Difference Between ClassNotFoundException & NoClassDefFoundError

Assume the following classes

program
  |-------->Test.java
  |--------->Demo.java

Test.java
----------
public class Test{}


Demo.java
-------------

public class Demo
{
public static void main(String s[])throws Exception
{

Class.forName("Test");  //line 1
Test t=new Test();  //line 2
}
}

Observation
ClassNotFoundException
---------------------------------
You keep only line 1(comment line 2),and compile and execute Demo.java,(Donot compile Test.java)it will give you ClassNotFoundException
What ever String you are passing in Class.forName("---") is a class name,and it should be there
solution:  Compile both the java files and then execute Demo class

NoClassDefFoundError
--------------------------------------
Keep Only line 2(comment line 1),and compile and Execute Demo.java.The program successfully compiles &     Executes.It is because,when ur class Demo is using class Test,then No need to compile Test.java,when you compile Demo.java,it auto compiles Test.java

So when will I get Error?  Just remove . from ur classpath & recompile


In classpath generally we add  ;.  at last.
This is because,when the JVM looks for the classes,it looks in classpath and when it sees .  , it  understands that it has to look in the current working directory also.

But if you remove . from classpath,then here auto-compilation will not take place,means you have to compile all the classfiles using javac *.java.

Even after you have compiled both the classes,if u try to execute it,it will still show you NoClassDefFoundError,because u have removed . from classpath variable


Now comment both line1 & line 2,and again recompile only Demo.java
Compilation will be successful,but While execution it will give you same NoClassDefFoundError


Solution:  include ;.  at your classpath

use jdk 1.6 to compile the java file as jdk1.3

assert is a keyword introduce at jdk 1.4,You cannot use it as an identifier in jdk1.4 +.
However if you have used it as an identifier,and you want to compile the class,then you need to go back to jdk 1.3.

But you have only jdk 1.6 with you.Still then you can compile your java class,as if you are using jdk 1.3

 Demo.java

 public class Demo

{

public static void main(String[] args) 

{

 boolean assert = true;

if(assert) System.out.println("if is true");

}

}

compile using jdk 1.4+ will give you these error

-------------------------------------------------
>javac Demo.java
Demo.java:5: as of release 1.4, 'assert' is a keyword, and may not be used as an
 identifier
(use -source 1.3 or lower to use 'assert' as an identifier)
                 boolean assert = true;
                         ^
Demo.java:6: as of release 1.4, 'assert' is a keyword, and may not be used as an
 identifier
(use -source 1.3 or lower to use 'assert' as an identifier)
                if(assert) System.out.println("if is true");

To compile as jdk1.3 although you are using jdk 1.4+
--------------------------------------------------------
>javac -source 1.3 Demo.java

Working with Trigger in Oracle

I want to implement autoincrement feature to my employee table.Built in facility is not available but we can write our own trigger concept to do so.

1. Create table in Oracle
------------

 create table Employee1
  (
    empid number primary key,
    empname varchar2(10),
    empsal number(10,2)
   );

2.Create a Sequence which will generate unique empid each time

SQL> CREATE SEQUENCE   EMPID_SEQ  MINVALUE 101 MAXVALUE 1000 INCREMENT BY 1 START WITH 101 NOCACHE   NOCYCLE  NOORDER;

3.Write a Trigger,which will insert the value from empid_seq to empid column of Employee1 table

SQL>
CREATE OR REPLACE TRIGGER EMPID_TRIG
before insert on EMPLOYEE1
for each row
begin
select EMPID_SEQ.nextval into :new.EMPID from dual;
end;
/
ALTER TRIGGER EMPID_TRIG ENABLE;

4.Insert record into table without EmpId.Notice that EmpId is autofilled by sequence value.
SQL>  insert into employee1(empname,empsal) values('suraj',1000);
SQL>   insert into employee1(empname,empsal) values('raj',200);




5>View Record to see the autoIncrement working
SQL> select * from employee1;

     EMPID EMPNAME        EMPSAL
---------- ---------- ----------
       101 suraj            1000
       102 raj               200

Running javaProgram without main() not possible in JDK1.7 latest Version

Running Java Program without main() has become an old story now.
So far we used to write the program like this
StaticDemo.java

public class StaticDemo

{

   static

    { 

        System.out.println("Hello");

