XML Interview Questions

Describe the differences between XML and HTML.

It's amazing how many developers claim to be proficient programming with XML, yet do not understand the basic differences between XML and HTML. Anyone with a fundamental grasp of XML should be able describe some of the main differences outlined in the table below.

Differences Between XML and HTML

XML HTML User definable tags Defined set of tags designed for web display Content driven Format driven End tags required for well formed documents End tags not required Quotes required around attributes values Quotes not required Slash required in empty tags Slash not required

Describe the role that XSL can play when dynamically generating HTML pages from a relational database.

Even if candidates have never participated in a project involving this type of architecture, they should recognize it as one of the common uses of XML. Querying a database and then formatting the result set so that it can be validated as an XML document allows developers to translate the data into an HTML table using XSLT rules. Consequently, the format of the resulting HTML table can be modified without changing the database query or application code since the document rendering logic is isolated to the XSLT rules.

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

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.

What is DOM and how does it relate to XML?

The Document Object Model (DOM) is an interface specification maintained by the W3C DOM Workgroup that defines an application independent mechanism to access, parse, or update XML data. In simple terms it is a hierarchical model that allows developers to manipulate XML documents easily Any developer that has worked extensively with XML should be able to discuss the concept and use of DOM objects freely. Additionally, it is not unreasonable to expect advanced candidates to thoroughly understand its internal workings and be able to explain how DOM differs from an event-based interface like SAX.

What is SOAP and how does it relate to XML?

The Simple Object Access Protocol (SOAP) uses XML to define a protocol for the exchange of information in distributed computing environments. SOAP consists of three components: an envelope, a set of encoding rules, and a convention for representing remote procedure calls. Unless experience with SOAP is a direct requirement for the open position, knowing the specifics of the protocol, or how it can be used in conjunction with HTTP, is not as important as identifying it as a natural application of XML.

Can you walk us through the steps necessary to parse XML documents?

Superficially, this is a fairly basic question. However, the point is not to determine whether candidates understand the concept of a parser but rather have them walk through the process of parsing XML documents step-by-step. Determining whether a non-validating or validating parser is needed, choosing the appropriate parser, and handling errors are all important aspects to this process that should be included in the candidate's response.

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

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.

Using XSLT, how would you extract a specific attribute from an element in an XML document?

Successful candidates should recognize this as one of the most basic applications of XSLT. If they are not able to construct a reply similar to the example below, they should at least be able to identify the components necessary for this operation: xsl:template to match the appropriate XML element, xsl:value-of to select the attribute value, and the optional xsl:apply-templates to continue processing the document. Extract Attributes from XML Data Example 1. <xsl:template match="element-name">     Attribute Value:         <xsl:value-of select="@attribute"/>         <xsl:apply-templates/> </xsl:template>

When constructing an XML DTD, how do you create an external entity reference in an attribute value?

Every interview session should have at least one trick question. Although possible when using SGML, XML DTDs don't support defining external entity references in attribute values. It's more important for the candidate to respond to this question in a logical way than than the candidate know the somewhat obscure answer.

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

The way candidates answer this question may provide insight into their view of XML data. For those who view XML primarily as a way to denote structure for text files, a common 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. Obviously, some important areas of XML technologies were not included in this list -- namespaces, XPointer, XLink, and so on -- and should be added to the interviewer's set of questions if applicable to the particular position that the candidate is applying for. However, these questions in conjunction with others to assess soft skills (communication skills, ability to work on teams, leadership ability, etc.) will help determine how well candidates understand the fundamental principles of XML.

Why is XML such an important development?

It removes two constraints which were holding back Web developments:<br> 1. § dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;<br> 2. the complexity of full SGML, whose syntax allows many powerful but hard-to-program options.<br> § XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.

Is it possible to write the contents of org.w3c.dom.Document object into an .xml file?

Yes its possible. One to achieve this is by using Xerces. Xerces is an XML parser. You would use the following code org.apache.xml.serialize.OutputFormat format = new org.apache.xml.serialize.OutputFormat(myDocument); org.apache.xml.serialize.XMLSerializer output = new org.apache.xml.serialize.XMLSerializer(new FileOutputStream(new File("test.xml")), format); output.serialize(myDocument);

What is the difference between DOM and SAX? What would you use if an option is given?

DOM parses an XML document and returns an instance of org.w3c.dom.Document. This document object's tree must then be "walked" in order to process the different elements. DOM parses the ENTIRE Document into memory, and then makes it available to you. The size of the Document you can parse is limited to the memory available. SAX uses an event callback mechanism requiring you to code methods to handle events thrown by the parser as it encounters different entities within the XML document. SAX throws events as the Document is being parsed. Only the current element is actually in memory, so there is no limit to the size of a Document when using SAX. The specific parser technology that will be used will be determined by the requirements of your application. If you need the entire document represented, you will most likely use DOM builder implementation. If you only care about parts of the XML document and/or you only need to parse the document once, you might be better served using SAX implementation.

What is SOAP?

The Simple Object Access Protocol (SOAP) uses XML to define a protocol for the exchange of information in distributed computing environments. SOAP consists of three components: an envelope, a set of encoding rules, and a convention for representing remote procedure calls.

What is DOM?

The Document Object Model (DOM) is an interface specification maintained by the W3C DOM Workgroup that defines an application independent mechanism to access, parse, or update XML data. In simple terms it is a hierarchical model that allows developers to manipulate XML documents easily.

Can you walk us through the steps necessary to parse XML file?

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); factory.setValidating(true); DocumentBuilder domBuilder = factory.newDocumentBuilder(); Document doc = domBuilder.parse(XMLFile);

Is it necessary to validate XML file against a DTD?

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.

What is XPath?

XPath stands for XML Path Language XPath is a syntax for defining parts of an XML document XPath is used to navigate through elements and attributes in an XML document XPath contains a library of standard functions XPath is a major element in XSLT XPath is designed to be used by both XSLT and XPointer XPath is a W3C Standard

What is XSL?

XSLT - a language for transforming XML documents XSLT is used to transform an XML document into another XML document, or another type of document that is recognized by a browser, like HTML and XHTML. Normally XSLT does this by transforming each XML element into an (X)HTML element.

What is a DTD and a Schema?

The XML Document Type Declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition or DTD. The DTD can point to an external subset containing markup declarations, or can contain the markup declarations directly in an internal subset, or can even do both. A Schema is: XML Schemas express shared vocabularies and allow machines to carry out rules made by people. They provide a means for defining the structure, content and semantics of XML documents. Schemas are a richer and more powerful of describing information than what is possible with DTDs.