Enjoy the book,

The_Fortune_at_the_Bottom_of_the_Pyramid

” javac : Cannot find Symbol“

These errors are due to variables going out of their scope. Scoping errors come in many shapes and sizes. One common mistake happens when a variable is shadowed and two scopes overlaps. Just go through Scope of Variable to revise your concept on Java variables and their scopes.

The most common reason for scoping errors is when you attempt to access a variable that is not in scope. In this post, I will note down the three most common errors which appears due to the variables scope.

Attempting to access an instance variable from a Static Context :

class MyClass {

int x = 3;

public static void main ( String[] args ) {

x++;

// This would not compile because x is an ‘instance’ variable, it could not be accessed //from within a static context.

}

}

Accessing Local variable from Nested Method :

class MyClass {

public static void main ( String[] args) {

MyClass c = new MyClass(); // reference variable c referring to an Object of class MyClass

c.methodA();

}

void methodA() {

int x = 4 ;

methodB();

x++;  // Once methodB completes x comes back in scope.

}

void methodB () {

x++; // would not compile, In method B, scope of variables of method A (ie x) ends.

}

}

Using a block variable, out of the block :

void MyMethod() {

for (int i =0; i<=5 ; i++){

boolean test;  // By default a boolean variable is given the value ‘false’

}

System.out.print(test);  // would not compile, test variable has it’s scope only within //the for loop

}

In the last two examples the compiler will say something like this :

“cannot find symbol”

References :

1. Scope of Variable : http://ezdia.com/Scope_of_Variables_in_Java/Content.do?id=1300

2. Java Static Variables : http://ezdia.com/Java_Static_Variable/Content.do?id=1301

All the variables in Java have their scope or which may be called as the ‘Expiry date of a variable’

In terms of the Scope of Variable, we can classify the Java variables in 4 parts :

• Static Variables

Static variables have the longest scope among all. They are created when a class is loaded

and survives as long as the class stays loaded in the Java Virtual Machine (JVM)

• Instance Variables

They are next most lived variables. They are created when a new Instance is created and

survives until the instance is removed.

• Local Variables

Unlike Objects and Instance variables, Local variables are stored in the Stack of the

memory. These variables lives as long as the methods in which they are declared, remains

on the stack.

However, we can have local variables alive and still out of scope. For more information please read, Errors due to Scope of Variable.

• Block Variables

These variables are those which are defined within ‘for’ block or ‘if-else’ block. There scope

ends as soon as the command comes out of the block.

A programmer must take care of the errors which appears due to calls to the variables when the command goes out the Scope of Variable. One common mistakes occurs when one variable is shadowed and two scopes overlap. For learning more on shadowing, please read Errors due to Scope of Variable.

References :

1. Scope of Variables : http://ezdia.com/Scope_of_Variables_in_Java/Content.do?id=1300

2. Java Static Variable : http://ezdia.com/Java_Static_Variable/Content.do?id=1301

A foreign key in a My Sql database is a field (or fields) of a table that points to a primary key of an other table. Such keys are defined in a database to ensure that only fields that supposed to appear in a database may be permitted.

What is the need of a foreign key :

For a long time MySQL do not support foreign keys being afraid of the fact that it would erode the speed and performance of the RDBMS. But considering the fact that foreign key relationships can play an important role in catching data entry errors and may result in a stronger and better-integrated database, MySQL included the foreign keys.

In order to set up a foreign key relationship between two MySQL tables, three conditions must be met:

1. Both tables must be of the InnoDB table type.
2. The fields used in the foreign key relationship must be indexed.
3. The fields used in the foreign key relationship must be similar in data type.

Following examples may help you understand the need and use of foreign keys.

EXAMPLE :

We have two tables, say Students and Marks.

Students Table :-

Roll No.                  Name                        Surname

001                       Sulabh                      Jain

002                       Pratik                      Verma

003                       Kuldeep                     Kumar

004                       Anupam                      Jain

Marks Table :-

S No                         Name                         Roll No                    Subject                        Marks

1                                Sulabh                       001                          Maths                           87

2                                Sulabh                      001                           English                         76

3                                Kuldeep                   003                           English                         81

4                                Pratik                        002                          Chemistry                   72

In the Marks table, the field Roll No is a foreign key referenced to the Roll No field of Students table.

