Extending Yourself - XML

One of the coolest things about schemas is that they allow you to extend previously defined datatypes to create new sub-types, in much the same way as OOP programmers extend existing classes. These sub-types will automatically inherit all the characteristics of the base type, but may also be further customized to specific requirements.

In order to better understand this, let's modify the example on the previous page to derive a new datatype from the base "starWarsEntity" type.

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<!-- define 
a complex type -->
<xsd:complexType name="starWarsEntity">
<xsd:sequence>
<xsd:element 
name="name" type="xsd:string"/>
<xsd:element name="species" type="xsd:string"/>
<xsd:element 
name="language" type="xsd:string"/>
<xsd:element name="home" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
<!-- 
extend base to derive a new sub-type -->
<xsd:complexType name="Wookie">
<xsd:complexContent>
<xsd:extension 
base="starWarsEntity">
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>
<!-- 
define the root element and its contents -->
<xsd:element name="gallery">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
name="character"
type="starWarsEntity" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>

In order to extend an existing datatype, you need to use the <xsd:extension> element, which goes hand-in-hand with a "base" attribute - this "base" attribute contains the name of the parent type (which must, obviously, exist).

What use is this, you ask? Not much at the moment...but look how easy it becomes to add species-specific attributes to the derived datatype:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<!-- define 
a complex type -->
<xsd:complexType name="starWarsEntity">
<xsd:sequence>
<xsd:element 
name="name" type="xsd:string"/>
<xsd:element name="species" type="xsd:string"/>
<xsd:element 
name="language" type="xsd:string"/>
<xsd:element name="home" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
<!-- 
extend base to derive a new sub-type -->
<xsd:complexType name="Wookie">
<xsd:complexContent>
<xsd:extension 
base="starWarsEntity">
<xsd:sequence>
<xsd:element name="battlecry"
type="xsd:string"/>
</xsd:sequence>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>
<!-- 
define the root element and its contents -->
<xsd:element name="gallery">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
name="character"
type="starWarsEntity" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>

So Wookies now have an additional characteristic - "battlecry".

Since it's so easy, let's derive a couple more:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<!-- define 
a complex type -->
<xsd:complexType name="starWarsEntity">
<xsd:sequence>
<xsd:element 
name="name" type="xsd:string"/>
<xsd:element name="species" type="xsd:string"/>
<xsd:element 
name="language" type="xsd:string"/>
<xsd:element name="home" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
<!-- 
extend base to derive a new sub-type -->
<xsd:complexType name="Wookie">
<xsd:complexContent>
<xsd:extension 
base="starWarsEntity">
<xsd:sequence>
<xsd:element name="battlecry"
type="xsd:string"/>
</xsd:sequence>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>
<xsd:complexType 
name="Human">
<xsd:complexContent>
<xsd:extension base="starWarsEntity">
<xsd:sequence>
<xsd:element 
name="gender"
type="xsd:string"/>
</xsd:sequence>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>
<xsd:complexType 
name="Ewok">
<xsd:complexContent>
<xsd:extension base="starWarsEntity">
<xsd:sequence>
<xsd:element 
name="vehicle"
type="xsd:string"/>
<xsd:element name="society"
type="xsd:string"/>
</xsd:sequence>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>
<!-- 
define the root element and its contents -->
<xsd:element name="gallery">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
name="character"
type="starWarsEntity" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>

So Humans now include the additional characteristic "gender", while Ewoks include the characteristics "vehicle" and "society". Each of these derived datatypes thus carries its own special modifications, while simultaneously including all the characteristics of the base "starWarsEntity" datatype.

Now, how do I make my XML document use these derived datatypes? Pretty simple - just add the "type" attribute to each "character" element, so that the XML validator knows to look at the derived datatype rather than the base type. Here's how:

<?xml version="1.0" encoding="UTF-8"?>
<gallery xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="starwars.xsd">
<character 
xsi:type="Human">
<name>Luke Skywalker</name>
<species>Human</species>
<language>Basic</language>
<home>Tatooine</home>
<gender>Male</gender>
</character>
<character 
xsi:type="Wookie">
<name>Chewbacca</name>
<species>Wookie</species>
<language>Shyriiwook</language>
<home>Kashyyyk</home>
<battlecry>AARGH!</battlecry>
</character>
<character 
xsi:type="Ewok">
<name>Chief Chirpa</name>
<species>Ewok</species>
<language>Ewok</language>
<home>Endor</home>
<vehicle>Glider</vehicle>
<society>Tribal</society>
</character>
</gallery>

Simple, huh?

>>> More XML Articles          >>> More By Harish Kamath, (c) Melonfire

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