Top 10 XML Interview Questions and Answers for Beginners

XML Interview questions are very popular in various programming job interviews, including Java interviews for web developers. XML is a mature technology and is often used as a standard for transporting data from one platform to another. XML Interview questions contain questions from various XML technologies like XSLT which is used to transform XML files, XPATH, XQuery, and fundamentals of XML e.g. DTD or Schema. In this article, we will see the top  10 frequently asked XML Interview questions and answers from the above topics. These questions are mostly asked in various Java interviews but they are equally useful in other programming interviews like C, C++, Scala, or any other programming language.

Since XML is not tied with any programming language and like SQL its one of the desired skills in the programmer, it makes sense to practice some XML questions before appearing in any technical job interview

XML Interview Questions and Answers

Here is my list of some common and frequently asked Interview questions on XML technologies.  Questions on this the list is not very tough but touches some important areas of XML technologies e.g. DTD, XML Schema, XSLT transformations, XPATH evaluation, XML binding, XML parsers and fundamentals of XML e.g. namespace, validation, attribute, elements etc.

Question 1: What is XML?

Answer: XML stands for Extensible Markup language which means you can extend XML based upon your needs. You can define custom tags like <books>, <orders>, etc in XML easily as opposed to other markup languages like HTML where you need to work with predefined tags e.g. <p> and you can not use user defined tag. 

Though structure of XML can be standardize by making use of DTD and XML Schema. XML is mostly used to transfer data from one system to another e.g. between client and server in enterprise applications.

Question 2: Difference between DTD and XML Schema?

Answer : There are couple of differences between DTD and XML Schema e.g. DTD is not written using XML while XML schema are xml documents in itself, which means existing XML tools like XML parsers can be used to work with XML schema. 

Also XML schema is designed after DTD and it offer more types to map different types of data in XML documents. On the other hand DTD stands for Document Type definition and was a legacy way to define structure of XML documents.

Question 3: What is XPath ?

Answer : XPath is an XML technology which is used to retrieve element from XML documents. Since XML documents are structured, XPath expression can be used to locate and retrieve elements, attributes or value from XML files. XPath is similar to SQL in terms of retrieving data from XML but it has it's own syntax and rules. See here to know more about How to use XPath to retrieve data from XML documents.

Question 4: What is XSLT?

Answer: XSLT is another popular XML technology to transform one XML file to other XML, HTML or any other format. XSLT is like a language which specifies its own syntax, functions and operator to transform XML documents. Usually transformation is done by XSLT Engine which reads instruction written using XSLT syntax in XML style sheets or XSL files. 

XSLT also makes extensive use of recursion to perform the transformation. One of the popular example of using XSLT is for displaying data present in XML files as HTML pages. XSLT is also very handy to transforming one XML file into another XML document.

Question 5: What is element and attribute in XML?

Answer : This can be best explained by an example. let's see a simple XML snippet

<Orders>
  <Order id="123">
     <Symbol> 6758.T</Symbol>
     <Price> 2300</Price>
  <Order>
<Orders>

In this sample XML id is an attribute of <Order> element. Here <Symbol>, <Price> and <Orders> are also other elements but they don't have any attribute.

Question 6: What is the meaning of well formed XML ?

Answer: Another interesting XML interview question that most appeared in telephonic interviews. A well formed XML means an XML document which is syntactically correct e.g. it has a root element, all open tags are closed properly, attributes are in quotes etc.  If an XML is not well formed, it may not be processed and parsed correctly by various XML parsers.

Question 7: What is XML namespace? Why it's important?

Answer : XML namespace are similar to package in Java and used to provide a way to avoid conflict between two xml tags of same name but different sources. XML namespace is defined using xmlns attribute at top of the XML document and has following syntax  xmlns:prefix="URI". later that prefix is used along with actual tag in XML documents. Here is an example of using XML namespace :

<root xmlns:inst="http://instruments.com/inst"
  <inst:phone>
      <inst:number>837363223</inst:number>
   </inst:phone>
</root>

Question 8: Difference between DOM and SAX parser?

Answer: This is another very popular XML interview question, not just in XML world but also on Java world. The main difference between DOM and SAX parsers is the way they parse XML documents. DOM creates an in-memory tree representation of XML documents during parsing while SAX is an event-driven parser. See the Difference between DOM and SAX parser for a more detailed answer to this question.

Question 9: What is a CDATA section in XML?

Answer : I like this XML Interview questions for its simplicity and importance, yet many programmer doesn't know much about it. CDATA stands for character data and has special instruction for XML parsers. Since XML parser parse all text in XML document e.g. <name>This is name of person</name>  here even though value of tag <name> will be parsed because it may contain XML tags e.g. <name><firstname>First Name</firstname></name>. CDATA section is not parsed by XML parser. CDATA section starts with "<![CDATA[" and finishes with "]]>".

Question 10: What is XML Data Binding in Java?

Answer: XML binding in Java refers to creating Java classes and object from XML documents and then modifying XML documents using Java programming language. JAXB, Java API for XML binding provides a convenient way to bind XML documents with Java objects. 

Another alternative for XML binding is using an open-source library e.g. XML Beans. One of the biggest advantages of XML binding in Java is to leverage Java programming capability to create and modify XML documents.

This list of XML Interview questions and answers are collected from programmers but useful to anyone who is working in XML technologies. The importance of XML technologies like XPath, XSLT, XQuery is only going to increase because of the platform-independent nature of XML and its popularity in transmitting data over cross platforms. 

Though XML has disadvantage like verbosity and size but its highly useful in web services and transmitting data from one system to another where bandwidth and speed is of secondary concern.