Creating a table with a foreign key :

CREATE TABLE `marks`

( `S No` int(10) unsigned NOT NULL AUTO_INCREMENT,

`Name` varchar(45) NOT NULL,

`Roll No` int(10) unsigned NOT NULL,

`Subject` varchar(45) NOT NULL,

`Marks` int(10) unsigned NOT NULL,

PRIMARY KEY (`S No`), KEY `Roll No` (`Roll No`),

CONSTRAINT `Roll No` FOREIGN KEY (`Roll No`)

REFERENCES `Students` (`Roll No`) )

ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1;

Foreign Key Constraints :

Try the following query,

INSERT INTO TABLE  Marks (S No, Name, Roll No, Subject, Marks) values (5, ‘Anupam’, 005, ‘Quantum’, 81);

(Notice that there is no value like 005 in the Roll No column of Students table to which the Roll No column of Marks table is referenced)

If we try to enter a value for the field Roll No in II table (Marks) which do not exists in the Roll No column of I table (Students), in such a case we will get an error like,

Cannot add or update a child row: a foreign key constraint fails

How to drop a FOREIGN KEY :

We use the ALTER TABLE command with the DROP FOREIGN KEY clause, for dropping a foreign key,

ALTER TABLE  Marks  DROP FOREIGN KEY Roll No ;
Query OK, 1 row affected (0.11 sec)
Records: 1  Duplicates: 0  Warnings: 0

Whether any key is a foreign key or not, could be easily checked by using the SHOW CREATE TABLE query.

References :

1. SQL Foreign Keys, http://ezdia.com/SQL_Foreign_Key/Content.do?id=1294

2. Referential Integrity, http://ezdia.com/Referential_Integrity_in_databases/Content.do?id=1295

Netbeans :

The latest edition which I have worked upon is Netbeans 6.7.1

The NetBeans IDE is an open-source integrated development environment. NetBeans IDE supports development of all Java application types (Java SE including JavaFX, (Java ME, web, EJB and mobile applications) out of the box. Among other features are an Ant-based project system, Maven support, refactorings, version control (supporting CVS, Subversion, Mercurial and Clearcase).

Features of Netbeans 6.7,

• The newest features in NetBeans 6.7 involve the integration of Project Kenai – a collaborative environment for developers to host their open-source projects – native Maven support and Hudson integration.(Read more on Kenai .. http://ezdia.com/Netbeans_and_Kenai/Content.do?id=997)
• It offers enhancements for Java, PHP, Ruby, JavaScript, Groovy and C/C++
• It brings support for SVG Rich Components, remote debugging in Ruby and the latest version of GlassFish

Minor updates with Netbeans 6.7.1 are that it does add support for the latest features of JavaFX with JavaFX SDK 1.2 being bundled with the IDE. It also incorporates bug fixes included in Patch 1 for the NetBeans 6.7 release.

Netbeans 6.8 is also released

Why Netbeans and not Eclipse ?

• Netbeans is very fast (atleast on Linux, it is way faster than Eclipse, One can say that it should approach Eclipse on Windows)

License: From July 2006 through 2007, NetBeans IDE was licensed under Sun’s Common Development and Distribution License (CDDL), a license based on the Mozilla Public License (MPL). In October 2007, Sun announced that NetBeans would henceforth be offered under a dual license of the CDDL and the GPL version 2 licenses, with the GPL linking exception for GNU Classpath

Download Links :

http://netbeans.org/downloads/

MySQL :

MySQL Client Version 5.1.11 (it is the latest)

MySQL is the world’s most popular open source database software, with over 100 million copies of its software downloaded or distributed throughout its history. With its superior speed, reliability, and ease of use, MySQL has become the preferred choice for Web, Web 2.0, SaaS, ISV, Telecom companies and forward-thinking corporate IT Managers because it eliminates the major problems associated with downtime, maintenance and administration for modern, online applications.

