Introduction to XML XML – Extensible Markup Language

Outline XML Overview XML Components DTD – Document type definition

XML Overview Resources – WWW consortium (W3C) Home page on XML – XML 1.0 Spec – O’Reilly publishing XML resources

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

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

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

XML Components Parents, children, and siblings Chevrolet Corvette

XML Components Root element – Only element in document with no parent – Each document can have only 1 root element – Invalid document: …

XML Components Attributes – Name-value pair(s) assigned to tag – Identification element has 2 attributes and 1 child 123

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

XML Components Comments – Same as in HTML – Comments CAN NOT appear: Inside a tag Inside another comment

XML Components XML Declaration (the following is used in Android): 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

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

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

DTD – Document type definitions Example

DTD – Element specifications SymbolMeaning no symbolelement must appear exactly once ?optional but if used, must appear exactly once *element appears zero or more times +element must appear at least once |choice of elements – but only one can be used,sequence of elements that can be used (each element used according to its symbol)

DTD – Specifications example Example

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