Java with mysql

1. To connect java into database, we need some driver to connect. I have used jdbc driver.

2. In java.sql package contain DriverManager class, it handles driver.

3. After this driver successfully connected to data base, it returns connection object.

   DriverManager.getConnection(String databaseUrl, String username, String password);

   datbaseUrl  -   Url which represents database connection

                   Example, jdbc:mysql://localhost:3306/Uma

                   Uma - Data base name

   username   -    username of the mysql

   password   -    Password of the mysql


4.  Use this connection object, we can create a statement for executing mysql queries.

    Statement st = Conn_object.createStatement()  -   create a statement with out any specification

    [ We can create statement with some specification like resultset scrollable, etc ]


5. statement class contains methods to run queries. We can get the query result using ResultSet.

  
   ResultSet rs = st.exceuteQuery(String query)  -   Simply returns the result, not update in datbase

                  st.updateQuery(String query)   -   Update the query result into database



Some methods in ResultSet :

    We can get values from ResultSet using some methods in ResultSet.


1. To get the column value using getXXX()

   Here XXX is the column data type like getInt(), getString()

2. To check wheather or not a column is null, using wasNull()

      if (rs.wasNull()) {
          // resultset contains null value
      }

3.  Use JDBC operation inside try block and catch the exception which is thrown. TO get more details about exception,

JDBC provides some methods,  e.getMessage(), e.getErrorCode()

    catch (Exception e) {
        // print error message using  e.getMessage()
        // print error number using e.getErrorCode()   
    }


More Info :

http://www.kitebird.com/articles/jdbc.html

Difference between JDK, JRE and JVM

JDK:

1. Java development kit contains tools for developing java programs such as compiler (javac.exe) whihc converts source code inro byte code,
Interpreter, Java Application launcher (java.exe) which converts byte coe into mechine code.


JRE :

1. Java Runtime Environment contains JVM, class libraries and other supported files. It does not contain any developer tools such as compiler, interpreter. Actually JVM runs the program using library files and other supported files that are provided by JRE.

JVM :

1. JVM is in both JRE and JDK.

2. JVM runs the java program using library files in jre.

3. JVM interpretes the byte code into machine code.

4. JVM is called "virtual" because it creates machine interface which does not depend on underlying operatiog system and machine hardware architecture.

[ JVM interpretes byte code into machine code which does not depend on underlying operating system ]



-

Enhanced for loop

Enhanced for loop:

1. This new feature added in Java1.5

2. Also callied as "for each loop".

3. It allows as to iterate through collection without having to create a iterator or without having to calculate beginning and end conditions for counter variable.

For example, 

In normal for loop,

class sample {
    public static void main(String args) {
        int[] var = {1, 9, 18, 27 };
        for( int i = 0; i < var.length; i++) {
            System.out.println("values :" +var[i]);
        }
    }
}
       
For each loop,

class sample {
    public static void main(String args) {
        int[] var = {1, 9, 18, 27 };
        for( int i : var) {
            System.out.println("values :" +var[i]);
        }
    }
}

Here, no need to check var length. Its easy to adapt for loop to for each loop.


But, It contains some disadvantages, follows

1. for each loop iterate only sequencialy. cant get first and third value using for each. [ Step value can not be incremented ]

2. Backward traversal is not possible.



Reference :

http://www.javabeat.net/articles/32-new-features-in-java-50-1.html

Flex Basics

Flex basics:

Use of Name space in mxml :

1. To specify language version

2. To provide scope for mapping XML tag name to ActionScipt class name or CSS style tag to ActionScript class name

3. And provide details to compiler, where local sources located for custom component.


Name space :

1. Flex 2 and 3 -> MXML 2006  which contains language version and mapping for flex framework component

2. In Gumbo, new language features are added and updated to MXML 2009 [ xmlns:fx="http://ns.adobe.com/mxml/2009" ]

3. Components are described in their own namespaces,

    -> Flex 3 or MX components are in mxml library namespace. [ xmlns:mx="library://ns.adobe.com/flex/mx" ]

    -> New Gumbo spark components are in spark component name space. [ xmlns:s="library://ns.adobe.com/flex/spark" ]


For flex 3,

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

<mx:Application xmlns:mx="http://ns.adobe.com/mxml/2006">


For flex 4, Gumbo spark component,

<?xml verison="1.0" encoding="utf-8"?>

<s:Applicaiton xmlns:fx="http://ns.adobe.com/mxml/2009" xmlns:s="library://ns.adobe.com/flex/spark" xmlns:mx="library://ns.adobe.com/flex/mx">


                                                                                - will continue in next post..