• MySQL is the world’s most popular open source database software, with over 100 million copies of its software downloaded or distributed throughout its history
• With its superior speed, reliability, and ease of use, MySQL has become the preferred choice for Web, Web 2.0, SaaS, ISV, Telecom companies and forward-thinking corporate IT Managers.
• It eliminates the major problems associated with downtime, maintenance and administration for modern, online applications.
• MySQL is a key part of LAMP (Linux, Apache, MySQL, PHP / Perl / Python), the fast-growing open source enterprise software stack.

MySql advantages over Oracle :

MySQL is characterised as a fast, robust database with a good feature set, but one which lacks all the extras of something like Oracle … So if we are writing code that’s designed to be portable it’s a pretty good choice.  Admin / security are effective but the setup of these and othe features isn’t over-complicated.

Download Links :

http://dev.mysql.com/downloads/

Tortoise SVN version is 1.6.6.

TortoiseSVN 1.6.6, Build 17493 – 32 Bit
Subversion 1.6.6,
apr 1.3.8
apr-utils 1.3.9
neon 0.28.6
OpenSSL 0.9.8k 25 Mar 2009
zlib 1.2.3

This software includes software developed by collabnet (www.Collab.Net)

This software includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit (www.openssl.org)

Open-source website – http://tortoisesvn.net/downloads

Tortoise Advantages over other CVS Clients:-

1. Subversion gives us the confidence that when we commit, everything went into the repository.
2. The ability to back out changes before going to production–using an activity branching model, we can allow developers to branch per activity and only merge to the main source base after code reviews have been performed.
3. If there are problems, we have one revision we can back out that includes the full change-set for that change. While the repository level re-visioning was a shift for my developers to make that didn’t happen immediately, it begins to make sense when an activity had to be removed from the build-it’s easier to see what people are working on as the commits hit our internal commit mailing list. Since we tag each release, we’re able to determine which source code contributed to a release.
4. It’s easier to see what people are working on as the commits hit our internal commit mailing list. Since we tag each release, we’re able to determine which source code contributed to a release.

Openfire 3.6.4 (Chat Server)

Openfire is an open source real time communication (RTC) server. Openfire uses the only widely adopted open protocol for instant messaging and group chat, XMPP (also called Jabber). Openfire is incredibly easy to setup and administer, but offers rock-solid security and performance. Jive Software leads the open source source project at Igniterealtime.org  and uses the technology to power real- time features in their Clearspace products.
Openfire and Spark use the open XMPP protocol (also called “Jabber”), the only broadly-adopted instant messaging protocol and an approved standard by the IETF. With many server implementations and dozens of clients, it’s also the only protocol with proven interoperability. The XMPP protocol is supported and continually enhanced by the active XMPP Software Foundation community.

Opensource Website - http://www.igniterealtime.org/projects/openfire/index.jsp

References:

1. Netbeans and Kenai : http://www.ezdia.com/Netbeans_and_Kenai/Content.do?id=997
2. Netbeans Keyboard Shortcut keys : http://www.ezdia.com/Netbeans_Keyboard_Shortcut_keys/Content.do?id=1119
3. About MySQL : http://www.ezdia.com/About_My_SQL/Content.do?id=1168
4. Books on MySQL : http://www.ezdia.com/Recommended_Books_of_MySQL/Content.do?id=60
5. PHP and MySQL Guide for beginners : http://www.ezdia.com/PHP_and_MySQL_Guide_for_Beginners/Content.do?id=601
6. Tortoise SVN Downloads: http://www.ezdia.com/TortoiseSVN_downloads/Content.do?id=1169

Converting a String Array to an ArrayList,  is sometimes a tricky job.  If you have a Array of Strings with you and you want to return a ArrayList, you can do it by following command,

ArrayList al = new ArrayList ( Arrays.asList ( StringArray )  );

References :

1. Array Library Methods,  http://ezdia.com/Array_Library_Methods/Content.do?id=1141

2. Array Class and methods,  http://ezdia.com/Array_Class_and_Methods/Content.do?id=1142

You can not define it like,

Logger logger = new Logger();

The right way of doing so is,

Logger logger = Logger.getLogger(ClassName.class);

where, ClassName is the name of the class in which you are defining the Logger object.

