OO Programming/Design articles and blogs.

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Why Frameworks Sucks

Encapsulation is not information hiding

Excellent article describing the difference between Encapsulation and Information hiding with real life example. Read it.

Example of Deadlock in Swing

http://www.javaspecialists.co.za/archive/Issue101.html

ROWNUM Pseudocolumn with ORDER BY

In Oracle to limit the number of rows return we use ROWNUM in the WHERE clause. Suppose if we want first 10 rows of emp table, then the sql query will be something like

SELECT id, name FROM emp WHERE ROWNUM < = 10

And if we want sorted result then might be ( as i thought for first time)

SELECT id, name FROM emp WHERE ROWNUM < = 10 ORDER BY name

The above query does not work as we want. The WHERE clause is applied before the ORDER BY clause. Oracle picks the first 10 rows and then applies ORDER BY on them. We need to first sort the data and then select the first 10 rows. Here is the query:

SELECT id, name FROM (SELECT id, name FROM emp ORDER BY name) WHERE ROWNUM<=10

You have to do this if the column in ORDER BY clause is non-primary key. If the column is primary key then simple query will do. Look at the below sql query

SELECT id, name FROM emp WHERE ROWNUM < = 10 ORDER BY id DESC

here id is primary key.

Also one more thing i noticed that we cannot use greater than(>) operator with ROWNUM. The query returns zero rows.

Design Vs. Architecture

Weblog showing the difference between Design and Architecture.

http://www.artima.com/weblogs/viewpost.jsp?thread=84070

Thinking about objects

Nice weblog, discussing whether one can make sense of the objects in the langauge independent way.

http://www.artima.com/weblogs/viewpost.jsp?thread=85308

Overriding validate() method for Containers.

I found an interesting problem when overriding the validate() method of the JPanel. I wanted to do some operation when the size of the parent changes. But when the program was executed nothing happened. The validate() method was not called. Where was the problem???????

Actually the answer lies in the validateTree() method of Container class . The validate() method is called only for the Containers which are Window otherwise the validateTree() method is called. So the flow is something like this :

validate() method of Container calls the validateTree(), which in turn calls the validate() if Container is Window otherwise validateTree().

So if your Component is Window then override validate() method otherwise override validateTree() method.

Using SAX Parser from validating XML document in Java 1.4

By default the SAX parser does not validate the XML document against DTD.
To validate a XML document, call setValidating(true) on SAXParserFactory instance.

Your handler class must override error(SAXParseException e) method of the DefaultHandler to throw an exception (see the example below). If you don't do this your xml document will be parsed and client(user/other program) will not know about the exception. The SAX parser calls this method if any error occurs while parsing. It is the responsibility of the handler writer to make this exception available to the external world.

The default implementation of error method is empty(i.e. it does not perform any operation).

In xml document DOCTYPE should be included. The parser takes the DTD from the DOCTYPE.

Example:

The below given java example prints the element in the console as they are encountered. If the example is executed with validbooks.xml no exception is thrown. But if the invalidbooks.xml is used exception will be thrown for <abc> tag.

If the error method is removed from the code, both the xml documents(invalidbooks.xml and validbooks.xml)is parsered without any exception.

DTD:

<?xml version="1.0" encoding="UTF-8"?>

<!ELEMENT age (#PCDATA)>

<!ELEMENT book (person+)>

<!ELEMENT first (#PCDATA)>

<!ELEMENT last (#PCDATA)>

<!ELEMENT person (first,last,age)> 

validbooks.xml

<?xml version="1.0"?>


<!DOCTYPE book SYSTEM "books.dtd">

<book>

	<person>

		<first>Kiran</first>

		<last>Pai</last>

		<age>22</age>

	</person>

	<person>

		<first>Bill</first>

		<last>Gates</last>

		<age>46</age>

	</person>

	<person>

		<first>Steve</first>

		<last>Jobs</last>

		<age>40</age>

	</person>

</book>

invalidbooks.xml

<?xml version="1.0"?>

<!DOCTYPE book SYSTEM "books.dtd">

<book>

	<person>

		<first>Kiran</first>

		<last>Pai</last>

		<age>22</age>

	</person>

	<person>

		<first>Bill</first>

		<last>Gates</last>

		<age>46</age>

	</person>

	<person>

		<abc></abc>

	</person>

	<person>

		<first>Steve</first>

		<last>Jobs</last>

		<age>40</age>

	</person>

</book>

Java Program:

import java.io.IOException;



import javax.xml.parsers.ParserConfigurationException;

import javax.xml.parsers.SAXParser;

import javax.xml.parsers.SAXParserFactory;



import org.xml.sax.Attributes;

import org.xml.sax.SAXException;

import org.xml.sax.SAXParseException;

import org.xml.sax.helpers.DefaultHandler;



public class SAXTest extends DefaultHandler

{

	public void startElement(String namespaceURI, String sName, String qName, Attributes attrs)

	{

		System.out.println("<" + qName + ">");

	}

	

	public void endElement(String namespaceURI, String sName, String qName)

	{

		System.out.println("");

	}

	

	public void error(SAXParseException e)throws SAXParseException

	{		

		throw e;

	}



	public static void main(String[] args)

	{

		DefaultHandler handler = new SAXTest();

		

		SAXParserFactory factory = SAXParserFactory.newInstance();

		factory.setValidating(true);

		try

		{

			SAXParser parser = factory.newSAXParser();

			System.out.println(parser.isValidating());

			parser.parse("books.xml", handler);

		}

		catch (ParserConfigurationException e)

		{

			e.printStackTrace();

		}

		catch (SAXException e)

		{

			e.printStackTrace();

		}

		catch (IOException e)

		{

			e.printStackTrace();

		}

	}

}

Headless in Java

Headless support is for application that want to process images but run on server which does not has display. Click
herefor more information.

Note: Java 1.4 onwards support headless and does not require X server.