Developers Arena

ETL Tools

2011-09-15T01:20:00.001-07:00

ETL TOOLS

The times of increasing data-dependence forced a lot of companies to invest in complicated data warehousing systems. Their differentiation and incompatibility led to an uncontrolled growth of costs and time needed to coordinate all the processes. The ETL (Extract, transform, load) tools were created to simplify the data management with simultaneous reduction of absorbed effort.

Depending on the needs of customers there are several types of tools.
One of them perform and supervise only selected stages of the ETL process like data migration tools(EtL Tools , “small t”tools) , data transformation tools(eTl Tools , “capital T”tools).Another are complete (ETL Tools ) and have many functions that are intended for processing large amounts of data or more complicated ETL projects.

Some of them like server engine tools execute many ETL steps at the same time from more than one developer , while other like client engine tools are simpler and execute ETL routines on the same machine as they are developed.
There are two more types. First called code base tools is a family of programing tools which allow you to work with many operating systems and programing languages.The second one called GUI base tools remove the coding layer and allow you to work without any knowledge (in theory) about coding languages.

How do the ETL tools work ? |ETL Tools Selection in Data Warehousing

2011-09-15T01:16:00.000-07:00

How do the ETL tools work?

The first task is data extraction from internal or external sources. After sending queries to the source system data may go indirectly to the database. However usually there is a need to monitor or gather more information and then go to Staging Area . Some tools extract only new or changed information automatically so we dont have to update it by our own.
The second task is transformation which is a broad category:
-transforming data into a stucture wich is required to continue the operation (extracted data has usually a sructure typicall to the source)
-sorting data
-connecting or separating
-cleansing
-checking quality

The third task is loading into a data warehouse.

As you can see the ETL Tools have many other capabilities (next to the main three: extraction , transformation and loading) like for instance sorting , filtering , data profiling , quality control, cleansing , monitoring , synchronization and consolidation.

ETL Process ,ETL Tools Selection in Data Warehousing

2011-09-15T01:14:00.000-07:00

ETL PROCESS

The three-stage ETL process and the ETL tools implementing the concept might be a response for the needs described above.

The ‘ETL’ shortcut comes from 'Extract, transform, and load' – the words that describe the idea of the system. The ETL tools were created to improve and facilitate data warehousing.

The Etl process consists of the following steps:

Initiation
Build reference data
Extract from sources
Validate
Transform
Load into stages tables
Audit reports
Publish
Archive
Clean up

Sometimes those steps are supervised and performed indirectly but its very time-consuming and may be not so accurate.
The purpose of using ETL Tools is to save the time and make the whole process more reliable.

Extract, transform, load |ETL Tools Selection in Data Warehousing

2011-09-15T01:09:00.000-07:00

Nowadays, most companies’ existence depends on data flow. When plenty of information is generally accessible and one can find almost everything he needs, managing became easier than ever before. The Internet simplifies cooperation – time needed to send and receive requested data gets shorter as more and more institutions computerize their resources. Also, the communication between separate corporation departments became easier – no one needs to send normal letters (or even the office boys) as the process is replaced by e-mails. Although the new ways of communication improved and facilitated managing, the ubiquitous computerization has its significant disadvantages.

The variety of data – as positive phenomenon as possible – got a little bit out of control. The unlimited growth of databases’ size caused mess that often slows down (or even disable) data finding process.

It’s all about effective information storing. Uncategorized data is assigned to different platforms and systems. As a consequence, finding wanted data brings a lot of troubles – user needs to know what data he administers, where it is located (and whether he has proper access), finally how to take them out.
Wrong was someone who thought that the hardest task was making decisions basing on data. No – finding data itself is often much more annoying. But users are not the only ones suffering for databases’ overgrowth. The IT departments – usually responsible for keeping the systems work – have to struggle with data in different formats and systems. ‘Keeping it alive’ is extremely time-consuming what delays the company’s work.
Slow (or sometimes omitted at all) transformation of data causes that it’s usually impossible to provide demanded information in demanded time. Formed divergence between data provided and data really existing in the moment of need harms the IT departments’ image.

To achieve better results, companies invest in external systems – computing power and resources. Not enough power causes lacks of synchronization of data. Transporting information between separate units lasts too long to work effectively. On the other side, computing power increasing – that might be an example solution – is expensive and lead to overgrowth of the operation costs.
Supposing that example company managed to prepare well-working database responsible for supporting designated operation. A lot of money and time got spent. Everything seems wonderful until it comes to another operation. Suddenly it appears that once created system doesn’t really fit the requirements of new operation and the best idea is to create a new system from the beginning. Yes, modifications might be made but there is no single developer common for all parts of the projects, so it demands cooperation of at least a few subjects – that hardly disables the idea.

Some List of ETL Tools
----------------------------

Here is a list of the most popular comercial and freeware(open-sources) ETL Tools.
Comercial ETL Tools:

IBM Infosphere DataStage
Informatica PowerCenter
Oracle Warehouse Builder (OWB)
Oracle Data Integrator (ODI)
SAS ETL Studio
Business Objects Data Integrator(BODI)
Microsoft SQL Server Integration Services(SSIS)
Ab Initio
Freeware, open source ETL tools:
Pentaho Data Integration (Kettle)
Talend Integrator Suite
CloverETL
Jasper ETL

What are different ways of Initiating Transaction in CICS?

2011-05-24T11:16:00.000-07:00

We can initiate the transation through MQ and TCP/IP and Terminal and consol.
We can initiate CICS Transaction By giving TRANSACTION ID By giving CICS START Command AUTOMATIC TASK INITIATION(ATI).

Other Methods:

1. By entering Transaction Id(4 bytes)
2.By Start command
3. By Return Trans Id (as in pseudo conv)
4. By Registering Tran ID in plt so that it will be automaticaly dusring CICS start
5. By ATI
6.PF PA keys as defined in PCT

How a CICS Transaction be initiated?

2011-05-24T11:15:00.000-07:00

Involving CICS Control programs and Control tables. a. Trans-id entered in terminal b. TCP with TCT recognizes incoming data c. SCP acquires the storage for the Terminal Input-Output Area (TIOA) d. KCP prepares control data for this task e.KCP, through PCT, tries to find the application program associated with the Trans-id. f. If PPT entry of the application program does not show the resident address of the program, KCP passes control to PCP, which fetches the application program from the load library and places it into the main storage. g. KCP passes control to the application program. h. Application program starts its processing

CEDF,CEMT,CEBR,CECI transactions in CICS

2011-05-24T11:14:00.000-07:00

CEDF :
CICS-supplied Execution Diagnostic Facility transaction. It provides interactive program execution and debugging functions of a CICS programs.
CEMT : CICS-supplied Extended Master Terminal transaction. It displays or manipulates CICS control environment interactively.
CEBR : CICS-supplied Temporary Storage Browse transaction. It displays the content of Temporary Storage Queue ( TSQ ).
CECI : CICS-supplied Command Interpreter transaction. It verifies the syntax of a CICS command and executes the command.

CICS Interview Questions for TCS,Infosys,IBM,HP and Other Companies

2011-05-24T11:10:00.000-07:00

How do you place the cursor on a particular position on the screen? –

Move -1 to the length attribute of the field aand use the CURSOR option.
2. Define the field with IC in the BMS map. Use CURSOR(n m)??

What are the two outputs created as a result of generation of a map? – The map copybook and the load module.
What is the difference between physical map and symbolic map? –

              The physical map is the load module and the symbolic map is the data structure.

5.     How do you protect a field from being overlaid? -
              What is the attribute byte? –
              Defines the display/transmission of field. most cases is an output field from the program.

How do you use extended attributes ? Define EXTATT=YES and the correct terminal type.
What are the 3 working storage fields used for every field on the map? – Length, attribute and input/output field.
What is MDT? What are FSET, FRSET ? Modified Data Tag. Bit in the attribute byte indicating modification of field on screen. Happens on an input operation.
FSET. Sets MDT on to ensure field is transmitted. Happens on an output operation.
FRSET. Resets MDT. Until this happens, field continues to be sent.
What is the use of DSECT parameter in BMS? Is the parameter to generate a symbolic map.
Do you receive the attribute byte in the symbolic map? On EOF yes.
How do you make your BMS maps case sensitive? Use ASIS???
What is effect on RECEIVE MAP when PF key is pressed? Data transmission may happen,
PA key is pressed? Data transmission will not happen.
What are SEND MAP MAPONLY & SEND MAP DATAONLY ? MAPONLY - to send the map alone, without any data. Eg: used for sending Menu screens.
DATAONLY - to send data alone, without sending the screen layout again. Eg: used for refreshing the screen data.
What is the difference between a PF key & a PA key ? PF keys wake up the task and transmit modified data, PA keys only wake up the task.
Name the macros used to define the following: MAP MAPSET FIELD
DFHMSD DFHMDI DFHMDF
Can you use OCCURS in a BMS map? If you do, what are the issues related with its use? Yes. cannot use group by clause???
Can you define multiple maps in a BMS mapset? Yes.
How is the storage determined in the symbolic map, if you have multiple maps? Storage for maps redefine the first. This means largest map has to be the first.
What is the meaning of BMS length of field = 0? Data was not entered in the field
Can you simply check if length = 0 for checking if a field was modified? No, not if ERASE EOF was used.

Difference between single quotes and double quotes in unix

2011-05-24T11:06:00.001-07:00

Enclosing characters in single quotes (`'') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

Enclosing characters in double quotes (`"') preserves the literal value of all characters within the quotes, with the exception of `$', ``', and `\'. The characters `$' and ``' retain their special meaning within double quotes (*note Shell Expansions::). The backslash retains its special meaning only when followed by one of the following characters: `$', ``', `"', `\', or `newline'. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash.

The special parameters `*' and `@' have special meaning when in double quotes (*note Shell Parameter Expansion::).

What is the difference between CI( CONTROL INTERVAL)and CA(CONTROL AREA)

2011-05-23T09:33:00.000-07:00

The CI is a single unit of transfer from secondary to main memory. Inside it will have freespace to store the records of fixed or variable size.
If u try to insert a new record to a ci which is not having any space to hold at that time the half of the record from this ci will be moved to some free ci. This splitting of ic in case of ci full is called CI splitt.
A collection of ci is called as CA. During the ci splitt if it is unable to find the free ci at that time the entire CA will be splitted into two and half will be moved to some free CA. This is called CA splitt. This is the primary mechanism of the VASM to store all the records inside your ksds in sorted order.

What is the procedure for running the batch job in mainframe?

2011-05-23T09:32:00.000-07:00

Friends let me explain with an example:--

EG:-Adding two numbers

Step-1:- We need to compile that program and that compiled program(i.e Error free code) we need to keep in one Load Library (EG:-ABCD.GEEK.LOADLIB)

Step-2:- Now we need to execute this program giving PGM=ADD(Program Name) and the Load Libray name(ABCD.GEEK.LOADLIB)

Step-3:- Now In SYSOUT we can view the results

That's it this the process how to compile and execute a batch program

How do you convert flat files to VSAM files

2011-05-23T09:30:00.000-07:00

Let's be a little more precise.We will take the tack that you will be creating a KSDS file.First, you need to know which field/s you will be using as the primary key and if you need, alternate keys. Sort the records by that field/s.Use IDCAMS REPRO to load the flat file into the VSAM file.

difference between Intra partition TDQ and Extra partition TDQ?

2011-05-23T09:28:00.001-07:00

INTRA PARTITION TD QUEUEs It is a group of sequential records which are produced by the same and / or different transactions within a CICS region. These Qs are stored in only one physical file ( VSAM ) in a CICS region, which is prepared by the system programmer. Once a record is read from a queue, the record will be logically removed from the queue; that is the record cannot be read again. EXTRA PARTITION TD QUEUEs It is a group of sequential records which interfaces between the transactions of the CICS region and the systems outside of CICS region. Each of these TDQs is a separate physical file, and it may be on the disk, tape, printer or plotter.