Loggers, Appenders and Layouts:

Log4j has three main components: loggers, appenders and layouts. These three types of components work together to enable developers to log messages according to message type and level, and to control at runtime how these messages are formatted and where they are reported.

Other methods of Logger Class :

Logger.getRootLogger -

The root logger resides at the top of the logger hierarchy. It is exceptional in two ways:

1. it always exists,
2. it cannot be retrieved by name.

It can be retrieved only by the above method provided.

Possible Levels assigned to Loggers,

The set of possible levels, that is:

TRACE,

DEBUG,

INFO,

WARN,

ERROR and

FATAL

One may define his own levels by sub-classing the Level class.

References:

1. Introduction to Logger, http://ezdia.com/Intorduction_to_Logger/Content.do?id=1139

2. Logger Class and Object, http://ezdia.com/Logger_Class_and_Object/Content.do?id=1140

I wrote a function yesterday, and defined an String array there. I defined it like,

String[] arr = null;

No error was shown by my IDE. I thought to move ahead. But when I tried to increase the size of my array, I was caught.

Now I learned the difference b/w Array & an Array List.

In Java, standard arrays are of a fixed length. After arrays are created, they cannot grow or shrink, which means that you must know in advance how many elements an array will hold. But, sometimes, you may not know until run time precisely how large of an array you need. To handle this situation, the collections framework defines ArrayList.

An Array List

• The ArrayList class extends AbstractList and implements the List interface.
• An ArrayList is a variable-length array of object references. That is, an ArrayList can dynamically increase or decrease in size.
• Array lists are created with an initial size. When this size is exceeded, the collection is automatically enlarged. When objects are removed, the array may be shrunk.

Constructors of Array List

• ArrayList( )
• ArrayList(Collection c)
• ArrayList(int capacity)

The first constructor builds an empty array list. The second constructor builds an array list that is initialized with the elements of the collection c. The third constructor builds an array list that has the specified initial capacity. The capacity grows automatically as elements are added to an array list.

The following program will show How an Array Lists grows and How we define an Array.

import java.util.*;
/**
*
* @author Sulabh
*/
public class ArrayListDemo {
public static void main(String args[]){
// Creating an Array List
ArrayList arrlist = new ArrayList();                   // Stating the initial size in not mandatory.
String[] arr = new String[2];                                   // We need to state the size of an Array.

System.out.println(“Size of Array List = “+arrlist.size());
System.out.println(“Size of String array = “+arr.length);

arrlist.add(“Sulabh”);
arrlist.add(“Pratik”);
arrlist.add(“Kuldeep”);

System.out.println(“Size of Array List = “+arrlist.size());
}
}

The output of the program will be :

Size of Array List = 0
Size of String array = 2
Size of Array List = 3

ensureCapacity()  for ArrayLists :

Although the capacity of an ArrayList object increases automatically as objects are stored in it, you can increase the capacity of an ArrayList object manually by calling ensureCapacity( )

arrlist.ensureCapacity( int minimunCapacity );
By increasing its capacity once, at the start, you can prevent several reallocations later. Because reallocations are costly in terms of time, preventing unnecessary ones improves performance.

References :

1. How to use Array Lists in Java : http://www.ezdia.com/How_to_use_Array_List_in_Java/Content.do?id=1022

2. Array Lists in Java : http://www.ezdia.com/Array_List_in_Java/Content.do?id=1023

3. Differences b/w Array & ArrayLists : http://www.ezdia.com/Array_and_ArrayLists/Content.do?id=1024

I searched for the solutions, and found many different ways of doing so.

For including another JSP into a JSP I used the following command in FirstJSP.jsp

<jsp:include page=”SecondJSP.jsp”/>

I searched for it and found many different ways of doing so.

