There is a centralized portal for people developing with XML languages at www.xml.org. The website was formed in 1999 by OASIS, the non-profit Organization for the Advancement of Structured Information Systems, to provide public access to XML information and XML Schemas. You can find everything there from tutorials to case studies.

The website www.w3schools.com/dtd/default.asp has a good DTD Tutorial under the title “Welcome to DTD School.” It also has links to many other tutorials of interest to XML developers.

If you're interested in getting more information on XML and the Java XML API, be aware there are a few items we haven't covered. First, there is an additional XML mark-up tag, known as a “processing instruction.” This is a piece of XML inherited from SGML, but not really a good fit. (SGML is the mother of all mark-up languages, too large and too complicated to ever get much use in the real world. XML and HTML are simplifications of SGML.) A processing instruction is used to link style sheets (regular HTML CSS style sheets) into documents. Second, the topics of both elements and attributes are deeper than we have room for here. We have not covered the third library in JAXP: the transformation library in javax.xml.transform.

There is an independently developed open source library called JDOM. This is a Java API that does the same job as W3C's DOM. It was created as a more object-oriented and easier to use alternative to DOM, and there is a good possibility it will be adopted into JAXP (or even replace it). It has been adopted as Java Specification Request number 102, if you want to check on its progress. Please see the JDOM website at www.jdom.org for the most up-to-date information.

IBM offers a series of free tutorials on their website. There are tutorials covering parsers, DOM, SAX, and the transformation library. Go to www.ibm.com/developerworks and click or search on XML. You have to register at the site, but it is free and quick.