transmitting information.

XML in Future Web Development

XML is going to be everywhere.We have been participating in XML development since its creation. It has been amazing to see how quickly the XML standard has been developed and how quickly a large number of software vendors have adopted the standard.We strongly believe that XML will be as important to the future of the Web as HTML has been to the foundation of the Web and that XML will be the most common tool for all data manipulation and data transmission.

XML can Separate Data from HTML

With XML, your data is stored outside your HTML.When HTML is used to display data, the data is stored inside your HTML. With XML, data can be stored in separate XML files. This way you can concentrate on using HTML for data layout and display, and be sure that changes in the underlying data will not require any changes to your HTML.XML data can also be stored inside HTML pages as "Data Islands". You can still concentrate on using HTML only for formatting and displaying the data.

XML is Used to Exchange Data

With XML, data can be exchanged between incompatible systems.In the real world, computer systems and databases contain data in incompatible formats. One of the most time-consuming challenges for developers has been to exchange data between such systems over the Internet.Converting the data to XML can greatly reduce this complexity and create data that can be read by many different types of applications.

XML and B2B

With XML, financial information can be exchanged over the Internet.Expect to see a lot about XML and B2B (Business To Business) in the near future.XML is going to be the main language for exchanging financial information between businesses over the Internet. A lot of interesting B2B applications are under development.

XML Can be Used to Create New Languages

XML is the mother of WAP and WML.The Wireless Markup Language (WML), used to markup Internet applications for handheld devices like mobile phones, is written in XML. The syntax rules of XML are very simple and very strict. The rules are very easy to learn, and very easy to use.Because of this, creating software that can read and manipulate XML is very easy.

An Example XML Document

XML documents use a self-describing and simple syntax. 
<?xml version="1.0" encoding="ISO-8859-1"?>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
The first line in the document - the XML declaration - defines the XML version and the character encoding used in the document. In this case the document conforms to the 1.0 specification of XML and uses the ISO-8859-1 (Latin-1/West European) character set. The next line describes the root element of the document (like it was saying: "this document is a note"): 
<note>
The next 4 lines describe 4 child elements of the root (to, from, heading, and body): 
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
And finally the last line defines the end of the root element: 
</note>
Can you detect from this example that the XML document contains a Note to Tove from Jani? Don't you agree that XML is pretty self-descriptive?

All XML Elements Must Have a Closing Tag

With XML, it is illegal to omit the closing tag.In HTML some elements do not have to have a closing tag. The following code is legal in HTML: 
<p>This is a paragraph
<p>This is another paragraph
In XML all elements must have a closing tag, like this: 
<p>This is a paragraph</p>
<p>This is another paragraph</p> 
Note: You might have noticed from the previous example that the XML declaration did not have a closing tag. This is not an error. The declaration is not a part of the XML document itself. It is not an XML element, and it should not have a closing tag.

XML Tags are Case Sensitive

Unlike HTML, XML tags are case sensitive.With XML, the tag <Letter> is different from the tag <letter>.Opening and closing tags must therefore be written with the same case: 
<Message>This is incorrect</message>
 
<message>This is correct</message>
 

XML Elements Must be Properly Nested

Improper nesting of tags makes no sense to XML.In HTML some elements can be improperly nested within each other like this: 
<b><i>This text is bold and italic</b></i>
In XML all elements must be properly nested within each other like this: 
<b><i>This text is bold and italic</i></b>
 

XML Documents Must Have a Root Element

All XML documents must contain a single tag pair to define a root element.All other elements must be within this root element.All elements can have sub elements (child elements). Sub elements must be correctly nested within their parent element: 
<root>
  <child>
    <subchild>.....</subchild>
  </child>
</root>
 

XML Attribute Values Must be Quoted

With XML, it is illegal to omit quotation marks around attribute values. XML elements can have attributes in name/value pairs just like in HTML. In XML the attribute value must always be quoted. Study the two XML documents below. The first one is incorrect, the second is correct: 
<?xml version="1.0" encoding="ISO-8859-1"?>
<note date=12/11/2002>
<to>Tove</to>
<from>Jani</from>
</note>
  
<?xml version="1.0" encoding="ISO-8859-1"?>
<note date="12/11/2002">
<to>Tove</to>
<from>Jani</from>
</note>

The error in the first document is that the date attribute in the note element is not quoted. This is correct: date="12/11/2002". This is incorrect: date=12/11/2002.

With XML, White Space is Preserved

With XML, the white space in your document is not truncated.This is unlike HTML. With HTML, a sentence like this:Hello              my name is Tove,will be displayed like this:Hello my name is Tove,because HTML reduces multiple, consecutive white space characters to a single white space.

With XML, CR / LF is Converted to LF

With XML, a new line is always stored as LF.Do you know what a typewriter is? Well, a typewriter is a mechanical device which was used last century to produce printed documents. :-)After you have typed one line of text on a typewriter, you have to manually return the printing carriage to the left margin position and manually feed the paper up one line.In Windows applications, a new line is normally stored as a pair of characters: carriage return (CR) and line feed (LF). The character pair bears some resemblance to the typewriter actions of setting a new line. In Unix applications, a new line is normally stored as a LF character. Macintosh applications use only a CR character to store a new line.

Comments in XML

The syntax for writing comments in XML is similar to that of HTML.<!-- This is a comment -->

There is Nothing Special About XML

There is nothing special about XML. It is just plain text with the addition of some XML tags enclosed in angle brackets. Software that can handle plain text can also handle XML. In a simple text editor, the XML tags will be visible and will not be handled specially. In an XML-aware application however, the XML tags can be handled specially. The tags may or may not be visible, or have a functional meaning, depending on the nature of the application.

XML Elements are Extensible

XML documents can be extended to carry more information.Look at the following XML NOTE example: 
<note>
<to>Tove</to>
<from>Jani</from>
<body>Don't forget me this weekend!</body>
</note>
Let's imagine that we created an application that extracted the <to>, <from>, and <body> elements from the XML document to produce this output:
MESSAGE To: Tove
From: JaniDon't forget me this weekend!
Imagine that the author of the XML document added some extra information to it: 
<note>
<date>2002-08-01</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
Should the application break or crash?No. The application should still be able to find the <to>, <from>, and <body> elements in the XML document and produce the same output.XML documents are Extensible.
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5 comments on "Learn XML"
Sort by: Most Recent Top Rated
  Commented by  imanpreetsingh, Student, Kathuria Group of Information Tech.    | 01 29 2009 16:51:17 +0000 
its intrestin...............thnx 4 sharing
  Commented by  imanpreetsingh, Student, Kathuria Group of Information Tech.    | 01 29 2009 16:50:55 +0000 
intresting
  Commented by  Prashant Gajjar, QA Engineer, Cybage Software Pvt Ltd.    | 09 03 2008 02:28:19 +0000 
Good. keep sending this techi info.
  Commented by  Darpan Sinha, Solution Architect, Fujitsu Consulting India Pvt Ltd    | 09 02 2008 00:22:18 +0000 
Nice explanations
  Commented by  Samir Nigam, Sr. Software Engineer , SRM TECHSOL Pvt. Ltd.    | 07 27 2008 22:51:30 +0000 
good one.
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