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XML Interview Questions Answers

Question 1 : Aren't XML, SGML, and HTML all the same thing?

Answer 1 : Not quite; SGML is the mother tongue, and has been used for describing thousands of different document types in many fields of human activity, from transcriptions of ancient Irish manuscripts to the technical documentation for stealth bombers, and from patients' clinical records to musical notation. SGML is very large and complex, however, and probably overkill for most common office desktop applications. XML is an abbreviated version of SGML, to make it easier to use over the Web, easier for you to define your own document types, and easier for programmers to write programs to handle them. It omits all the complex and less-used options of SGML in return for the benefits of being easier to write applications for, easier to understand, and more suited to delivery and interoperability over the Web. But it is still SGML, and XML files may still be processed in the same way as any other SGML file (see the question on XML software). HTML is just one of many SGML or XML applications—the one most frequently used on the Web. Technical readers may find it more useful to think of XML as being SGML-- rather than HTML++.

Question 2 : Do I have to know HTML or SGML before I learn XML?

Answer 2 : No, although it's useful because a lot of XML terminology and practice derives from two decades' experience of SGML. Be aware that ‘knowing HTML’ is not the same as ‘understanding SGML’. Although HTML was written as an SGML application, browsers ignore most of it (which is why so many useful things don't work), so just because something is done a certain way in HTML browsers does not mean it's correct, least of all in XML.

Question 3 : Does XML replace HTML?

Answer 3 : No. XML itself does not replace HTML. Instead, it provides an alternative which allows you to define your own set of markup elements. HTML is expected to remain in common use for some time to come, and the current version of HTML is in XML syntax. XML is designed to make the writing of DTDs much simpler than with full SGML. (See the question on DTDs for what one is and why you might want one.)

Question 4 : Give a few examples of types of applications that can benefit from using XML.

Answer 4 : There are literally thousands of applications that can benefit from XML technologies. The point of this question is not to have the candidate rattle off a laundry list of projects that they have worked on, but, rather, to allow the candidate to explain the rationale for choosing XML by citing a few real world examples. For instance, one appropriate answer is that XML allows content management systems to store documents independently of their format, which thereby reduces data redundancy. Another answer relates to B2B exchanges or supply chain management systems. In these instances, XML provides a mechanism for multiple companies to exchange data according to an agreed upon set of rules. A third common response involves wireless applications that require WML to render data on hand held devices.

Question 5 : Give some examples of XML DTDs or schemas that you have worked with.

Answer 5 : Although XML does not require data to be validated against a DTD, many of the benefits of using the technology are derived from being able to validate XML documents against business or technical architecture rules. Polling for the list of DTDs that developers have worked with provides insight to their general exposure to the technology. The ideal candidate will have knowledge of several of the commonly used DTDs such as FpML, DocBook, HRML, and RDF, as well as experience designing a custom DTD for a particular project where no standard existed.

Question 6 : How can I make my existing HTML files work in XML?

Answer 6 : Either convert them to conform to some new document type (with or without a DTD or Schema) and write a stylesheet to go with them; or edit them to conform to XHTML. It is necessary to convert existing HTML files because XML does not permit end-tag minimisation (missing , etc), unquoted attribute values, and a number of other SGML shortcuts which have been normal in most HTML DTDs. However, many HTML authoring tools already produce almost (but not quite) well-formed XML. You may be able to convert HTML to XHTML using the Dave Raggett's HTML Tidy program, which can clean up some of the formatting mess left behind by inadequate HTML editors, and even separate out some of the formatting to a stylesheet, but there is usually still some hand-editing to do.

Question 7 : How does XML handle white-space  in my documents?

Answer 7 : All white-space, including linebreaks, TAB characters, and normal spaces, even between ‘structural’ elements where no text can ever appear, is passed by the parser unchanged to the application (browser, formatter, viewer, converter, etc), identifying the context in which the white-space was found (element content, data content, or mixed content, if this information is available to the parser, eg from a DTD or Schema). This means it is the application's responsibility to decide what to do with such space, not the parser's: * insignificant white-space between structural elements (space which occurs where only element content is allowed, ie between other elements, where text data never occurs) will get passed to the application (in SGML this white-space gets suppressed, which is why you can put all that extra space in HTML documents and not worry about it) * significant white-space (space which occurs within elements which can contain text and markup mixed together, usually mixed content or PCDATA) will still get passed to the application exactly as under SGML. It is the application's responsibility to handle it correctly. The parser must inform the application that white-space has occurred in element content, if it can detect it. (Users of SGML will recognize that this information is not in the ESIS, but it is in the Grove.) <chapter> <title> My title for Chapter 1. </title> <para> text </para> </chapter>

Question 8 : How would you build a search engine for large volumes of XML data?

Answer 8 : answer is to build a full-text search and handle the data similarly to the way Internet portals handle HTML pages. Others consider XML as a standard way of transferring structured data between disparate systems. These candidates often describe some scheme of importing XML into a relational or object database and relying on the database's engine for searching. Lastly, candidates that have worked with vendors specializing in this area often say that the best way the handle this situation is to use a third party software package optimized for XML data.

Question 9 : If XML is just a subset of SGML, can I use XML files directly with existing SGML tools?

Answer 9 : Yes, provided you use up-to-date SGML software which knows about the WebSGML Adaptations TC to ISO 8879 (the features needed to support XML, such as the variant form for EMPTY elements; some aspects of the SGML Declaration such as NAMECASE GENERAL NO; multiple attribute token list declarations, etc). An alternative is to use an SGML DTD to let you create a fully-normalised SGML file, but one which does not use empty elements; and then remove the DocType Declaration so it becomes a well-formed DTDless XML file. Most SGML tools now handle XML files well, and provide an option switch between the two standards.

Question 10 : Is there an XML version of HTML?

Answer 10 : Yes, the W3C recommends using XHTML which is ‘a reformulation of HTML 4 in XML 1.0’. This specification defines HTML as an XML application, and provides three DTDs corresponding to the ones defined by HTML 4.* (Strict, Transitional, and Frameset). The semantics of the elements and their attributes are as defined in the W3C Recommendation for HTML 4. These semantics provide the foundation for future extensibility of XHTML. Compatibility with existing HTML browsers is possible by following a small set of guidelines (see the W3C site).

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