        System.out.exit(0); 

     }

}

And then >javac *.java
>java StaticDemo

It compiles and runs successfully,printing Hello

This works well in JDK1.7 old versions(build 1.7.0-ea-b19)

But From jdk 1.7(build1.7.0-ea-b85),It gives run time Exception

Error: Main method not found in class StaticDemo, please define the main method
   public static void main(String[] args)

The latest version while writing this article was build  jdk 1.7(build 1.7.0_03-b05),which also doesnot allow running java program without main();

So the Concept is:
When we say
java StaticDemo
Compiler understands that we are trying to execute the program,so before executing the static block,it first checks whether main() is there or not.If not,throws Exception

Email Validation Using RegularExpression

We can validate Email using regularExpression support given by JDK

  EmailValidation.java

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class EmailValidation{
     
    public static void main(String args[])
    {
      Pattern pattern;
      Matcher matcher;

String email="suraj20p@gmail.com";
      String EMAIL_PATTERN = "^[_A-Za-z0-9]+(\\.[_A-Za-z0-9]+)*@[A-Za-z0-9]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,})$";

   
     //compile our pattern
     pattern = Pattern.compile(EMAIL_PATTERN);
     //pass our email to matcher,so that it can be checked whether pattern is matching or not
     matcher = pattern.matcher(email);
    
     //check whether string is matched or not
      boolean result= matcher.matches();

      if(result)
           System.out.println("Email Validated Successfully");
      else System.out.println("Invalid Email Address");
    }
}

  Download Source Here

Creating a Formatted XML using Java

Lets Assume we want to create an XML file,
Eg:-

CreatXMLFile.java
----------------------
import java.io.*;
import javax.xml.parsers.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.*;
import javax.xml.transform.stream.*;
import org.w3c.dom.*;
public class CreatXMLFile
{  
public static void main(String[] args) throws Exception
 {   
    //creating factory
    DocumentBuilderFactory documentBuilderFactory =DocumentBuilderFactory.newInstance();
    //creating a builder
    DocumentBuilder documentBuilder =documentBuilderFactory.newDocumentBuilder(); 
    //creating a document
    Document document = documentBuilder.newDocument();
   
    String root = "SMS";

    //adding root Element
    Element rootElement = document.createElement(root);
    rootElement.setAttribute("area", "hyderabad");

    document.appendChild(rootElement); //adds <SMS> as root element

    String element ="Number";    
    String data = "12345";
   
    Element em = document.createElement(element);
    em.appendChild(document.createTextNode(data));
    rootElement.appendChild(em);  //creates <Number>12345</Number> and adds to root element<SMS>
/*
<SMS>
<Number>12345</Number>
</SMS>
*/
  
    element ="message";   
    data= "hello";
  
    em = document.createElement(element);  
    em.appendChild(document.createTextNode(data));
    rootElement.appendChild(em); //creates <message>hello</message> and adds to root element<SMS>
/*
<SMS>
<Number>12345</Number>
<message>hello</message>
</SMS>
*/
  
  
    em=document.createElement("delivered");
    Element childElement = document.createElement("status");
    childElement.appendChild(document.createTextNode("failed"));
    em.appendChild(childElement);
    rootElement.appendChild(em);//creates <delivered><status> failed</status></delivered> and adds to root element<SMS>

 /*<SMS>
<Number>12345</Number>
<message>hello</message>
<delivered>
      <status> failed</status>
</delivered>
</SMS>
    */
   
    //Used to Write the generated xml structure to  file or system console
    TransformerFactory transformerFactory = TransformerFactory.newInstance();
    Transformer transformer = transformerFactory.newTransformer();
    transformer.setOutputProperty(OutputKeys.INDENT, "yes");//arrange the xml in an idented format
    DOMSource source = new DOMSource(document);   
    File f=new File("d:/myfile.xml");
   
    StreamResult result =  new StreamResult(f);  //prints xml into file
    //StreamResult result =  new StreamResult(System.out);   
    transformer.transform(source, result);
 
 }
 }

Adding column to Existing Table

Syntax
alter table <tableName> add  <New_Column> <datatype_with_size>;



SQL>alter table employee add  empsal number(5);