What is Pseudo-conversation?How it is achieved in CICS Programs?

2011-05-23T09:27:00.001-07:00

The programming methodology in which the task will not wait for the terminal users response, but frees and resources after sending the message is call a PSEDUDO Conversational Technique. Terminating the task every time the application needs a response from the user and then starting the next transaction when the user presses a attention key is PSEUDO Conversational Processing.

Generally the I/O request requiring a user input takes 1000 times more time than CPU processing time. So, the idea is to free the current CPU resources till the user gives his response and initiate the same after receiving the response from user. In the meantime, CPU engages itself in processing other programs/transactions. This helps multitasking concept and hnce faster and concurrent processing.

What is the difference between the XCTL and LINK commands?

2011-05-23T09:26:00.001-07:00

XCTL : This command passes the control to the same level.Control does not return back after this command is executed.
Syntax:
EXEC CICS XCTL
PROGRAM(PGMNAME)
COMMAREA(DATA-NAME)
END-EXEC.

LINK: link transfers the control to the next lower level.Link command also expects the control to return back.
Syntax is same use link in place of XCTL.

Both XCTL and LINK are control transfer commands.

XCTL transfers the control to another program (called program) at the level below to the calling program. Control is never returned back to the called program.

LINK transfers the control to another program (called program) at the same of the calling program. Control is expected to return back to the called program.

CICS Error Code AIEV,IBM AEIV due to length error of COMMAREA on EXEC CICS LINK command

2011-05-23T09:25:00.000-07:00

Problem(Abstract)

You have a modified Node Error Program, DFHZNEP, that issues an EXEC CICS LINK command to another application. This program has worked for many years until upgrading to z/OS 1.8. Now, the program abends with abend code AEIV indicating that there is a length error (LENGERR).

Cause

EXEC CICS LINK command has LENGTH parameter coded incorrectly.

Diagnosing the problem

Your Link command is coded similar to the following:

EXEC CICS LINK PROGRAM('SSCCERR')
COMMAREA(NEPCABEG)
LENGTH(NEPCALEN)

NEPCALEN is defined using the DFHNEPCA macro as an equate:

NEPCALEN EQU *-NEPCABEG

Thus, NEPCALEN is equated to a full word binary value of x'0000009E', causing the length value to be picked up at location x'9E'. The CICS Translator loads the address of the equated field into the parameter list, and this actually yields an address of x'0000009E'.

Prior to upgrading to z/OS V1.8, location x'9E' either contained a half word of zeros or a half word value not having the left most bit turned on. After upgrading, this storage contains a half word value with the left most bit turned on, thus thought to be a negative value. The negative value causes LENGERR and abendAEIV.

Source :IBM

How can connect sql server 2005 with java in java?

2011-05-21T23:58:00.000-07:00

In order to connect MySQL database with Java one need to use below mentioned sample script:

<%@ page import="java.sql.*" %>
<%@ page import="com.mysql.jdbc.Driver" %>

<%!
Class.forName("com.mysql.jdbc.Driver").newInstance ();
java.sql.Connection conn;
conn = DriverManager.getConnection(
"jdbc:mysql:///?user=&password=") ;
%>
Use the JDBC functionality to open a connection to SQL Server.
You can read up on how here:

How to connect MySQL Database from Java

2011-05-21T23:56:00.000-07:00

Connect to an Oracle database with JDBC in Java

2011-05-21T23:54:00.000-07:00

JDBC mean Java Database Connectivity. In java, using JDBC drivers, we can connect to database.
Steps to connect to JDBC.
1) Load the driver, using Class.forName(DriverName);
2) Get the connection object, Connection con = Driver.getConnection(loaded driver name);
3) Create a SQL statement, Statement s = con.createStatement();
4) Create Resultset object using the statement created above, ResultSet rs = s.executeQuery("sql statement");

Iterate the result set to get all the values from the database.

Finally don't miss this
5) s.close();
6) con.close() ;

Example:

import java.sql.*;

public class TestDBOracle {

  public static void main(String[] args)
      throws ClassNotFoundException, SQLException
  {
    Class.forName("oracle.jdbc.driver.OracleDriver");
    //
    // or
    // DriverManager.registerDriver
    //        (new oracle.jdbc.driver.OracleDriver());

        String url = "jdbc:oracle:thin:@//server.local:1521/prod";
    //               jdbc:oracle:thin:@//host:port/service
    // or
    // String url = "jdbc:oracle:thin:@server.local:1521:prodsid";
    //               jdbc:oracle:thin:@host:port:SID
    //
    //  SID  - System ID of the Oracle server database instance.
 //         By default, Oracle Database 10g Express Edition
 //         creates one database instance called XE.
 //         ex : String url = "jdbc:oracle:thin:@myhost:1521:xe";



    Connection conn =
         DriverManager.getConnection(url,"scott","tiger");

    conn.setAutoCommit(false);
    Statement stmt = conn.createStatement();
    ResultSet rset =
         stmt.executeQuery("select BANNER from SYS.V_$VERSION");
    while (rset.next()) {
         System.out.println (rset.getString(1));
    }
    stmt.close();
    System.out.println ("Ok.");
  }
}

XML and using Xerces parser for Java to generate and parse XML

2011-05-21T23:51:00.000-07:00

his small tutorial introduces you to the basic concepts of XML and using Xerces parser for Java to generate and parse XML.
The intended audience are XML beginners with knowledge of Java.
DOM (Document Object Model ) parser - Tree Structure based API:
The Dom parser implements the dom api and it creates a DOM tree in memory for a XML document
5.2 SAX (Simple API For XML ) parser - Event Based API
The SAX parser implements the SAX API and it is event driven interface. As it parses it invokes the callback methods
5.3 When to use DOM parser

Manipulate the document
Traverse the document back and forth
Small XML files

Drawbacks of DOM parser
Consumes lot of memory 5.4 When to use SAX parser

No structural modification
Huge XML files

5.5 Validating And Non Validating
DOM and SAX can either be a validating or a non validating parser.
A validating parser checks the XML file against the rules imposed by DTD or XML Schema.
A non validating parser doesn't validate the XML file against a DTD or XML Schema.
Both Validating and non validating parser checks for the well formedness of the xml document

View Complete Tutorial here

How to create XML document using java,Java Architecture for XML Binding (JAXB)

2011-05-21T23:48:00.000-07:00

In this section, you will learn to create a XML document using the DOM APIs. This XML document uses 1.0 version and UTF-8 encoding.

To work with an XML document it is easy to do so, if you have a document object in place and the XML document loaded in it. Yes, java too has APIs for working with an XML document. With this API, you can navigate through the XML document, or create an XML document from the scratch.

import org.w3c.dom.Document;
import org.w3c.dom.DOMException;
import org.w3c.dom.Element;

DocumentBuilderFactory fact = DocumentBuilderFactory.newInstance();
DocumentBuilder bd = fact.newDocumentBuilder();
doc = bd.newDocument();
Element rt = (Element) doc.createElement("rtElement");
document.appendChild(root);
rt.appendChild( doc.createTextNode("Some") );
. . .
. . .

Proper imports are to be done to work with the classes and the methods needed for working with an XML document. The above code sample would give you an idea on how to go about it. Methods like createElement of the Element object are used to create elements.

And these elements are added to the root node using the method appendChild. Lots of sample codes are available in the internet on this topic.

Here is Java File: CreatXMLFile.java

Download this example.

How to Create Running Total Field in Crystal Reports

2011-05-21T23:43:00.000-07:00

Overview

A running total field in a Crystal report is a summary field that allows you to control how and when the summary is calculated, and when it is reset.
In this tutorial, you will create a running total field and insert it into a report.

View Complete Tutorial here

VB.NET Crystal Reports Summary Fields

2011-05-21T23:30:00.000-07:00

The following C# - Crystal Reports section describes how to add a summary field in the Crystal Reports
In the Crystal Reports designer view window, right click on the Report Footer , just below the Total field and select Insert -> Summary .

Then you will get a screen , select the Total from the combo box and select Sum from next Combo Box , and summary location Grand Total (Report Footer) . Click Ok button

Now you can see @Total is just below the Total field in the report Footer

Now the designing part is over and the next step is to call the Crystal Reports in C# and view it in Crystal Reports Viewer control .

Select the default form (Form1.cs) you created in C# and drag a button and a CrystalReportViewer control to your form .

You have to include CrystalDecisions.CrystalReports.Engine in your C# Source Code.

using CrystalDecisions.CrystalReports.Engine;

using CrystalDecisions.Shared;

How to create a login page in Aspnet with sqlserver

2011-05-21T23:21:00.000-07:00

First we have to create a database for storing the login information

Open the Sqlserver Management studio

Create a new database and change the database name as db_Logininformation

Create a New table in that database(db_Logininformation) and change the table name as tab_userinformation
Now, create the following fields

UserID int PrimaryKey
username varchar(50)
Password varchar(50)

Next, Enter the some usernames and paasword directlu in the database for checking purpose

and save the table

Next ,open the Microsoft visual studio 2008

Next,select the Aspnet web application and change the name as LoginPage

Next,come to the design page of LoginPage and drag and drop two labels,two textboxes and button

next,come to the code page of the Login.aspx.cs

Write the following codein the page event

protected void Button1_Click(object sender, EventArgs e)
{
con = new SqlConnection(ConfigurationManager.ConnectionStrings["connectionstring"].ToString());
con.Open();
com = new SqlCommand("select Password from userinformation where Username='" + txt_uname.Text + "'", con);
dr = com.ExecuteReader();
if (!dr.Read())
{
Response.write("Invalid User");
}
else
{
if (dr[0].ToString() == txt_pwd.Text)
{
Response.Redirect("~/mainPage.aspx");
}
else
{
Response.write("Wrong Password");
txt_pwd.Focus();
}
}
}

How to Create ASP.NET Session Login Without Cookies

2011-05-21T23:10:00.000-07:00

The following describes the easiest way I have found to force users to log into an ASP.NET website for each session but not require them to accept cookies. You must do the following things.

Create a Web.config file with the appropriate entries to allow session state management.
Create a well formed Global.asax file with the code below included in it.
Create a login page to authenticate users against a database or whatever method you desire.

' Fires when the session is started and sets the default loggedin state to ""

Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
Session("Loggedin") = ""
CheckLoggedIn()
End Sub

Collapse

' Called when the request has been process by the Request Handler and 

' HttpSessionState is available [This is the key piece of code that forces 

' the user is login check with each page request]

Sub Application_OnPostRequestHandlerExecute()
CheckLoggedIn()
End Sub

Collapse

'Check that the user is logged in.

Sub CheckLoggedIn()
'If the user is not logged in and you are not currently on the Login Page.

If Session("LoggedIn") = "" And InStr(Request.RawUrl, "Login.aspx") = 0 Then
Response.Redirect("~/Login/Login.aspx")
End If
End Sub

Finally create a Login.aspx file that authenticates the user. If the user is allowed in, set: Session("Loggedin") = "Yes"

That's all there is to it. Hope this helps! Enjoy!

