Intro to XML. Borrowed, with author s permission, from:

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1 Intro to XML Borrowed, with author s permission, from: ndroidintroduction/is389androidbasics.aspx

2 Part 1: XML Basics

3 Why XML Here? You need to understand the basics of XML to do much with Android All of the layout and configuration files are XML files

4 Overview XML is a very simple language The entire specification is only about 35 printed pages XML IS CASE SENSITIVE

5 The XML Document At the heart of XML is the XML document An XML document is a logical entity rather than a physical one One logical document can be stored in different physical files locally and on the Web and assembled somehow A well-formed subset of an XML document is called a document fragment An XML document contains markup (tags) and data Looks like HTML without predefined tags Elements contain 0, 1 or many attributes

6 Elements (Introduction) XML tags resemble HTML tags but they are not predefined That is, you give names to tags just as you give names to variables and other identifiers Starting tags appear as <tag> Ending tags appear as </tag> Empty elements (elements without data) can appear as <tag/> XML elements MUST have an ending tag A starting and ending tag (along with any text) makes up an element

7 Elements (Nesting) Elements can be nested These are hierarchical nodes Elements MUST be nested correctly though A child element must completely reside in its parent element There must be exactly one and only one root element

8 Elements (Nesting Example) <?xml version="1.0" encoding="utf-8"?>  <universe> Root element is <answer> <universe> 42 <!-- It's > </answer> </universe> Nested element is <answer>

9 XML Attributes (Introduction) Elements can have one or more attributes Same rules as HTML Attributes appear after the starting tag name One element cannot have two attributes of the same name One element can have many attributes though

10 XML Attributes (Syntax 1) Attributes appear as key=value pairs An attribute must have both a key and a value Attribute keys do not appear in quotation marks Attribute values must appear in quotation marks Remember, single and double quotes are interchangeable A space separates each key=value pair Attributes can appear in starting and empty element tags but cannot appear in an ending tag For a given element, each attribute key must differ

11 XML Attributes (Example) In the following example, id and size are attributes of the <universe> element <?xml version="1.0" encoding="utf-8"?> <universe id="1" size="infinite"> <answer>42</answer> </universe>

12 Comparing Attributes and Elements The following 2 documents are (roughly) equivalent <?xml version="1.0" encoding="utf-8"?> <student id="12345"> <name>bill</name> </student> <?xml version="1.0" encoding="utf-8"?> <student> <id>12345</id> <name>bill</name> </student>

13 PART 2 XML Namespaces

14 Namespace Caveats XML namespaces are the source of criticism and confusion XML namespaces are the source of many myths Java namespaces and XML namespaces are not the same thing In fact, they have nothing to do with each other

15 The Problem of Name Ambiguity Names can be ambiguous When we say Las Vegas do we mean Las Vegas, Nevada or Las Vegas, New Mexico How many cities have the name Springfield? When we say address, do we mean street address or Internet address? Name ambiguity occurs when two XML documents have elements of the same name but with different meanings (context)

16 The Problem of Name Ambiguity (Example) <?xml version="1.0" encoding="utf-8"?> <office> <address>123 Oak street</address> <address> </address> </office> The meaning of <address> differs between the two contexts

17 The Purpose of XML Namespaces (1) They eliminate name ambiguity in element names between documents Namespaces answer the question Are we talking about the same thing? XML namespaces were released about 1 year after XML itself was released

18 The Purpose of Namespaces (2) According to the W3C namespace specification: We envision applications of Extensible Markup Language (XML) where a single XML document may contain elements and attributes (here referred to as a "markup vocabulary") that are defined for and used by multiple software modules.

19 Implementation of XML Namespaces XML namespaces use URIs (IRIs) to qualify element names This allows us to use the same element name with a different URI thereby qualifying the element name The URI is just a globally unique name Formally, XML namespaces define a vocabulary or universe of names

20 The Implementation of Namespaces (1) Namespace names are defined through special attributes Any attribute starting with the prefix xmlns: is considered a prefix defining attribute The prefix following the attribute is the local abbreviation for the namespace The prefix name has no relevance although there are common naming conventions The attribute value is a unique URI

XML Namespace Example (Android) The prefix android is given to http://schemas.

21 XML Namespace Example (Android) The prefix android is given to The prefix tools is given to

22 Namespaces (Quotable Quotes) What Does a Namespace URL Locate? There is nothing at all at the end of a namespace URI, except perhaps a 404 Not Found error

23 Namespaces (Myths) IT S NOT NECESSARILY A WEB SITE IT S NOT A REFERENCE TO A PROGRAM ON THE WEB IT S NOT A POINTER TO A RESOURCE IT S JUST A NAME

24 Applications using XML namespaces XML namespaces are widely used in The Schema Definition Language (XSDL) The Extensible Stylesheet Language (XSL) The namespace can represent a vocabulary

25 Characteristics of Namespace URIs (Example) The following URIs ARE different: And so are the following:

26 Part 3: XML Parsing

27 Parsing: Natural Language Sentences aren t just sequences of words: we need to know how words group together to form meaningful units. E.g., in The book for this course costs a fortune, we need to represent the following facts: The book for this course serves to identify a single book (as would my book, Moby Dick, etc.) The verb costs has to agree with the noun book in number; in other languages, they would have to agree in gender or other properties.

28 Parsing: Natural Language S NP VP Det NP N P PP NP V Det NP N Det N The book for this course costs a fortune

29 Parsing: XML LinearLayout TextView LinearLayout Button Button

30 Parsing: Programming languages Natural languages give us some cues (intonation, punctuation) for parsing, but in general it is a very difficult computational problem (O(N 3 )), and an active research area. Natural language words also don t come with explicit tags (N, V, P): Part-Of-Speech (POS) tagging algorithms (Brill 1995) are the typical solution Programming languages (including XML) are designed to make parsing easier, but we want to use an existing package, like javax.xml.parsers.

31 XML Parsing: DOM vs. SAX Document Object Model:Build up tree representation of an entire document at once; then traverse the tree to find relevant tags. (This is the one I use!)

32 XML Parsing: DOM vs. SAX Simple API for XML: Doesn t build a big tree; instead, allows you to write callback methods (like listeners) that get called automatically when you traverse the document.

33 Addendum: JSON

34 Javascript Object Notation XML requires a lot of syntactic noise : makes it difficult to see the contents (data) from the notation (metadata tags). Javascript Object Notation (JSON) replaces tag words with curly braces

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