XML Interview Questions and Answers

What is the difference between XML and C or C++ or Java ?
C and C++ (and other languages like FORTRAN, or Pascal, or Visual Basic, or Java or hundreds more) are programming languages with which you specify calculations, actions, and decisions to be carried out in order:
mod curconfig[if left(date,6) = "01-Apr",
t.put "April googlel!",
f.put days('31102005','DDMMYYYY') -
days(sdate,'DDMMYYYY')
" more shopping days to Samhain"];
XML is a markup specification language with which you can design ways of describing information (text or data), usually for storage, transmission, or processing by a program. It says nothing about what you should do with the data (although your choice of element names may hint at what they are for):
<part num="DA42" models="LS AR DF HG KJ"
update="2001-11-22">
<name>Camshaft end bearing retention circlip</name>
<image drawing="RR98-dh37" type="SVG" x="476"
y="226"/> <maker id="RQ778">Ringtown Fasteners Ltd</maker>
<notes>Angle-nosed insertion tool <tool
id="GH25"/> is required for the removal
and replacement of this part.</notes>
</part>
On its own, an SGML or XML file (including HTML) doesn't do anything. It's a data format which just sits there until you run a program which does something with it.

Does XML replace HTML?
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.)

Do I have to know HTML or SGML before I learn XML?
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.

What does an XML document actually look like (inside)?
The basic structure of XML is similar to other applications of SGML, including HTML. The basic components can be seen in the following examples. An XML document starts with a Prolog:
1. The XML Declaration

which specifies that this is an XML document;
2. Optionally a Document Type Declaration

which identifies the type of document and says where the Document Type Description (DTD) is stored;
The Prolog is followed by the document instance:
1. A root element, which is the outermost (top level) element (start-tag plus end-tag) which encloses everything else: in the examples below the root elements are conversation and titlepage;
2. A structured mix of descriptive or prescriptive elements enclosing the character data content (text), and optionally any attributes (‘name=value’ pairs) inside some start-tags.
XML documents can be very simple, with straightforward nested markup of your own design:
<?xml version="1.0" standalone="yes"?>
<conversation><br>
<greeting>Hello, world!</greeting>
<response>Stop the planet, I want to get
off!</response>
</conversation>
Or they can be more complicated, with a Schema or question C.11, Document Type Description (DTD) or internal subset (local DTD changes in [square brackets]), and an arbitrarily complex nested structure:
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE titlepage
SYSTEM "http://www.google.bar/dtds/typo.dtd"
[<!ENTITY % active.links "INCLUDE">]>
<titlepage id="BG12273624">
<white-space type="vertical" amount="36"/>
<title font="Baskerville" alignment="centered"
size="24/30">Hello, world!</title>
<white-space type="vertical" amount="12"/>

<image location="http://www.google.bar/fleuron.eps"
type="URI" alignment="centered"/>
<white-space type="vertical" amount="24"/>
<author font="Baskerville" size="18/22"
style="italic">Vitam capias</author>
<white-space type="vertical" role="filler"/>
</titlepage>

Or they can be anywhere between: a lot will depend on how you want to define your document type (or whose you use) and what it will be used for. Database-generated or program-generated XML documents used in e-commerce is usually unformatted (not for human reading) and may use very long names or values, with multiple redundancy and sometimes no character data content at all, just values in attributes:
<?xml version="1.0"?> <ORDER-UPDATE AUTHMD5="4baf7d7cff5faa3ce67acf66ccda8248"
ORDER-UPDATE-ISSUE="193E22C2-EAF3-11D9-9736-CAFC705A30B3"
ORDER-UPDATE-DATE="2005-07-01T15:34:22.46" ORDER-UPDATE-DESTINATION="6B197E02-EAF3-11D9-85D5-997710D9978F"
ORDER-UPDATE-ORDERNO="8316ADEA-EAF3-11D9-9955-D289ECBC99F3">
<ORDER-UPDATE-DELTA-MODIFICATION-DETAIL ORDER-UPDATE-ID="BAC352437484">
<ORDER-UPDATE-DELTA-MODIFICATION-VALUE ORDER-UPDATE-ITEM="56"
ORDER-UPDATE-QUANTITY="2000"/>
</ORDER-UPDATE-DELTA-MODIFICATION-DETAIL>
</ORDER-UPDATE>

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