Create Login Page with ASP Connected to Database

2011-05-21T23:02:00.000-07:00

<%
dim username, password, loginButton
username=TRIM(Request("username"))
password=TRIM(Request("password"))
logButton=Request("loginButton")="Login"
if logButton then
   Dim Con, sql, rec
   set Con = Server.CreateObject("ADODB.Connection")
   Con.Open "DRIVER={Microsoft Access Driver (*.mdb)}; DBQ=" & Server.MapPath("database.mdb")
   'Select the record matching the username.
   sql = "SELECT * FROM tblusers WHERE UCase(username)='
"& UCase(username) & "' AND UCase(password)=' " & UCase(password) & " ' "
   set rec=Con.execute(sql)
   'If no match found, EOF is not true.
   if NOT rec.EOF then
      Response.Redirect("somepage.asp") 'Change to page redirect to after login
   else
      blankError="Invalid username." 'EOF is true, no match found.
   end if
end if
%>

<html>
<head>
<title>Login</title>
</head>
<body>
<form name="productForm" method="post" action="<%=Request.ServerVariables("URL")%>">
<center>
<table border =1>
<tr><td colspan="2">
<%

if blankError<>"" then
Response.Write("<center><font color='red' size='3'>"&blankError&"</font></center>")
end if
%>
</td></tr>
<tr>
<td><Strong><font face="courier new" size="3">Username:</font></strong></td>
<td><input type="text" name="username" size="35"></td>
</tr>
<tr>
<td><Strong><font face="courier new" size="3">Password</font></strong></td>
<td><input type="password" name="password" size="35"></td>
</tr>
<tr><td colspan="2" align="center"><input type="submit" name="loginButton" value="Login">
<input type="reset" name="reset" value="Clear"></td>
</tr>
</table>
</center>
</form>
</body>
</html>

Fixing the java.lang.OutOfMemoryError: PermGen space error.

2011-05-06T07:21:00.000-07:00

Fixing the java.lang.OutOfMemoryError: PermGen space error.

Start your JVM with -XX:MaxPermSize=XXXm where XXX is a number like 128, 256. This will allow the JVM to allocate XXX megabytes of memory for the PermGen space if that space is needed. Here are the quick guides to configuring memory options for some popular containers. If you still have issues see my other posts about tracking down your possible memory leaks.

Explaining java.lang.OutOfMemoryError: PermGen space

2011-05-06T07:20:00.000-07:00

Most probably, a lot of Java developers have seen OutOfMemory error one time or other. However these errors come in different forms and shapes. The more common is: "Exception in thread "main" java.lang.OutOfMemoryError: Java heap space" and indicates that the Heap utilization has exceeded the value set by -Xmx. This is not the only error message, of this type, however.
One more interesting flavor of the same error message, less common but hence even more troublesome is: "java.lang.OutOfMemoryError: PermGen space". Most of the memory profiler tools are unable to detect this problem, so it is
even more troublesome and therefor - interesting.
To understand this error message and fix it, we have to remember that, for optimized, more efficient garbage-collecting Java Heap is managed in generations - memory segments holding objects of different ages. Garbage collection algorithms in each generation are different. Objects are allocated in a generation for younger objects - the Young Generation, and because of infant mortality most objects die there. When the young generation fills up it causes a Minor Collection. Assuming high infant mortality, minor collections are garbage-collected frequently. Some surviving objects are moved to a Tenured Generation. When the Tenured Generation needs to be collected there is a Major Collection that is often much slower because it involves all live objects. Each generation contains variables of different length of life and different GC policies are applied to them.
There is a third generation too - Permanent Generation. The permanent generation is special because it holds meta-data describing user classes (classes that are not part of the Java language). Examples of such meta-data are objects describing
classes and methods and they are stored in the Permanent Generation. Applications with large code-base can quickly fill up this segment of the heap which will cause java.lang.OutOfMemoryError: PermGen no matter how high your -Xmx and how much memory you have on the machine.
Sun JVMs allow you to resize the different generations of the heap, including the permanent generation. On a Sun JVM (1.3.1 and above) you can configure the initial permanent generation size and the maximum permanent generation size.

To set a new initial size on Sun JVM use the -XX:PermSize=64m option when starting the virtual machine. To set the maximum permanent generation size use -XX:MaxPermSize=128m option. If you set the initial size and maximum size to equal values you may be able to avoid some full garbage collections that may occur if/when the permanent generation needs to be resized. The default values differ from among different versions but for Sun JVMs upper limit is typically 64MB.
Some of the default values for Sun JVMs are listed below.

JDK 1.3.1_06	Initial Size	Maximum Size
Client JVM	1MB	32MB
Server JVM	1MB	64MB

JDK 1.4.1_01	Initial Size	Maximum Size
Client JVM	4MB	64MB
Server JVM	4MB	64MB

JDK 1.4.2	Initial Size	Maximum Size
Client JVM	4MB	64MB
Server JVM	16MB	64MB

Java JVM GC, PermGen, and Memory Options

2011-05-06T07:19:00.000-07:00

when I ran into a situation with some code where we were running out of PermGen space I had to do some reading. In the JVM, PermGen holds the metadata about classes that have been loaded/created. This information is garbage collected like the other parts of the heap, however there are rough edges that can prevent this from happening, class loaders in particular (which I won’t discuss here.) Generally, the amount of PermGen space needed is small in relation to the rest of the heap and default JVM values should work for you.

Java Exhausted Result Set / Oracle & Websphere 5.1|java.sql.SQLException: Exhausted Resultset

2011-05-06T07:17:00.000-07:00

I am experiencing some problems with a simple query on a thin oracle driver. I am using a connection from the application server connection pool on a cluster machine. Basically this is my code:

try {
conn = DAOFactory.getConnection();
stmt = conn.prepareStatement(SELECT_STATEMENT);
stmt.setInt(1, var);
stmt.setFetchSize(max);
rs = stmt.executeQuery();

while( rs.next() ) {
long id = rs.getLong(1);
}

All this I incorporate into a try/catch block like this:

} catch(SQLException e) {
// some exception handling
} finally {
try {
if(stmt!=null)
stmt.close();
if(rs!=null)
rs.close();
if(conn!=null)
conn.close();
} catch (SQLException e1) {
// some exception handling
}
}

After some runs I get sporadically the “Exhausted Resultset” Exception at line rs.getLong(1). This indicates, that my result set is closed. This will imply that rs.next() returns true even if there are no results! This kind of strange behaviour I have already noticed by some other users.

My question: How can I ensure that rs.next() will return false if the result set is empty? Does anybody else encounter this problem before and have a smooth solution?

How to sort list in reverse order

2011-04-30T00:58:00.003-07:00

To sort the elements of the List in the reverse natural order of the strings, get a reverse Comparator from the Collections class with reverseOrder(). Then, pass the reverse Comparator to the sort() method.
List list = new ArrayList();
Comparator comp = Collections.reverseOrder();
Collections.sort(list, comp)

What is difference between Arrays and ArrayList?

2011-04-30T00:58:00.001-07:00

Arrays are created of fix size whereas ArrayList is of not fix size. It means that once array is declared as :
int [] intArray= new int[6];
intArray[7] // will give ArraysOutOfBoundException.
Also the size of array cannot be incremented or decremented. But with arrayList the size is variable.
Once the array is created elements cannot be added or deleted from it. But with ArrayList the elements can be added and deleted at runtime.
List list = new ArrayList();
list.add(1);
list.add(3);
list.remove(0) // will remove the element from the 1st location.
ArrayList is one dimensional but array can be multidimensional.
int[][][] intArray= new int[3][2][1]; // 3 dimensional array

What is difference between HashMap and HashTable?

2011-04-30T00:57:00.002-07:00

Both collections implements Map. Both collections store value as key-value pairs. The key differences between the two are
1. 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.
2. 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. • Fail-safe - “if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException”
3. HashMap permits null values and only one null key, while Hashtable doesn't allow key or value as null.

How can Arraylist be synchronized without using Vector?

2011-04-30T00:57:00.000-07:00

Arraylist can be synchronized using:
• Collection.synchronizedList(List list)
Other collections can be synchronized:
• Collection.synchronizedMap(Map map)
• Collection.synchronizedCollection(Collection c)

What is difference between ArrayList and vector?

2011-04-30T00:56:00.000-07:00

Synchronization - ArrayList is not thread-safe whereas Vector is thread-safe. In Vector class each method like add(), get(int i) is surrounded with a synchronized block and thus making Vector class thread-safe.
Data growth - Internally, both the ArrayList and Vector hold onto their contents using an Array. When an element is inserted into an ArrayList or a Vector, the object will need to expand its internal array if it runs out of room. A Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent.

Oracle PL/SQL WHERE CURRENT OF & FOR UPDATE

2011-04-30T00:54:00.000-07:00

WHERE CURRENT OF & FOR UPDATE

The WHERE CURRENT OF clause is used in some UPDATE and DELETE statements.

The WHERE CURRENT OF clause in an UPDATE or DELETE statement states that the most recent row fetched from the table should be updated or deleted. We must declare the cursor with the FOR UPDATE clause to use this feature.

Inside a cursor loop, WHERE CURRENT OF allows the current row to be directly updated.

When the session opens a cursor with the FOR UPDATE clause, all rows in the return set will hold row-level exclusive locks. Other sessions can only query the rows, but they cannot update, delete, or select with FOR UPDATE.

Oracle provides the FOR UPDATE clause in SQL syntax to allow the developer to lock a set of Oracle rows for the duration of a transaction.

The syntax of using the WHERE CURRENT OF clause in UPDATE and DELETE statements follows:

WHERE [CURRENT OF cursor_name | search_condition]

Oracle PL/SQL: SQL ROLLUP

2011-04-30T00:49:00.001-07:00

SQL ROLLUP

You can use WITH ROLLUP to generate a summary row for each group.

Example 1 of ROLLUP

Suppose we have the following table - Sales:

EmpId	Yr	Sales
1	2005	12000.00
1	2006	18000.00
1	2007	25000.00
2	2005	15000.00
2	2006	6000.00
3	2006	20000.00
3	2007	24000.00

A simple Group by results in:

SELECT Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY Yr

Yr	Sales
2005	27000.00
2006	44000.00
2007	49000.00

A simple group by with ROLLUP results in:

SELECT Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY Yr WITH ROLLUP

Yr	Sales
2005	27000.00
2006	44000.00
2007	49000.00
NULL	120000.00

Another example of group by with ROLLUP:

SELECT EmpId, Yr, SUM(Sales) AS Sales
FROM Sales
GROUP BY EmpId, Yr WITH ROLLUP

EmpId	Yr	Sales
1	2005	12000.00
1	2006	18000.00
1	2007	25000.00
1	NULL	55000.00
2	2005	15000.00
2	2006	6000.00
2	NULL	21000.00
3	2006	20000.00
3	2007	24000.00
3	NULL	44000.00
NULL	NULL	120000.00

Dataset table row count - OLEDB Data Source VB.Net

2011-04-30T00:45:00.000-07:00

The DataSet contains copy of the data we requested through the SQL statement. The DataSet consists of DataTableCollection and their DataRelationCollection. The DataTableCollection contains zero or more DataTable objects. The data inside Table is in the form of Rows and Columns . The following VB.NET source code shows how to find the number of rows in a table that resides in the Dataset from OLEDB Data Source.

Imports System.Data.OleDb
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)

Handles Button1.Click
        Dim connetionString As String
        Dim connection As OleDbConnection
        Dim oledbAdapter As OleDbDataAdapter
        Dim ds As New DataSet
        Dim sql As String

        connetionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=Your mdb filename;"
        sql = "Your SQL Statement Here"

        connection = New OleDbConnection(connetionString)
        Try
            connection.Open()
            oledbAdapter = New OleDbDataAdapter(sql, connection)
            oledbAdapter.Fill(ds, "OLEDB Temp Table")
            oledbAdapter.Dispose()
            connection.Close()

            MsgBox("number of Row(s)   -   " & ds.Tables(0).Rows.Count)

        Catch ex As Exception
            MsgBox("Can not open connection ! ")
        End Try
    End Sub
End Class

Using named arguments in JavaScript functions

2011-04-20T07:26:00.000-07:00

Normally, a JavaScript function takes an list of arguments, with the order of each argument predetermined. As an example, the parseInt() function takes two arguments, a string representing a number, and the radix of that number. You need to specify those in exactly that order, first a string, then the number, or at the very least, just the first argument. However, ponder a function where you wish to specify only the second argument, or both of them in arbitrary order - the deficiency of the JavaScript arguments model prohibits this. In this tutorial, I'll teach you how to modify JavaScript functions to accept named arguments, with which you can specify in any order, as named arguments don't rely on order, but instead their name when passing into functions.
JavaScript functions do not natively support naming arguments, so the simplest way to simulate this functionality is via object literals. Let me illustrate with a custom parseInt() function that can accept its two arguments in any order:

JavaScript Function with Named Arguments
// Define function to take one "argument", which is in fact an object:
function fnParseInt( oArg ){ 
return parseInt( oArg.number, oArg.radix );
}

// Which you then call like this (pass in an object literal):
fnParseInt( { number : 'afy', radix : 36 } );

The key to the above is passing in an object literal as the function's sole parameter instead of separate, "authentic" parameter(s). Now, this is a pretty useless example, but it illustrates my point of creating a function that can accept arguments in any order and via a more intuitive name:value format. Such functions are much more robust than standard ones, not to mention user friendly in cases where the function takes on a lot of parameters.

