Java Tech Page 4u

HTML 5 Media Elements With Example

noreply@blogger.com (GS) — Thu, 01 Dec 2011 17:35:00 PST

The new HTML5 media elements are ..

<audio> : For audio content, sounds, music or other audio streams
<video> : For video content, such as a movie clip or other video streams
<source> : For media resources for media elements, defined inside video or audio elements
<embed> : For embedded content, such as a plug-in
<track> : For text tracks used in media players

Audio example :

Video example :

Linking (including) an external CSS (style sheet) to a HTML page

noreply@blogger.com (GS) — Thu, 24 Nov 2011 09:07:00 PST

An external style sheet can be linked to the page with a <link> tag.

Example link tag which includes an external style sheet is as follows..

The advantage of doing this is that the same style sheet can be used in every page on your site. And the entire site's styling can be managed by changing just this one file.

When the CSS is defined inside the html page, the css classes can be declared with in the <style> tags, but we do not put <style> tags inside an external style sheet.

What is CSS? Where is CSS Used? and an example HTML that uses CSS

noreply@blogger.com (GS) — Thu, 24 Nov 2011 08:57:00 PST

CSS is nothing but Cascading Style Sheets, these are used to separate the content from design of the HTML page, so that in future if we want to change the design and layout of the HTML page, without disturbing the content, it will become any easy job. That is we can change the layout / design by simply modifying the CSS.

The CSS can be declared with in the html file or in a separate CSS file.

The structure of the simple CSS is as follows,

.leftcol
{
float: left;
width: 33%;
background:#908800;
}
.middlecol
{
float: left;
width: 34%;
background:#effefe;
}
.rightcol
{
float:left;
width: 33%;
background:#dddd11;
}

In the above example the "leftcol","middlecol","rightcol" are the css class names, which defines its own styling, these classes need to be applied on the HTML elements to see these styling effects.

simple html that uses the above declared css class

<html>

<head>

<style type="text/css">
.leftcol
{
float: left;
width: 33%;
background:#908800;
}
.middlecol
{
float: left;
width: 34%;
background:#effefe;
}
.rightcol
{
float:left;
width: 33%;
background:#dddd11;
}
</style>

</head>

<body>

<div class="leftcol">this is a left div ...</div>
<div class="middlecol">this is a middle div ...</div>
<div class="rightcol">this is a right div ...</div>

</body>

</html>

What is the difference between Dojo and jQuery

noreply@blogger.com (GS) — Thu, 24 Nov 2011 08:53:00 PST

jQuery is a JavaScript Library that simplifies HTML document traversing(DOM manipulation), event handling, animating, and Ajax interactions for rapid web development. jQuery is designed to change the way that you write JavaScript.

Dojo is a more comprehensive toolkit that offers the above features, but also includes many other pieces that facilitate building rich web applications

Simple SAX parser example (count number of elements in an xml file)

noreply@blogger.com (GS) — Mon, 21 Nov 2011 06:37:00 PST

Example Xml File :

<?xml version = "1.0" ?>

<Employees>

<Employee>

<Emp_Id> 1 </Emp_Id>

<Emp_Name> Bob </Emp_Name>

<Emp_E-mail> bob@hotmail.com </Emp_E-mail>

</Employee>

<Employee>

<Emp_Id> 2 </Emp_Id>

<Emp_Name> Nick </Emp_Name>

<Emp_E-mail> nick@yahoo.com </Emp_E-mail>

</Employee>

<Employee>

<Emp_Id> 3 </Emp_Id>

<Emp_Name> John </Emp_Name>

<Emp_E-mail> john@gmail.com </Emp_E-mail>

</Employee>

</Employees>

Java code to parse the above xml file and provides the elements count

import javax.xml.parsers.*;
import org.xml.sax.*;
import org.xml.sax.helpers.*;
import java.io.*;

public class ElementCount{
int startTag = 0;
public static String ele;
public static void main(String args[])throws IOException {
BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Enter XML file name:");
String xmlFile = bf.readLine();
File file = new File(xmlFile);
if (file.exists()){
System.out.print("Enter tag name for which the count should be calculated:");
ele = bf.readLine();
ElementCount tagCount = new ElementCount(xmlFile);
}
else{
System.out.println("File not found!");
}
}
public ElementCount(String str){
try{
SAXParserFactory parserFact = SAXParserFactory.newInstance();
SAXParser parser = parserFact.newSAXParser();
DefaultHandler dHandler = new DefaultHandler(){
public void startElement(String uri, String name, String element,
Attributes atri)throws SAXException{
if (element.equals(ele)){
startTag++;
}
}
public void endDocument(){
System.out.println("Total elements: " + startTag);
}
};
parser.parse(str,dHandler);
}
catch (Exception e){
e.printStackTrace();
}
}
}