1. JSP:Param

<jsp:include page=“SecondJSP.jsp” />

<jsp:param name=“name” value=“Sulabh Jain” />

<jsp:param name=“age” value=“24″ />

</jsp:include>

The above code will help in setting the values of param1 and param2 as value1 and value2

which can be called or retrieved in SecondJSP.jsp using,

<%

String myName= request.getParameter(“name”)

String myAge= request.getParameter(“age”)

%>

These can be printed in JSP using ‘equals to’ sign (=) with percentage tags,

My Name  is <%=myName%>
My   Age    is <%=myAge%>

2. JSP:Forward

It also behaves similar to jsp:param

<jsp: forward page=”SecondJSP.jsp”>
<jsp: param name=”name” value=”Amar Patel”/>
<jsp: param name=”age” value=”15″/>
</jsp: forward>

Limitations of above two methods :

This method of passing parameters can be used only for Static variables. For eg.
if we have a dynamic variable declared in JSP we cannot pass it using jsp:param

Here it got little difficult,

Now I need to pass dynamic variables which I defined in the first JSP.
The third method provides solution to it.

3. request.setAttribute

Here we will use an object of javax.servlet.http.HttpServletRequest class named as request.
calling the public abstract void setAttribute(java.lang.String name, java.lang.Object o) method we can set the String name as an attribute.

<%

String name = “Sulabh Jain”;

request.setAttribute(“myName”, name);

%>

Now we can easily get in in SecondJSP as

<%

String nameInNewJSP = request.getAttribute(“myName”).toString();

%>

toString is used here to convert the object into string.

References:

1. Passing parameters, http://www.ezdia.com/Passing_parameters_to_JSP_using_jsp%3Aparam/Content.do?id=968

2. Calling a new JSP, http://www.ezdia.com/Calling_a_new_JSP/Content.do?id=969

3. JSP exercises, http://www.ezdia.com/JSP_Exercises/Content.do?id=806

Please post your comments, on the above provided code, and suggest if there exists better or more options.

Thanks

As I click on a div, one Div opens right below it, and when I click on it again the Div must hide again.

I am providing the HTML code for such functionality.

Hidden Div for row one is appearing

<!DOCTYPE HTML PUBLIC “-//W3C//DTD HTML 4.0 Transitional//EN”>
<HTML>
<HEAD>
<TITLE> Opening and Hiding Divs on Mouse Clicks </TITLE>
<script>
function showHiddenDiv(divid){
var visibility = document.getElementById(divid).style.display;
if(visibility == “none”){
document.getElementById(divid).style.display = ”;
}
else{
document.getElementById(divid).style.display = ‘none’;
}
}
</script>
</HEAD>

<BODY>
<table width=”350″ border=”1″ datasrc=”#cdcat”>

<tr><td id=”row1″ onclick=”showHiddenDiv(‘hiddenRow1′);” align=”center”>Row One</td></tr>
<tr id=”hiddenRow1″ style=”display:none” height=”50″><td align=”center”>”hidden content for row one”</td></tr>
<tr><td id=”row2″ onclick=”showHiddenDiv(‘hiddenRow2′);” align=”center”>Row Two</td></tr>
<tr id=”hiddenRow2″ style=”display:none” height=”50″><td align=”center”>”hidden content for row two”</td></tr>
<tr><td id=”row3″ onclick=”showHiddenDiv(‘hiddenRow3′);” align=”center”>Row Three</td></tr>
<tr id=”hiddenRow3″ style=”display:none” height=”50″><td align=”center”>”hidden content for row three”</td></tr>
<tr><td id=”row4″ onclick=”showHiddenDiv(‘hiddenRow4′);” align=”center”>Row Four</td></tr>
<tr id=”hiddenRow4″ style=”display:none” height=”50″><td align=”center”>”hidden content for row four”</td></tr>

</table>
</BODY>
</HTML>

This code has been used at various places and uses simplest functions possible.

function showHiddenDiv(divid) is used for displaying and hiding the div whose Id has been passed as divid where ever the function is called.

document.getElementById(divid).style.display is used to get the display style status of the Div whose Id is divid. It will be “none” is Div is hidden & it will be blank if the div is Displayed.

References :

1. Drop Down Menu boxes : http://www.ezdia.com/Drop_down_menus_in_HTML/Content.do?id=495

2. All about element display in html : http://www.ezdia.com/All_about_showelementbyid_in_html/Content.do?id=946

3. Working examples with explanations : http://www.ezdia.com/Favorite_Javascripts_for_designers/Content.do?id=947