Difference between Views & Materialized views in Oracle

2011-04-20T07:24:00.000-07:00

Difference between Views & Materialized views

Materialized views are disk based and update periodically base upon the query definition.

Views are virtual only and run the query definition each time they are accessed.

Views evaluate the data in the tables underlying the view definition at the time the view is queried. It is a logical view of your tables, with no data stored anywhere else. The upside of a view is that it will always return the latest data to you. The downside of a view is that its performance depends on how good a select statement the view is based on. If the select statement used by the view joins many tables, or uses joins based on non-indexed columns, the view could perform poorly.

Materialized views are similar to regular views, in that they are a logical view of your data (based on a select statement), however, the underlying query resultset has been saved to a table. The upside of this is that when you query a materialized view, you are querying a table, which may also be indexed. In addition, because all the joins have been resolved at materialized view refresh time, you pay the price of the join once (or as often as you refresh your materialized view), rather than each time you select from the materialized view. In addition, with query rewrite enabled, Oracle can optimize a query that selects from the source of your materialized view in such a way that it instead reads from your materialized view. In situations where you create materialized views as forms of aggregate tables, or as copies of frequently executed queries, this can greatly speed up the response time of your end user application. The downside though is that the data you get back from the materialized view is only as up to date as the last time the materialized view has been refreshed.

Materialized views can be set to refresh manually, on a set schedule, or based on the database detecting a change in data from one of the underlying tables. Materialized views can be incrementally updated by combining them with materialized view logs, which act as change data capture sources on the underlying tables.

Materialized views are most often used in data warehousing / business intelligence applications where querying large fact tables with thousands of millions of rows would result in query response times that resulted in an unusable application

Difference between HAVING CLAUSE & WHERE CLAUSE in Oracle

2011-04-20T07:22:00.000-07:00

SQL - Difference between HAVING CLAUSE & WHERE CLAUSE

1. HAVING specifies a search condition for a group or an aggregate function used in SELECT statement. The WHERE clause specifies the criteria which individual records must meet to be selected by a query. It can be used without the GROUP BY clause. The HAVING clause cannot be used without the GROUP BY clause.

2. The WHERE clause selects rows before grouping. The HAVING clause selects rows after grouping.

3. The WHERE clause cannot contain aggregate functions. The HAVING clause can contain aggregate functions

Disabling Triggers in Oracle PL/SQL

2011-04-18T08:21:00.002-07:00

Disabling Triggers

To disable or enable a trigger:

alter trigger {disable | enable};

Viewing Defined Triggers in Oracle

2011-04-18T08:21:00.000-07:00

Viewing Defined Triggers

To view all the defined triggers, use:

select name_of_trigger from user_triggers;

For more details on a particular trigger:

select trigger_type, triggering_event, table_name, referencing_names, trigger_body
from user_triggers
where trigger_name = '';

Displaying Trigger Errors in Oracle

2011-04-18T08:19:00.001-07:00

Displaying Trigger Errors

If we get a message Warning: Trigger created with compilation errors. you can check the error messages with:

Show errors trigger ;

You can also type, SHO ERR (SHOW ERRORS) to see the most recent compilation error.

Types of Triggers in Oracle

2011-04-18T08:18:00.000-07:00

Row Triggers and Statement Triggers: A statement trigger is fired once on behalf of the triggering statement, regardless of the number of rows in the table that the triggering statement affects. A row trigger fires once for each row affected by the triggering event.

BEFORE and AFTER Triggers: BEFORE triggers run the trigger action before the triggering statement is run. AFTER triggers run the trigger action after the triggering statement is run.

INSTEAD OF Triggers: INSTEAD OF triggers describe how to perform insert, update, and delete operations against views that are too complex to support these operations natively. INSTEAD OF triggers allow applications to use a view as the sole interface for all SQL operations (insert, delete, update and select).

Triggers on System Events and User Events: You can use triggers to publish information about database events to subscribers. System events are for example Database startup and shutdown, Data Guard role transitions etc and User Events are User logon and logoff, DDL statements (CREATE, ALTER, and DROP) etc

Example of PL/SQL trigger in Oracle

2011-04-18T08:17:00.000-07:00

In the below example line 2 is A triggering event or statement, lines 4-9 are A trigger action.

Example of creating a trigger based on the following two tables:

CREATE TABLE T1 (a INTEGER);
CREATE TABLE T2 (b INTEGER);

We will create a trigger that may insert a tuple into T2 when a tuple is inserted into T1. The trigger checks if the inserted row in T1 is has a value less than 5 only then a tuple is inserted in T2.

1 CREATE TRIGGER tr1
2 AFTER INSERT ON T1
3 REFERENCING NEW AS newRow
4 FOR EACH ROW
5 WHEN (newRow.a <= 5)
6 BEGIN
7 INSERT INTO T2
VALUES(:newRow.a);
8 END tr1;
9 .
10 run;

PL/SQL Triggers in Oracle

2011-04-18T08:15:00.000-07:00

PL/SQL Triggers

A PL/SQL trigger is a construct in PL/SQL that runs or "triggered" on event of changes being made to a table in the database. The triggering event is a INSERT, UPDATE or DELETE done on a table. The trigger can be made so it can be "fired" either BEFORE or AFTER the Data Manipulation Language is executed.

A database trigger is a block of code that is automatically executed in response to certain events.
Triggers are executed implicitly whenever the triggering event happens.
The triggering event is an INSERT, DELETE, or UPDATE command.
The timing can be either BEFORE or AFTER, INSTEAD OF trigger.

The trigger can be either row-level or statement-level, where the former fires once for each row affected by the triggering statement and the latter fires once for the whole statement.

You can write triggers that fire whenever one of the following operations occurs:

DML statements (INSERT, UPDATE, DELETE) on a particular table or view, issued by any user
DDL statements (CREATE or ALTER primarily) issued either by a particular schema/user or by any schema/user in the database
Database events, such as logon/logoff, errors, or startup/shutdown, also issued either by a particular schema/user or by any schema/user in the database

A trigger has three basic parts:

A triggering event or statement
A trigger restriction
A trigger action

Oracle PL/SQL Collections

2011-04-18T08:14:00.000-07:00

Records

Table Based Records Cursor Based Records Programmer-defined Records PL/SQL Tables

Varrays

Nested Tables

PL/SQL Function Example 2 in Oracle

2011-04-17T03:12:00.001-07:00

Example 2 of PL/SQL Function:

create or replace function find_area
(Len in number, Wid in number)
return number
as
varea number;
begin
varea := Len * Wid;
return varea;
end;

SQL> select find_area (10, 30) area from dual;

AREA
---------
300

PL/SQL Function Example in Oracle

2011-04-17T03:11:00.001-07:00

Example 1 of PL/SQL Function:

Creating a Function: Examples The following statement creates the function Ask_Balance on the sample table cust_orders (the PL/SQL is in italics):

CREATE FUNCTION Ask_Balance(acc_num IN NUMBER) 
   RETURN NUMBER 
   IS acc_bal NUMBER(11,2);
   BEGIN 
      SELECT order_total 
      INTO acc_bal 
      FROM cust_orders 
      WHERE customer_id = acc_num; 
      RETURN(acc_bal); 
    END;
/

The Ask_Balance function returns the balance of a specified account.

When you call the function, you must specify the argument acc_num, the number of the account whose balance is sought. The datatype of acc_num is NUMBER.

The function returns the account balance. The RETURN clause of the CREATE FUNCTION statement specifies the datatype of the return value to be NUMBER.

The function uses a SELECT statement to select the balance column from the row identified by the argument acc_num in the cust_orders table. The function uses a RETURN statement to return this value to the environment in which the function is called.

The function created in the preceding example can be used in a SQL statement. For example:

SELECT Ask_Balance(165) FROM DUAL;

Ask_Balance(165)
------------
2519

[Source]

Syntax of PL/SQL function in Oracle

2011-04-17T03:10:00.000-07:00

PL/SQL Function

There are two types of PL/SQL blocks: named block and an anonymous block.
There are two types of named blocks: Functions and Procedures

FUNCTION name [(parameter[, parameter, …])] RETURN
datatype IS
[local declarations]
BEGIN
executable statements
[EXCEPTION
exception handlers]
END [name];

PL/SQL Function in Oracle

2011-04-17T03:09:00.000-07:00

PL/SQL Function

There are two types of PL/SQL blocks: named block and an anonymous block.
There are two types of named blocks: Functions and Procedures

In PL/SQL a Function takes zero or more parameter values and returns one value.

Syntax of PL/SQL Procedure in Oracle

2011-04-17T03:06:00.000-07:00

PROCEDURE name [(parameter[, parameter, …])] IS
[local declarations]
BEGIN
executable statements
[EXCEPTION
exception handlers]
END [name];

Example 1 of PL/SQL Procedure

create or replace procedure get_area
(Len in number, Wid in number, Area out number)
as
begin
Area := Len * Wid;
end;

SQL> variable area1 number;
SQL> execute get_area (10, 50, :area1);
PL/SQL procedure successfully completed.
SQL> print area1
AREA1
---------
500

Example 2 of PL/SQL Procedure

The following statement creates the procedure remove_emp in the schema hr. The PL/SQL is shown in italics:

CREATE PROCEDURE remove_emp (employee_id NUMBER) AS
tot_emps NUMBER;
BEGIN
DELETE FROM employees
WHERE employees.employee_id = remove_emp.employee_id;
tot_emps := tot_emps - 1;
END;
/

The remove_emp procedure removes a specified employee. When you call the procedure, you must specify the employee_id of the employee to be removed.

The procedure uses a DELETE statement to remove from the employees table the row of employee_id. [Source]

PL/SQL Procedures In Oracle

2011-04-17T03:05:00.001-07:00

There are two types of PL/SQL blocks: named block and an anonymous block.
There are two types of named blocks: Functions and Procedures

A stored procedure is a PL/SQL block that accepts zero or more parameters as input (IN), output (OUT), or both (INOUT). PL/SQL Procedures do not return a value; instead the INOUT parameter or OUT parameter may be used to pass a value from the procedure. Procedures cannot be used in SQL statements; they are invoked using the EXECUTE command or called inside a PL/SQL block.

%TYPE and %ROWTYPE in Oracle

2011-04-17T03:04:00.000-07:00

%TYPE is used to declare a variable that is of the same type as a specified table’s column.

Emp_number emp_name.emp_nameno%type;

%ROWTYPE is used to declare a record (variable that represents the entire row of a table).

Emp_record emp_name%rowtype;

Another example of declaring variable:

Declare
name varchar2(30);
Select emp_name_name into name from emp_name where emp_nameno = 203456;
Begin
Null;
End;

Any DML statements should be after Begin statement;

Begin
Delete from emp_name where emp_nameno = 24545459;
Commit;
End;

Declaring PL/SQL variables and constants in Oracle

2011-04-17T03:02:00.000-07:00

Example of declaring Variableiables:

Variable1 varchar2(100);
Join_date Date;
Variable2 number default 5;
Variable3 number not null := 2;

Not Null means a value may change but it can never be assigned Null.

