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In SGML, HTML and XML documents, the logical constructs known as character data and attribute values consist of sequences of characters, in which each character can manifest directly (representing itself), or can be represented by a series of characters called a character reference, of which there are two types: a numeric character reference and a character entity reference.
Code that uses this iterator can test the current item (to tell, for example, whether it is a start-tag or end-tag, or text), and inspect its attributes (local name, namespace, values of XML attributes, value of text, etc.), and can also move the iterator to the next item. The code can thus extract information from the document as it traverses it.
Content and attribute declarations cannot depend on attributes or element context (this was also listed as a central problem of DTD). It is not 100% self-describing (as a trivial example, see the previous point), even though that was an initial design requirement.
W3C XML Schema is complex and hard to learn, although that is partially because it tries to do more than mere validation (see PSVI). Although being written in XML is an advantage, it is also a disadvantage in some ways. The W3C XML Schema language, in particular, can be quite verbose, while a DTD can be terse and relatively easily editable.
As attribute nodes named "xmlns" or "xmlns:xxx", exactly as the namespaces are written in the source XML document. This is the model presented by DOM. As namespace declarations: distinguished from attributes, but corresponding one-to-one with the relevant attributes in the source XML document. This is the model presented by JDOM.
XML documents must contain a root element (one that is the parent of all other elements). All elements in an XML document can contain sub elements, text and attributes. The tree represented by an XML document starts at the root element and branches to the lowest level of elements.
Within the root element, apart from any number of attributes and other elements, there may also be more optional text, comments, processing instructions and whitespace. A more expanded example of an XML document follows, demonstrating some of these extra nodes along with a single rootElement element.
In Document Type Definition (DTD) files for SGML and XML, an attribute value may be designated as being of type CDATA: arbitrary character data. Within a CDATA-type attribute, character and entity reference markup is allowed and will be processed when the document is read. For example, if an XML DTD contains