Friday, June 26, 2009

Multiple inheritance in Java

I have always missed true multiple inheritance in Java (like, in C++). For e.g., we are not able to define a class as follows in Java:

class X extends A, B, C {


Though, I do not see any inheritance use case which cannot be solved by the current Java facilities, but I would love to have this facility in Java. I think, the most latest Java version (1.7) doesn't have this feature.

One workaround I can see, for multiple inheritance, is to define a class like following:

class X {
A a;
B b;
C c;

i.e., we could create private class members inside X (whose functionality we want to use in class X).

Though this might serve purpose for some of the cases, but it's not true multiple inheritance! This I think, is actually aggregation pattern.

Of course, Java has multiple inheritance of interfaces. But that is inheritance of method signatures, and not of implementation.

I guess, keeping the number of base classes to one, Java is much simpler syntactically, and has a simpler compiler implementation. Though I agree, that having a simple syntax (as the current Java inheritance facilities) which is powerful enough, and can solve many use cases is better, than having a complex syntactical facility, which might serve even more uses cases, but could also lead to semantically difficult programs, which may be difficult to maintain and debug, as complexity of the problem domain increases.

Sunday, June 21, 2009

Primitive long a subtype of float

The Java language specification defines, that primitive "long" is a subtype of primitive "float" (ref,

But XML Schema Datatypes spec, shows no relationship between xs:float and xs:long (
ref: XML Schema 1.0 data types, XML Schema 1.1 data types).

I'm a little confused, that which concept is correct (Java's definition of this data-type inheritance, or XML Schema). I seem to be in favor of XML Schema definition. But perhaps, XML Schema type system is for XML oriented data, and Java type system is for a wider class of applications. But I'm not sure, if this is the reason for the differences of definitions in Java language spec and XML Schema.

Saturday, June 20, 2009

Became Eclipse WTP committer

I was nominated as Eclipse WTP project committer, for the WTP Source Editing subproject.

As per the voting process, for becoming an Eclipse project committer, the Eclipse WTP Source Editing team, granted me project committership on 18, Jun 2009.

My contributions to PsychoPath XPath 2.0 engine (which is one of the components in WTP Source Editing tooling), helped me become a committer to this project.

I am happy to be included in the Eclipse WTP team. I look forward to contribute more to PsychoPath, and other WTP components. Apart from PsychoPath, I look forward to work on WTP XSL components in near future.

Monday, June 15, 2009

Running first XSLT 2.0 stylesheet with IBM XSLT 2.0 engine

I could run my first XSLT 2.0 stylesheet with IBM XSLT 2.0 engine (ref, WAS XML Feature Pack Open Beta).

I tried the following XSLT 2.0 stylesheet, using xsl:for-each-group instruction, which worked well with the IBM XSLT engine.

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl=""

<xsl:output method="xml" indent="yes" />

<xsl:template match="books">
<xsl:for-each-group select="book" group-by="author">
<author name="{current-grouping-key()}">
<xsl:for-each select="current-group()">
<xsl:copy-of select="name" />
<xsl:copy-of select="publisher" />


Saturday, June 13, 2009

XML Schema validation with Xerces-J

Hiranya Jayathilaka raised an interesting discussion some time ago on xerces-dev list, that how we could validate an XML Schema 1.0 document using Xerces-J. Hiranya was looking for a solution using a Java API with Xerces-J. We were looking for verifying the correctness of the Schema document, and not doing an XML instance document validation.

I'm providing a summary of the discussion we had on the list, and the conclusions we made.

There are basically three ways of doing this:

1. Using a JAXP SchemaFactory
Using this technique, we do something like below:

SchemaFactory sf = SchemaFactory.newInstance ..
sf.setErrorHandler ..
Schema s = sf.newSchema(new StreamSource(schemapath));

The 'SchemaFactory.newSchema' call would not succeed if XML Schema has a grammar error.

2. Using XSLoader
Using this technique, we do something like below:

XSLoaderImpl xsLoader = new XSLoaderImpl();
XSModel xsModel = xsLoader.loadURI(xsdUri);

Michael Glavassevich suggested, how we could add an error handler to this mechanism:

DOMErrorHandler myErrorHandler = ...;

XSImplementation xsImpl = (XSImplementation) registry.getDOMImplementation("XS-Loader");
XSLoader xsLoader = xsImpl.createXSLoader(null);

DOMConfiguration config = xsLoader.getConfig();
config.setParameter("error-handler", myErrorHandler); // <-- set the error handler

3. Using XMLGrammarPreparser
Using this technique, we do something like below (thanks to Hiranya Jayathilaka for sharing this code):

XMLGrammarPreparser preparser = new XMLGrammarPreparser();
preparser.registerPreparser(XMLGrammarDescription.XML_SCHEMA, null);
preparser.setFeature("", true);
preparser.setFeature("", true);
preparser.setFeature("", true);
preparser.setErrorHandler(new MyErrorHandler());
Grammar g = preparser.preparseGrammar(XMLGrammarDescription.XML_SCHEMA, new XMLInputSource(null, xsdUrl, null));

Michael Glavassevich provided a nice comparison of these three approaches:
SchemaFactory - it is an entry point into the JAXP Validation API for loading schemas for validation. If it was a user asking I'd recommend SchemaFactory of the three choices since it's in Java 5+ and would work in environments where Xerces isn't available.
XSLoader- it is an entry point into the XML Schema API for obtaining an XSModel for analysis/processing of the component model.
XMLGrammarPreparser - it provides API for preparsing schemas and DTDs for use in grammar caching (i.e. a lower-level alternative to SchemaFactory).