Variable4 varchar2(20) := Null;
Variable5 varchar2(20) default Null;

Example of declaring Constants:

Variable_constant constant number := 100;

Constants cannot be changed.

You must initialize constants at the time of declaration.

Oracle Syntax of a PL/SQL Block

2011-04-17T03:00:00.000-07:00

DECLARE
Variable_declarations
BEGIN
Program_code
EXCEPTION
Exception_handlers
END;

Below is the basic structure of the PL/SQL program:

Set serveroutput on
Var1 varchar2(20);
Begin
Var1 := ‘welcome’;
Dbms_output.put_line(var1);
Exception
When others then
Dbms_output.put_line(‘It is an exception’);
End;
/
In the declaration section all the variables and constants are defined.

In PL/SQL all the errors are handled in the Exception block.
Begin and End are mandatory statements indicating begin and end of the PL/SQL Block.
Variables and Constants must be declared first before they can be used.
The declaration of variables and constants are alike, but constant definitions must contain the keyword CONSTANT and must be assigned a value as part of the definition. Later on any attempts to assign a value to a constant will result in an error message.
Values can be assigned to variables directly using the “:=” assignment operator, by way of a SELECT ... INTO statement or When used as OUT or IN OUT parameter from a procedure.

Oracle PL/SQL Introduction |Oracle PL/SQL Tutorial

2011-04-17T02:59:00.000-07:00

PL/SQL stands for Procedural Language/SQL. PL/SQL is Oracle's Procedural Language extension to SQL. PL/SQL expands SQL by adding constructs found in procedural languages, resulting in a structural language that is more powerful than SQL. Basically it runs on the database server, but a few Oracle products such as Developer/2000 also contain a PL/SQL engine that resides on the client. Thus, you can run your PL/SQL code on either the client or the server depending on which is more suitable for the task at hand.

Unlike SQL, PL/SQL is procedural, not declarative.

A declarative (non-procedural) programming language is a language that allows the programmer to state the task to be accomplished without specifying the procedures needed to carry it out.

A Procedural programming language is a language in which programs largely consist of a series of commands to assign values to objects.

The basic unit in PL/SQL is a block. All PL/SQL programs are made up of blocks, which can be nested within each other. Typically, each block performs a logical action in the program. PL/SQL is Block Structured.

NavigableSet in Java 6.0

2011-04-11T13:43:00.000-07:00

NavigableSet API is included in the SCJP 6.0 certification exam. This article explains few important methods with simple example program. NavigableSet is the subinterface of SortedSet. This interface defines methods for finding the element in a list. For example lower() method used for finding the element which is less than the given value. Look into the following example:

package NavigableTutorial.net;

import java.util.ArrayList;
import java.util.List;
import java.util.NavigableSet;
import java.util.TreeSet;

public class NavigableSetTutorial1 {
    public static void main(String args[]){
        List list = new ArrayList();
        list.add(3);
        list.add(4);
        list.add(1);
        list.add(8);
        list.add(7);
        list.add(10);

        NavigableSet navigableSet = new TreeSet(list);
        System.out.println(navigableSet.lower(8));
        System.out.println(navigableSet.higher(8));

    }
}

Output:
7
10

In the above code, NavigableSet.lower() method is used for reteriving the value which is less than '8' in the list. Same way NavigableSet.higher() is used for reteriving the value greater than '8' in the list.

Java 6 - NavigableSet -headSet and tailSet Example Java Collections Framework

2011-04-11T13:42:00.000-07:00

The below example explains the headSet and tailSet methods of the NavigableSet.

package examples;

import java.util.NavigableSet;
import java.util.TreeSet;
public class NavigableSetExample {

public static void main(String[] args) {
NavigableSet set = new TreeSet();
// Add elements in to the Navigable Set
set.add(1);
set.add(2);
set.add(3);
System.out.println(set);

// Head Set
System.out.println("Head Set :" + set.headSet(2));
System.out.println("Head Set including given integer: "+ set.headSet(2, true));

// Tail Set
System.out.println("Tail Set" + set.tailSet(2));
System.out.println("Tail Set including given integer: " + set.tailSet(2, false));
}

}

[1, 2, 3]
Head Set :[1]
Head Set including given integer: [1, 2]
Tail Set[2, 3]
Tail Set including given integer: [3]

NavigableSet - pollFirst and pollLast example Java Collections

2011-04-11T13:40:00.000-07:00

pollFirst(), pollLast() methods of NavigableSet can be used to remove first and last element of the Set respectively. The below program explains the pollFirst() and pollLast()

package examples;

import java.util.NavigableSet;
import java.util.TreeSet;

public class NavigableSetExample {

public static void main(String args[]){
NavigableSet navigableSet = new TreeSet();

navigableSet.add(1);
navigableSet.add(2);

System.out.println("Set : " + navigableSet);

navigableSet.pollFirst();

System.out.println("Set After pollFirst " + navigableSet);

navigableSet.add(3);
navigableSet.add(4);

System.out.println("Set " + navigableSet);

navigableSet.pollLast();

System.out.println("Set after pollLast " +navigableSet);

}

}

Output:

Set : [1, 2]
Set After pollFirst [2]
Set [2, 3, 4]
Set after pollLast [2, 3]

Java Collections Tutorial Navigable Map Example

2011-04-11T13:39:00.000-07:00

package JAVATUTORIAL;
import java.util.*;
import java.util.concurrent.*;

public class NavigableMapTutorial{
    public static void main(String[] args) {
        System.out.println("Navigable Map Example!\n");
        NavigableMap navMapObj = new
                           ConcurrentSkipListMap();
        navMapObj.put(1, "S");
        navMapObj.put(2, "February");
        navMapObj.put(3, "March");
        navMapObj.put(4, "April");
        navMapObj.put(5, "May");
        navMapObj.put(6, "June");
        navMapObj.put(7, "July");
        navMapObj.put(8, "August");
        navMapObj.put(9, "September");
        navMapObj.put(10, "October");
        navMapObj.put(11, "November");
        navMapObj.put(12, "December");
        //Displaying all data
        System.out.println("Data in Navigable Map is : " +
                                              navMapObj.descendingMap()+"\n");
        //Retrieving first data
        System.out.print("First data: " + navMapObj.firstEntry()+"\n");
        //Retrieving last data
        System.out.print("Last data: " + navMapObj.lastEntry()+"\n\n");
        //Retrieving the nreatest less than or equal to the given key
        System.out.print("Nearest less than or equal to the given key: "
                                + navMapObj.floorEntry(5)+"\n");
        //Retrieving the greatest key strictly less than the given key
        System.out.println("Retrieving the greatest key strictly less than the given key"+ navMapObj.lowerEntry(3));
        //Retrieving a key-value associated with the least key
                        //strictly greater than the given key
        System.out.println("Retriving data from navigable map greter than   the given key: " + navMapObj.higherEntry(5)+"\n");
        //Removing first
        System.out.println("Removing First: " + navMapObj.pollFirstEntry());
        //Removing last
        System.out.println("Removing Last: " + navMapObj.pollLastEntry()+"\n");
        //Displaying all data
        System.out.println("Now data: " + navMapObj.descendingMap());
    }
    }

Description of program:

The following program helps you in inserting, removing and retrieving the data from the NavigableMap. It uses the put() method to add the element. If you want to retrieve the data at first and last position from the NavigableMap, you use the firstEntry() and lastEntry() methods. The descendingMap() method represents all data to the NavigableMap in descending order.

You can retrieve the nearest less than or equal to the given number and the greatest key strictly less than the given number floorEntry() and lowerEntry() methods. And you retrieve a key-value associated with the least key strictly greater than the given key, you use the higherEntry() method. The pollFirstEntry() method removes the first data from the NavigableMap and pollLastEntry() method also removes the data at the last position from the NavigableMap.

JDK 6 Collections-related APIs & Developer Guides |Java 6.0 Collection Framework

2011-04-11T13:37:00.000-07:00

Java 6.0 New Collection APIs an Overview
The following are the new collection APIs introduced in Java 6.0. I listes them as Interfaces and classes.
New Interfaces

Deque
BlockingDeque
NavigableSet
NavigableMap

New Classes

ArrayDeque
LinkedBlockingDeque
ConcurrentSkipListSet
ConcurrentSkipListMap
AbstractMap.SimpleEntry
AbstractMap.SimpleImmutableEntry

Updated Classes in Java 6.0

LinkedList
TreeSet
TreeMap
Collections

Advantages and Disadvantages of the Java Collection Framework

2011-04-11T13:34:00.000-07:00

A collection is simply an object that groups multiple elements into a single unit. It is also called as a container sometimes. It is used to store, retrieve, manipulate, and communicate aggregate data. Typically, it represents data items that form a natural group and allows duplicate elements while others do not. It consists of both ordered and unordered elements.

The primary advantages of a collections framework are that it:

Reduces programming effort by providing useful data structures and algorithms so you don't have to write them yourself.
Increases performance by providing high-performance implementations of useful data structures and algorithms. Because the various implementations of each interface are interchangeable, programs can be easily tuned by switching implementations.
Provides interoperability between unrelated APIs by establishing a common language to pass collections back and forth.
Reduces the effort required to learn APIs by eliminating the need to learn multiple ad hoc collection APIs.
Reduces the effort required to design and implement APIs by eliminating the need to produce ad hoc collections APIs.
Fosters software reuse by providing a standard interface for collections and algorithms to manipulate them.

Disadvantages of collections framework:

It must cast to correct type.
It can't be done compile-time type checking.

What is Collection Framework in Java?

2011-04-11T13:32:00.000-07:00

A collection is an object that represents a group of objects (such as the familiar Vector class). A collections framework is a unified architecture for representing and manipulating collections, allowing them to be manipulated independently of the details of their representation.

collections is a group of objects known as its elements. Basically it is a package of data structures that includes ArrayLists, LinkedLists, HashSets, etc. A collection is simply an object that groups multiple elements into a single unit. It is also called as a container sometimes. It is used to store, retrieve, manipulate, and communicate aggregate data. Typically, it represents data items that form a natural group and allows duplicate elements while others do not. It consists of both ordered and unordered elements. There is no direct implementation of this interface however SDK provides implementations of more specific sub interfaces like Set and List. The manipulation and passing of collections is done by this interface.

How to configure AUTOTRACE in SQLPLUS for database user?

2011-04-08T22:55:00.000-07:00

1. Change directory path and connect with SYSTEM user to database
C:\>cd c:\oracle\product\10.1.0\db_1\rdbms\admin

C:\Oracle\product\10.1.0\Db_1\RDBMS\ADMIN>sqlplus /nolog

SQL*Plus: Release 9.0.1.0.1 - Production on Tue Oct 16 17:08:20 2007

(c) Copyright 2001 Oracle Corporation. All rights reserved.

SQL> conn system/manager
Connected.

2. run UTLXPLAN.SQL script for plan table.
SQL> @utlxplan.sql

3. Create public synonym for plan table or grant all privilege.
SQL> create public synonym plan_table for plan_table;

SQL> grant all on plan_table to public;

Grant succeeded.

4. Exit and again change directory path and connect with SYS user to database
SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.0.1.1.1 - Production
With the Partitioning option
JServer Release 9.0.1.1.1 - Production

C:\Oracle\product\10.1.0\Db_1\RDBMS\ADMIN>cd\

C:\>cd c:\oracle\product\10.1.0\db_1\sqlplus\admin

C:\Oracle\product\10.1.0\Db_1\sqlplus\admin>sqlplus /nolog

SQL*Plus: Release 9.0.1.0.1 - Production on Tue Oct 16 17:12:07 2007

(c) Copyright 2001 Oracle Corporation. All rights reserved.

SQL> conn sys as sysdba
Enter password:
Connected.

