Introduction to XML

1. Introduction to XML XML – Extensible Markup Language

2. Outline • XML Overview • XML Components • DTD – Document type definition

3. XML Overview • Resources • WWW consortium (W3C) Home page on XML • http://www.w3.org/XML • XML 1.0 Spec • http://www.w3.org/TR/REC-xml/ • O’Reilly publishing XML resources • http://www.xml.com

4. XML Overview • What is the connection between XML and Android? • Manifest File • Register activities • Required permissions • Other aspects of the application • Developing Activities • Declarative • XML – Google’s preferred method • Code is more concise • Less likely to change • Visual Editor (generates XML) – editor still unwieldy • Procedural – within Java Code

5. XML Overview • XML represents the contents of data – not presentation • To incorporate presentation, style sheets can be used

6. XML Components • XML elements and tags <Car> Chevrolet Corvette </Car> • In this example: • One element: Car • Car element starts and ends with tags • XML is case–sensitive (unlike HTML)

7. XML Components • Parents, children, and siblings <Car> <Identification> <Make>Chevrolet</Make> <Model>Corvette</Model> <VIN>123</VIN> </Identification> <Engine> <Displacement>6.2</Displacement> <Cylinders>8</Cylinders> </Engine> </Car>

8. XML Components • Root element • Only element in document with no parent • Each document can have only 1 root element • Invalid document: <car>…</car> <boat>…</boat>

9. XML Components • Attributes • Name-value pair(s) assigned to tag • Identification element has 2 attributes and 1 child <Car> <Identification Make=“Chevrolet” Model=“Corvette”> <VIN>123</VIN> </Identification> </Car>

10. XML Components • When to use attributes and when to use children? • Attributes are for metadata of element • Children are for information about element • Subject of heated debate • No clear way to tell what is metadata and what is information • Android – both used in specific instances

11. XML Components • Comments • Same as in HTML <!-- This is a comment --> • Comments CAN NOT appear: • Inside a tag • Inside another comment

12. XML Components • XML Declaration (the following is used in Android): <?XML version=“1.0” encoding=“UTF-8”?> • Version is the version of XML being used • 1.0 is current version • Encoding is the type of text • UTF-8 is default • Variable width: 1 – 4 bytes • Superset of ASCII • Others • ASCII, UTF-16, others

13. XML Components • Checking for well-formedness • Start tags must have matching end tags • Elements may nest, but not overlap • There must be exactly one root element • Attribute values must be quoted • An element cannot have two same-named attributes • Comments cannot be inside tags

14. DTD – Document type definitions • Written in formal syntax • Allow for describing: • What elements may appear • What elements’ contents and attributes are • Usually stored in separate file • .dtd extension is typical • Information can be contained in associated .xml file

15. DTD – Document type definitions • Example <!ELEMENT Car (Identification, Engine)> <!ELEMENT Identification (Make, Model, VIN)> <!ELEMENT Make (#PCDATA)> <!ELEMENT Model (#PCDATA)> <!ELEMENT VIN (#PCDATA)> <!ELEMENT Engine (Displacement, Cylinders)> <!ELEMENT Displacement (#PCDATA)> <!ELEMENT Cylinders (#PCDATA)>

16. DTD – Element specifications

17. DTD – Specifications example • Example <!ELEMENT Student (Identification, GradYear)> <!ELEMENT Identification (Name, Town?, Job*)> <!ELEMENT Name (#PCDATA)> <!ELEMENT Town (#PCDATA)> <!ELEMENT Job (#PCDATA)>

18. Full example • On public drive • Example • Car.xml with embedded DTD • Car.xml with separate DTD file