What is a constructor and difference between this and super

noreply@blogger.com (GS) — Mon, 21 Nov 2011 06:34:00 PST

A constructor is a special type of function that gets called every time you initialize a class, constructor has the same name as class name. Constructor has no return type not even void. We can pass the parameters to the constructor.

-this() is used to invoke a constructor of the same class.

-super() is used to invoke a super class constructor.

Constructor is called immediately after the object is created before the new operator completes.

For example, say you have this class called Person:
class Person
{
public Person ()
{
Console.WriteLine("We're making a new person");
}
}

Now you can create a new object of type Person, like this:
Person Tim= new person();

The program will write the line "We're making a new person" to the screen.

Constructor can use the access modifiers public, protected, private or have no access modifier
Constructor can not use the modifiers abstract, static, final, native, synchronized or strictfp
Constructor can be overloaded, we cannot override.
You cannot use this() and Super() in the same constructor.
You can not have both super() and this() invocation in the same constructor.
Either super() or this() should be the first statement in the constructor.

What is the difference between SAX and DOM parsers?

noreply@blogger.com (GS) — Mon, 21 Nov 2011 06:31:00 PST

DOM Parser:

A DOM (Document Object Model) parser creates a tree structure in memory from an input document and then waits for requests from client.
A DOM parser always serves the client application with the entire document no matter how much is actually needed by the client.
A DOM parser is rich in functionality. It creates a DOM tree in memory and allows you to access any part of the document repeatedly and allows you to modify the DOM tree. But it is space inefficient when the document is huge, and it takes a little bit longer to learn how to work with it.

SAX Parser:

A SAX (Simple API for XML) parser does not create any internal structure. Instead, it takes the occurrences of components of an input document as events, and tells the client what it reads as it reads through the input document.
A SAX parser serves the client application always only with pieces of the document at any given time.
A SAX parser, however, is much more space efficient in case of a big input document (because it creates no internal structure). What’s more, it runs faster and is easier to learn than DOM parser because its API is really simple. But from the functionality point of view, it provides a fewer functions, which means that the users themselves have to take care of more, such as creating their own data structures.

What are the differences between finalize(), final and finally

noreply@blogger.com (GS) — Mon, 21 Nov 2011 06:30:00 PST

finalize() – method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state. Should not be used to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to release these non-memory resources through the finalize() method.
finally – The finally block always executes when the try block exits, except System.exit(0) call. This ensures that the finally block is executed even if an unexpected exception occurs. It allows the programmer to avoid having clean up code accidentally bypassed by a return, continue, or break. Keeping the clean up code in a finally block is always a good practice, even when no exceptions are anticipated.
final – This is useful in several ways...

When it is used with variable declaration, it becomes constant
When it is used with method, the method cant be overridden by the child class
When it is used with the class, the class cant have child classes.

What are the differences between HashMap and Hashtable?

noreply@blogger.com (GS) — Mon, 21 Nov 2011 06:29:00 PST

Both provide key-value access to data. The Hashtable is one of the original collection classes in Java. HashMap is part of the new Collections Framework, added with Java 2, v1.2.

The key difference between the two is that access to the Hashtable is synchronized on the table while access to the HashMap isn't. You can add it, but it isn't there by default.

Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't. If you change the map while iterating, you'll know.

And, a third difference is that HashMap permits null values in it, while Hashtable doesn't.

Synchronizing a HashMap

noreply@blogger.com (GS) — Mon, 21 Nov 2011 06:27:00 PST

How to make hashmap operations synchronized?

For JDK 5 or after, we can use ConcurrentHashMap . This is an efficient implementation of a concurrent hash map using different locks for different portions of the HashMap . This reduces the contention of the lock and hence increase throughput tremendously as compared to a synchronized Map.

For JDK versions before JDK5 there is a back port of the same, if we do not want to use that, then we can use Collections.synchronizedMap() method to create a synchronized map. The new map will be backed with the original map and all the access will be synchronized on a single monitor. Also, we have to do external synchronization while iterating over the map entries.