5. run plustrce script and script must be run with SYS user.
SQL> @plustrce
SQL>
SQL> drop role plustrace;

Role dropped.

SQL> create role plustrace;

Role created.

SQL>
SQL> grant select on v_$sesstat to plustrace;

Grant succeeded.

SQL> grant select on v_$statname to plustrace;

Grant succeeded.

SQL> grant select on v_$mystat to plustrace;

Grant succeeded.

SQL> grant plustrace to dba with admin option;

Grant succeeded.

SQL> set echo off

6. Grant plustrace role to public.
SQL> grant plustrace to public;

Grant succeeded.

Samba Configuration for Linux Directory access from windows|Linux In a Windows Network with SAMBA

2011-04-08T22:52:00.000-07:00

Kindly go through the below mentioned site

http://www.computernetworkingnotes.com/rhce_certification/samba_server.htm

step
1. install rpm package for smb (samba)
2. edit smb.conf (path= /etc/samba/smb.conf)
and add the directory name and user which you want to share
3. create same user as windows who access the directory from windows.
for example: windows user is "admin" which will access the linux shared directory.
then we need to create the same user in linux server also with the same password.

4. add user to smb
$smbpasswd -a [username]

5. restart smb services
$service smb restart
or
$service /sbin/samba/smb restart

ORA-00020: maximum number of processes (%s) exceeded in Oracle 10g,Oracle 11g

2011-04-08T22:49:00.000-07:00

ORA-20 "maximum number of processes (%s) exceeded"

Cause: An ORA-20 "maximum number of processes (%s) exceeded" occurs when the number of OS processes for the instance exceeds the PROCESSES parameter

and i am not able to connect to sys as sysdba user too.
C:\>sqlplus / as sysdba

SQL*Plus: Release 11.2.0.2.0 Production on Wed Jan 26 08:58:09 2011

Copyright (c) 1982, 2010, Oracle. All rights reserved.

ERROR:
ORA-00020: maximum number of processes (150) exceeded

but there is a alternative which we can use to connect to sys as sysdba user.

C:\>sqlplus -prelim / as sysdba

SQL*Plus: Release 11.2.0.2.0 Production on Wed Jan 26 08:58:54 2011

Copyright (c) 1982, 2010, Oracle. All rights reserved.

Solution:
1. Increase the processes parameter Or
2. Kill KBCOOK process at OS level.

$ ps -ef | grep KBCOOK

oracle 17555 1 0 04:48 ? 00:00:00 ora_pmon_KBCOOK
oracle 17557 1 0 04:48 ? 00:00:00 ora_vktm_KBCOOK
oracle 17561 1 0 04:48 ? 00:00:00 ora_diag_KBCOOK
oracle 17563 1 0 04:48 ? 00:00:00 ora_dbrm_KBCOOK
oracle 17565 1 0 04:48 ? 00:00:00 ora_psp0_KBCOOK
oracle 17569 1 0 04:48 ? 00:00:00 ora_dia0_KBCOOK
oracle 17571 1 0 04:48 ? 00:00:00 ora_mman_KBCOOK
oracle 17573 1 0 04:48 ? 00:00:00 ora_dbw0_KBCOOK
oracle 17575 1 0 04:48 ? 00:00:00 ora_lgwr_KBCOOK
oracle 17577 1 0 04:48 ? 00:00:00 ora_ckpt_KBCOOK
oracle 17579 1 0 04:48 ? 00:00:00 ora_smon_KBCOOK
oracle 17581 1 0 04:48 ? 00:00:00 ora_reco_KBCOOK
oracle 17583 1 0 04:48 ? 00:00:00 ora_mmon_KBCOOK
oracle 17585 1 0 04:48 ? 00:00:00 ora_mmnl_KBCOOK
oracle 17587 1 0 04:48 ? 00:00:00 ora_d000_KBCOOK
oracle 17589 1 0 04:48 ? 00:00:00 ora_s000_KBCOOK
oracle 17608 1 0 04:48 ? 00:00:00 ora_smco_KBCOOK
oracle 17610 1 0 04:48 ? 00:00:00 ora_fbda_KBCOOK
oracle 17612 1 0 04:48 ? 00:00:00 ora_qmnc_KBCOOK
oracle 17641 1 0 04:48 ? 00:00:00 ora_q000_KBCOOK
oracle 17671 1 0 04:49 ? 00:00:00 ora_q001_KBCOOK
oracle 17761 1 0 04:50 ? 00:00:00 oracleKBCOOK (LOCAL=NO)
oracle 18480 1 0 04:58 ? 00:00:00 ora_w000_KBCOOK
oracle 18608 17126 0 05:00 pts/1 00:00:00 grep KBCOOK

$ kill -9 17761

$ ps -ef | grep KBCOOK

oracle 17555 1 0 04:48 ? 00:00:00 ora_pmon_KBCOOK
oracle 17557 1 0 04:48 ? 00:00:00 ora_vktm_KBCOOK
oracle 17561 1 0 04:48 ? 00:00:00 ora_diag_KBCOOK
oracle 17563 1 0 04:48 ? 00:00:00 ora_dbrm_KBCOOK
oracle 17565 1 0 04:48 ? 00:00:00 ora_psp0_KBCOOK
oracle 17569 1 0 04:48 ? 00:00:00 ora_dia0_KBCOOK
oracle 17571 1 0 04:48 ? 00:00:00 ora_mman_KBCOOK
oracle 17573 1 0 04:48 ? 00:00:00 ora_dbw0_KBCOOK
oracle 17575 1 0 04:48 ? 00:00:00 ora_lgwr_KBCOOK
oracle 17577 1 0 04:48 ? 00:00:00 ora_ckpt_KBCOOK
oracle 17579 1 0 04:48 ? 00:00:00 ora_smon_KBCOOK
oracle 17581 1 0 04:48 ? 00:00:00 ora_reco_KBCOOK
oracle 17583 1 0 04:48 ? 00:00:00 ora_mmon_KBCOOK
oracle 17585 1 0 04:48 ? 00:00:00 ora_mmnl_KBCOOK

Refer Metalink Note: [ID 825045.1] for more details.

ORA-16006: audit_trail destination incompatible with database open mode on redhat 5.2 10gr2 (10.2.0.4)

2011-04-08T22:46:00.000-07:00

ORA-16006: audit_trail destination incompatible with database open mode
while i was trying to open standby database got below error message

ORA-16006: audit_trail destination incompatible with database open mode

at little search on metalink found below note: ID 835638.1

solution:

SQL>alter system set audit_trail=OS scope=spfile;

How to Create AWR Report in Oracle 10g?

2011-04-08T22:43:00.000-07:00

he remaining procedures in the dbms_workload_repository package are awr_report_text and awr_report_html, which generate the AWR report for the specified snapshot range in text or HTML formats, respectively. The following script segment shows how to retrieve the AWR text report for any snapshot range or duration:

SELECT

output

FROM TABLE(dbms_workload_repository.awr_report_text (45634523,1,5600,5612 ));

The sample output below shows the typical report generated for AWR data. The output displays shows the four arguments to the awr_report_text stored procedure:

§ The database ID is 37933856.

§ The instance number for RAC is 1.

§ The starting snapshot number is5600

§ The ending snapshot number is5612

AWR Reports |Workload Repository Reports in Oracle 10g

2011-04-08T22:39:00.000-07:00

Oracle provide two scripts to produce workload repository reports (awrrpt.sql and awrrpti.sql). They are similar in format to the statspack reports and give the option of HTML or plain text formats. The two reports give essential the same output but the awrrpti.sql allows you to select a single instance. The reports can be generated as follows:

@$ORACLE_HOME/rdbms/admin/awrrpt.sql
@$ORACLE_HOME/rdbms/admin/awrrpti.sql

The scripts prompt you to enter the report format (html or text), the start snapshot id, the end snapshot id and the report filename. The resulting report can be opend in a browser or text editor accordingly.

What are the Workload Repository Views AWR Views in Oracle 10g?

2011-04-08T22:36:00.000-07:00

Workload Repository Views

The following workload repository views are available:

V$ACTIVE_SESSION_HISTORY - Displays the active session history (ASH) sampled every second.
V$METRIC - Displays metric information.
V$METRICNAME - Displays the metrics associated with each metric group.
V$METRIC_HISTORY - Displays historical metrics.
V$METRICGROUP - Displays all metrics groups.
DBA_HIST_ACTIVE_SESS_HISTORY - Displays the history contents of the active session history.
DBA_HIST_BASELINE - Displays baseline information.
DBA_HIST_DATABASE_INSTANCE - Displays database environment information.
DBA_HIST_SNAPSHOT - Displays snapshot information.
DBA_HIST_SQL_PLAN - Displays SQL execution plans.
DBA_HIST_WR_CONTROL - Displays AWR settings.

What is AWR Report in Oracle 10g?

2011-04-08T22:33:00.000-07:00

Automatic Workload Repository (AWR) in Oracle Database 10g

Oracle have provided many performance gathering and reporting tools over the years. Originally the UTLBSTAT/UTLESTAT scripts were used to monitor performance metrics. Oracle8i introduced the Statspack functionality which Oracle9i extended. In Oracle 10g statspack has evolved into the Automatic Workload Repository (AWR).

AWR Features

The AWR is used to collect performance statistics including:

Wait events used to identify performance problems.
Time model statistics indicating the amount of DB time associated with a process from the V$SESS_TIME_MODEL and V$SYS_TIME_MODEL views.
Active Session History (ASH) statistics from the V$ACTIVE_SESSION_HISTORY view.
Some system and session statistics from the V$SYSSTAT and V$SESSTAT views.
Object usage statistics.
Resource intensive SQL statements.

The repository is a source of information for several other Oracle 10g features including:

Automatic Database Diagnostic Monitor
SQL Tuning Advisor
Undo Advisor
Segment Advisora

Inheritance,Inheritance in Java,Java Inheritance

2011-04-07T05:31:00.000-07:00

Q2) What is inheritance?

Ans) Inheritance is the property which allows a Child class to inherit some properties from its parent class. In Java this is achieved by using extends keyword. Only properties with access modifier public and protected can be accessed in child class.

public class Parent{
public String parentName;
public int parentage;
public String familyName;
}
public class Child extends Parent{
public String childName;
public int childAge;
public void printMyName(){
System.out.println(“ My name is “+ chidName+” “ +familyName)
}
}

In above example the child has inherit its family name from the parent class just by inheriting the class.

UNIX interview questions

2011-04-07T05:29:00.001-07:00

What are the main differences between Apache 1.x and 2.x?
What does the “route” command do?
What are the read/write/execute bits on a directory mean?
What does iostat do?

what does vmstat do?
What does netstat do?
What is the most graceful way to bring a system into single user mode?
How do you determine disk usage?
What is AWK?
What is SED?
What is the difference between binaries in /bin, and /usr/bin?
What is a dynamically linked file?
What is a statically linked file?

Java Polymorphism - OOPS |Java - Polymorphism

2011-04-07T05:28:00.000-07:00

Q1) What is polymorphism?

Ans) Polymorphism gives us the ultimate flexibility in extensibility. The abiltiy to define more than one function with the same name is called Polymorphism. In java,c++ there are two type of polymorphism: compile time polymorphism (overloading) and runtime polymorphism (overriding).

When you override methods, JVM determines the proper methods to call at the program’s run time, not at the compile time. Overriding occurs when a class method has the same name and signature as a method in parent class.
Overloading occurs when several methods have same names with

Overloading is determined at the compile time.
Different method signature and different number or type of parameters.
Same method signature but different number of parameters.
Same method signature and same number of parameters but of different type

Example of Overloading
     int add(int a,int b)
     float add(float a,int b)
     float add(int a ,float b)
     void add(float a)
     int add(int a)
     void add(int a)                 //error conflict with the method int add(int a)

Example: Overloading

Class BookDetails{
            String title;
            String publisher;
            float price;

setBook(String title){
}
setBook(String title, String publisher){
}
setBook(String title, String publisher,float price){
}

}

Example: Overriding

class BookDetails{
String title;

setBook(String title){ }
}

class ScienceBook extends BookDetails{
setBook(String title){} //overriding

setBook(String title, String publisher,float price){ } //overloading

}

Java interview questions & answers -Set 4

2011-04-07T05:21:00.000-07:00

What is the difference between an applet and a servlet?- a) Servlets are to servers what applets are to browsers. b) Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.
What is the difference between doPost and doGet methods?- a) doGet() method is used to get information, while doPost() method is used for posting information. b) doGet() requests can’t send large amount of information and is limited to 240-255 characters. However, doPost()requests passes all of its data, of unlimited length. c) A doGet() request is appended to the request URL in a query string and this allows the exchange is visible to the client, whereas a doPost() request passes directly over the socket connection as part of its HTTP request body and the exchange are invisible to the client.
What is the life cycle of a servlet?- Each Servlet has the same life cycle: a) A server loads and initializes the servlet by init () method. b) The servlet handles zero or more client’s requests through service() method. c) The server removes the servlet through destroy() method.
Who is loading the init() method of servlet?- Web server
What are the different servers available for developing and deploying Servlets?- a) Java Web Server b) JRun g) Apache Server h) Netscape Information Server i) Web Logic
How many ways can we track client and what are they?- The servlet API provides two ways to track client state and they are: a) Using Session tracking and b) Using Cookies.
What is session tracking and how do you track a user session in servlets?- Session tracking is a mechanism that servlets use to maintain state about a series requests from the same user across some period of time. The methods used for session tracking are: a) User Authentication - occurs when a web server restricts access to some of its resources to only those clients that log in using a recognized username and password. b) Hidden form fields - fields are added to an HTML form that are not displayed in the client’s browser. When the form containing the fields is submitted, the fields are sent back to the server. c) URL rewriting - every URL that the user clicks on is dynamically modified or rewritten to include extra information. The extra information can be in the form of extra path information, added parameters or some custom, server-specific URL change. d) Cookies - a bit of information that is sent by a web server to a browser and which can later be read back from that browser. e) HttpSession- places a limit on the number of sessions that can exist in memory. This limit is set in the session. maxresidents property.
What is Server-Side Includes (SSI)?- Server-Side Includes allows embedding servlets within HTML pages using a special servlet tag. In many servlets that support servlets, a page can be processed by the server to include output from servlets at certain points inside the HTML page. This is accomplished using a special internal SSINCLUDE, which processes the servlet tags. SSINCLUDE servlet will be invoked whenever a file with an. shtml extension is requested. So HTML files that include server-side includes must be stored with an . shtml extension.
What are cookies and how will you use them?- Cookies are a mechanism that a servlet uses to have a client hold a small amount of state-information associated with the user. a) Create a cookie with the Cookie constructor: public Cookie(String name, String value) b) A servlet can send a cookie to the client by passing a Cookie object to the addCookie() method of HttpServletResponse: public void HttpServletResponse. addCookie(Cookie cookie) c) A servlet retrieves cookies by calling the getCookies() method of HttpServletRequest: public Cookie[ ] HttpServletRequest. getCookie().
Is it possible to communicate from an applet to servlet and how many ways and how?- Yes, there are three ways to communicate from an applet to servlet and they are: a) HTTP Communication(Text-based and object-based) b) Socket Communication c) RMI Communication
What is connection pooling?- With servlets, opening a database connection is a major bottleneck because we are creating and tearing down a new connection for every page request and the time taken to create connection will be more. Creating a connection pool is an ideal approach for a complicated servlet. With a connection pool, we can duplicate only the resources we need to duplicate rather than the entire servlet. A connection pool can also intelligently manage the size of the pool and make sure each connection remains valid. A number of connection pool packages are currently available. Some like DbConnectionBroker are freely available from Java Exchange Works by creating an object that dispenses connections and connection Ids on request. The ConnectionPool class maintains a Hastable, using Connection objects as keys and Boolean values as stored values. The Boolean value indicates whether a connection is in use or not. A program calls getConnection() method of the ConnectionPool for getting Connection object it can use; it calls returnConnection() to give the connection back to the pool.
Why should we go for interservlet communication?- Servlets running together in the same server communicate with each other in several ways. The three major reasons to use interservlet communication are: a) Direct servlet manipulation - allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object) b) Servlet reuse - allows the servlet to reuse the public methods of another servlet. c) Servlet collaboration - requires to communicate with each other by sharing specific information (through method invocation)
Is it possible to call servlet with parameters in the URL?- Yes. You can call a servlet with parameters in the syntax as (?Param1 = xxx || m2 = yyy).
What is Servlet chaining?- Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request. In servlet chaining, one servlet’s output is piped to the next servlet’s input. This process continues until the last servlet is reached. Its output is then sent back to the client.
How do servlets handle multiple simultaneous requests?- The server has multiple threads that are available to handle requests. When a request comes in, it is assigned to a thread, which calls a service method (for example: doGet(), doPost() and service()) of the servlet. For this reason, a single servlet object can have its service methods called by many threads at once.
What is the difference between TCP/IP and UDP?- TCP/IP is a two-way communication between the client and the server and it is a reliable and there is a confirmation regarding reaching the message to the destination. It is like a phone call. UDP is a one-way communication only between the client and the server and it is not a reliable and there is no confirmation regarding reaching the message to the destination. It is like a postal mail.
What is Inet address?- Every computer connected to a network has an IP address. An IP address is a number that uniquely identifies each computer on the Net. An IP address is a 32-bit number.
What is Domain Naming Service(DNS)?- It is very difficult to remember a set of numbers(IP address) to connect to the Internet. The Domain Naming Service(DNS) is used to overcome this problem. It maps one particular IP address to a string of characters. For example, www. mascom. com implies com is the domain name reserved for US commercial sites, moscom is the name of the company and www is the name of the specific computer, which is mascom’s server.
What is URL?- URL stands for Uniform Resource Locator and it points to resource files on the Internet. URL has four components: http://www. address. com:80/index.html, where http - protocol name, address - IP address or host name, 80 - port number and index.html - file path.
What is RMI and steps involved in developing an RMI object?- Remote Method Invocation (RMI) allows java object that executes on one machine and to invoke the method of a Java object to execute on another machine. The steps involved in developing an RMI object are: a) Define the interfaces b) Implementing these interfaces c) Compile the interfaces and their implementations with the java compiler d) Compile the server implementation with RMI compiler e) Run the RMI registry f) Run the application
What is RMI architecture?- RMI architecture consists of four layers and each layer performs specific functions: a) Application layer - contains the actual object definition. b) Proxy layer - consists of stub and skeleton. c) Remote Reference layer - gets the stream of bytes from the transport layer and sends it to the proxy layer. d) Transportation layer - responsible for handling the actual machine-to-machine communication.
what is UnicastRemoteObject?- All remote objects must extend UnicastRemoteObject, which provides functionality that is needed to make objects available from remote machines.
Explain the methods, rebind() and lookup() in Naming class?- rebind() of the Naming class(found in java. rmi) is used to update the RMI registry on the server machine. Naming. rebind(”AddSever”, AddServerImpl); lookup() of the Naming class accepts one argument, the rmi URL and returns a reference to an object of type AddServerImpl.
What is a Java Bean?- A Java Bean is a software component that has been designed to be reusable in a variety of different environments.
What is a Jar file?- Jar file allows to efficiently deploying a set of classes and their associated resources. The elements in a jar file are compressed, which makes downloading a Jar file much faster than separately downloading several uncompressed files. The package java. util. zip contains classes that read and write jar files.
What is BDK?- BDK, Bean Development Kit is a tool that enables to create, configure and connect a set of set of Beans and it can be used to test Beans without writing a code.
What is JSP?- JSP is a dynamic scripting capability for web pages that allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix traditionally ends with .jsp to indicate to the web server that the file is a JSP files. JSP is a server side technology - you can’t do any client side validation with it. The advantages are: a) The JSP assists in making the HTML more functional. Servlets on the other hand allow outputting of HTML but it is a tedious process. b) It is easy to make a change and then let the JSP capability of the web server you are using deal with compiling it into a servlet and running it.
What are JSP scripting elements?- JSP scripting elements lets to insert Java code into the servlet that will be generated from the current JSP page. There are three forms: a) Expressions of the form <%= expression %> that are evaluated and inserted into the output, b) Scriptlets of the form<% code %>that are inserted into the servlet’s service method, and c) Declarations of the form <%! Code %>that are inserted into the body of the servlet class, outside of any existing methods.
What are JSP Directives?- A JSP directive affects the overall structure of the servlet class. It usually has the following form:<%@ directive attribute=”value” %> However, you can also combine multiple attribute settings for a single directive, as follows:<%@ directive attribute1=”value1″ attribute 2=”value2″ . . . attributeN =”valueN” %> There are two main types of directive: page, which lets to do things like import classes, customize the servlet superclass, and the like; and include, which lets to insert a file into the servlet class at the time the JSP file is translated into a servlet
What are Predefined variables or implicit objects?- To simplify code in JSP expressions and scriptlets, we can use eight automatically defined variables, sometimes called implicit objects. They are request, response, out, session, application, config, pageContext, and page.
What are JSP ACTIONS?- JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include: jsp:include - Include a file at the time the page is requested. jsp:useBean - Find or instantiate a JavaBean. jsp:setProperty - Set the property of a JavaBean. jsp:getProperty - Insert the property of a JavaBean into the output. jsp:forward - Forward the requester to a newpage. Jsp: plugin - Generate browser-specific code that makes an OBJECT or EMBED
How do you pass data (including JavaBeans) to a JSP from a servlet?- (1) Request Lifetime: Using this technique to pass beans, a request dispatcher (using either “include” or forward”) can be called. This bean will disappear after processing this request has been completed. Servlet: request. setAttribute(”theBean”, myBean); RequestDispatcher rd = getServletContext(). getRequestDispatcher(”thepage. jsp”); rd. forward(request, response); JSP PAGE:(2) Session Lifetime: Using this technique to pass beans that are relevant to a particular session (such as in individual user login) over a number of requests. This bean will disappear when the session is invalidated or it times out, or when you remove it. Servlet: HttpSession session = request. getSession(true); session. putValue(”theBean”, myBean); /* You can do a request dispatcher here, or just let the bean be visible on the next request */ JSP Page: 3) Application Lifetime: Using this technique to pass beans that are relevant to all servlets and JSP pages in a particular app, for all users. For example, I use this to make a JDBC connection pool object available to the various servlets and JSP pages in my apps. This bean will disappear when the servlet engine is shut down, or when you remove it. Servlet: GetServletContext(). setAttribute(”theBean”, myBean); JSP PAGE:
How can I set a cookie in JSP?- response. setHeader(”Set-Cookie”, “cookie string”); To give the response-object to a bean, write a method setResponse (HttpServletResponse response) - to the bean, and in jsp-file:<% bean. setResponse (response); %>
How can I delete a cookie with JSP?- Say that I have a cookie called “foo, ” that I set a while ago & I want it to go away. I simply: <% Cookie killCookie = new Cookie(”foo”, null); KillCookie. setPath(”/”); killCookie. setMaxAge(0); response. addCookie(killCookie); %>
How are Servlets and JSP Pages related?- JSP pages are focused around HTML (or XML) with Java codes and JSP tags inside them. When a web server that has JSP support is asked for a JSP page, it checks to see if it has already compiled the page into a servlet. Thus, JSP pages become servlets and are transformed into pure Java and then compiled, loaded into the server and executed.

Java interview questions & answers -Set 4

2011-04-07T05:18:00.000-07:00

Which containers use a Border layout as their default layout?- Window, Frame and Dialog classes use a BorderLayout as their layout.
Which containers use a Flow layout as their default layout?- Panel and Applet classes use the FlowLayout as their default layout.
What are wrapper classes?- Wrapper classes are classes that allow primitive types to be accessed as objects.
What are Vector, Hashtable, LinkedList and Enumeration?- Vector : The Vector class provides the capability to implement a growable array of objects. Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores objects in a dictionary using hash codes as the object’s keys. Hash codes are integer values that identify objects. LinkedList: Removing or inserting elements in the middle of an array can be done using LinkedList. A LinkedList stores each object in a separate link whereas an array stores object references in consecutive locations. Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It has two methods, namely hasMoreElements() and nextElement(). HasMoreElemnts() tests if this enumeration has more elements and nextElement method returns successive elements of the series.
What is the difference between set and list?- Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.
What is a stream and what are the types of Streams and classes of the Streams?- A Stream is an abstraction that either produces or consumes information. There are two types of Streams and they are: Byte Streams: Provide a convenient means for handling input and output of bytes. Character Streams: Provide a convenient means for handling input & output of characters. Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream. Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.
What is the difference between Reader/Writer and InputStream/Output Stream?- The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented.
What is an I/O filter?- An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
What is serialization and deserialization?- Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
What is JDBC?- JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces to enable programs to write pure Java Database applications.
What are drivers available?- a) JDBC-ODBC Bridge driver b) Native API Partly-Java driver c) JDBC-Net Pure Java driver d) Native-Protocol Pure Java driver
What is the difference between JDBC and ODBC?- a) OBDC is for Microsoft and JDBC is for Java applications. b) ODBC can’t be directly used with Java because it uses a C interface. c) ODBC makes use of pointers which have been removed totally from Java. d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required. e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms. f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.
What are the types of JDBC Driver Models and explain them?- There are two types of JDBC Driver Models and they are: a) Two tier model and b) Three tier model Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server. Three tier model: A middle tier is introduced in this model. The functions of this model are: a) Collection of SQL statements from the client and handing it over to the database, b) Receiving results from database to the client and c) Maintaining control over accessing and updating of the above.
What are the steps involved for making a connection with a database or how do you connect to a database?a) Loading the driver : To load the driver, Class. forName() method is used. Class. forName(”sun. jdbc. odbc. JdbcOdbcDriver”); When the driver is loaded, it registers itself with the java. sql. DriverManager class as an available database driver. b) Making a connection with database: To open a connection to a given database, DriverManager. getConnection() method is used. Connection con = DriverManager. getConnection (”jdbc:odbc:somedb”, “user”, “password”); c) Executing SQL statements : To execute a SQL query, java. sql. statements class is used. createStatement() method of Connection to obtain a new Statement object. Statement stmt = con. createStatement(); A query that returns data can be executed using the executeQuery() method of Statement. This method executes the statement and returns a java. sql. ResultSet that encapsulates the retrieved data: ResultSet rs = stmt. executeQuery(”SELECT * FROM some table”); d) Process the results : ResultSet returns one row at a time. Next() method of ResultSet object can be called to move to the next row. The getString() and getObject() methods are used for retrieving column values: while(rs. next()) { String event = rs. getString(”event”); Object count = (Integer) rs. getObject(”count”);
What type of driver did you use in project?- JDBC-ODBC Bridge driver (is a driver that uses native(C language) libraries and makes calls to an existing ODBC driver to access a database engine).
What are the types of statements in JDBC?- Statement: to be used createStatement() method for executing single SQL statement PreparedStatement — To be used preparedStatement() method for executing same SQL statement over and over. CallableStatement — To be used prepareCall() method for multiple SQL statements over and over.
What is stored procedure?- Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task. Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored procedures can be compiled and executed with different parameters and results and may have any combination of input/output parameters.
How to create and call stored procedures?- To create stored procedures: Create procedure procedurename (specify in, out and in out parameters) BEGIN Any multiple SQL statement; END; To call stored procedures: CallableStatement csmt = con. prepareCall(”{call procedure name(?,?)}”); csmt. registerOutParameter(column no. , data type); csmt. setInt(column no. , column name) csmt. execute();
What is servlet?- Servlets are modules that extend request/response-oriented servers, such as java-enabled web servers. For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the business logic used to update a company’s order database.
What are the classes and interfaces for servlets?- There are two packages in servlets and they are javax. servlet

Java interview questions & answers -Set 3

2011-04-07T05:16:00.000-07:00

What is synchronization?- Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.
When you will synchronize a piece of your code?- When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.
What is deadlock?- When two threads are waiting each other and can’t precede the program is said to be deadlock.
What is daemon thread and which method is used to create the daemon thread?- Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
Are there any global variables in Java, which can be accessed by other part of your program?- No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.
What is an applet?- Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.
What is the difference between applications and applets?- a)Application must be run on local machine whereas applet needs no explicit installation on local machine. b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser. d)Application starts execution with its main method whereas applet starts execution with its init method. e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface.
How does applet recognize the height and width?- Using getParameters() method.
When do you use codebase in applet?- When the applet class file is not in the same directory, codebase is used.
What is the lifecycle of an applet?- init() method - Can be called when an applet is first loaded start() method - Can be called each time an applet is started. paint() method - Can be called when the applet is minimized or maximized. stop() method - Can be used when the browser moves off the applet’s page. destroy() method - Can be called when the browser is finished with the applet.
How do you set security in applets?- using setSecurityManager() method
What is an event and what are the models available for event handling?- An event is an event object that describes a state of change in a source. In other words, event occurs when an action is generated, like pressing button, clicking mouse, selecting a list, etc. There are two types of models for handling events and they are: a) event-inheritance model and b) event-delegation model
What are the advantages of the model over the event-inheritance model?- The event-delegation model has two advantages over the event-inheritance model. They are: a)It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component’s design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.
What is source and listener?- source : A source is an object that generates an event. This occurs when the internal state of that object changes in some way. listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with one or more sources to receive notifications about specific types of events. Second, it must implement methods to receive and process these notifications.
What is adapter class?- An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act listener by extending one of the adapter classes and implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged()and mouseMoved(). The signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested in only mouse drag events, then you could simply extend MouseMotionAdapter and implement mouseDragged() .
What is meant by controls and what are different types of controls in AWT?- Controls are components that allow a user to interact with your application and the AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components. These controls are subclasses of Component.
What is the difference between choice and list?- A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items.
What is the difference between scrollbar and scrollpane?- A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own events and perform its own scrolling.
What is a layout manager and what are different types of layout managers available in java AWT?- A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.
How are the elements of different layouts organized?- FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion. BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards. GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid. GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

Java interview questions & answers -Set 2

2011-04-07T05:15:00.000-07:00

What is meant by Inheritance and what are its advantages?- Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.
What is the difference between this() and super()?- this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.
What is the difference between superclass and subclass?- A super class is a class that is inherited whereas sub class is a class that does the inheriting.
What modifiers may be used with top-level class?- public, abstract and final can be used for top-level class.
What are inner class and anonymous class?- Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private. Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.
What is a package?- A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.
What is a reflection package?- java. lang. reflect package has the ability to analyze itself in runtime.
What is interface and its use?- Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it. Interfaces are useful for: a)Declaring methods that one or more classes are expected to implement b)Capturing similarities between unrelated classes without forcing a class relationship. c)Determining an object’s programming interface without revealing the actual body of the class.
What is an abstract class?- An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.
What is the difference between Integer and int?- a) Integer is a class defined in the java. lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other. b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.
What is a cloneable interface and how many methods does it contain?- It is not having any method because it is a TAGGED or MARKER interface.
What is the difference between abstract class and interface?- a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract. b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods. c) Abstract class must have subclasses whereas interface can’t have subclasses.
Can you have an inner class inside a method and what variables can you access?- Yes, we can have an inner class inside a method and final variables can be accessed.
What is the difference between String and String Buffer?- a) String objects are constants and immutable whereas StringBuffer objects are not. b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.
What is the difference between Array and vector?- Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.
What is the difference between exception and error?- The exception class defines mild error conditions that your program encounters. Exceptions can occur when trying to open the file, which does not exist, the network connection is disrupted, operands being manipulated are out of prescribed ranges, the class file you are interested in loading is missing. The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered.
What is the difference between process and thread?- Process is a program in execution whereas thread is a separate path of execution in a program.
What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?- Multithreading is the mechanism in which more than one thread run independent of each other within the process. wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class. wait() : When a thread executes a call to wait() method, it surrenders the object lock and enters into a waiting state. notify() or notifyAll() : To remove a thread from the waiting state, some other thread must make a call to notify() or notifyAll() method on the same object.
What is the class and interface in java to create thread and which is the most advantageous method?- Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.
What are the states associated in the thread?- Thread contains ready, running, waiting and dead states.

Java interview questions & answers -Set 1

2011-04-07T05:13:00.000-07:00

overs everything from basics to JDBC connectivity, AWT and JSP.

What is the difference between procedural and object-oriented programs?- a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOP program, unit of program is object, which is nothing but combination of data and code. b) In procedural program, data is exposed to the whole program whereas in OOPs program, it is accessible with in the object and which in turn assures the security of the code.
What are Encapsulation, Inheritance and Polymorphism?- Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse. Inheritance is the process by which one object acquires the properties of another object. Polymorphism is the feature that allows one interface to be used for general class actions.
What is the difference between Assignment and Initialization?- Assignment can be done as many times as desired whereas initialization can be done only once.

What is OOPs?- Object oriented programming organizes a program around its data, i. e. , objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.
What are Class, Constructor and Primitive data types?- Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform. Constructor is a special kind of method that determines how an object is initialized when created. Primitive data types are 8 types and they are: byte, short, int, long, float, double, boolean, char.
What is an Object and how do you allocate memory to it?- Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
What is the difference between constructor and method?- Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
What are methods and how are they defined?- Methods are functions that operate on instances of classes in which they are defined. Objects can communicate with each other using methods and can call methods in other classes. Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method. A method’s signature is a combination of the first three parts mentioned above.
What is the use of bin and lib in JDK?- Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.
What is casting?- Casting is used to convert the value of one type to another.
How many ways can an argument be passed to a subroutine and explain them?- An argument can be passed in two ways. They are passing by value and passing by reference. Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.
What is the difference between an argument and a parameter?- While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.
What are different types of access modifiers?- public: Any thing declared as public can be accessed from anywhere. private: Any thing declared as private can’t be seen outside of its class. protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages. default modifier : Can be accessed only to classes in the same package.
What is final, finalize() and finally?- final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can’t be overridden. A final variable can’t change from its initialized value. finalize() : finalize() method is used just before an object is destroyed and can be called just prior to garbage collection. finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.
What is UNICODE?- Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.
What is Garbage Collection and how to call it explicitly?- When an object is no longer referred to by any variable, java automatically reclaims memory used by that object. This is known as garbage collection. System. gc() method may be used to call it explicitly.
What is finalize() method?- finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.
What are Transient and Volatile Modifiers?- Transient: The transient modifier applies to variables only and it is not stored as part of its object’s Persistent state. Transient variables are not serialized. Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.
What is method overloading and method overriding?- Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.
What is difference between overloading and overriding?- a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method. b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass. c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces the superclass. d) Overloading must have different method signatures whereas overriding must have same signature.

Java Package Interview questions |Java lang package interview questions

2011-04-07T05:10:00.000-07:00

Which package is always imported by default?
No. It is by default loaded internally by the JVM. The java.lang package is always imported by default.
Can I import same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains anything about it. And the JVM will internally load the class only once no matter how many times you import the same class.
Does importing a package imports the sub packages as well? E.g. Does importing com.bob.* also import com.bob.code.*?
No you will have to import the sub packages explicitly. Importing com.bob.* will import classes in the package bob only. It will not import any class in any of its sub package’s.

Explain the usage of Java packages.
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
For example: The Java API is grouped into libraries of related classes and interfaces; these libraries are known as package.
Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.BOB compile?
Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying, cannot resolve symbol.

Java Garbage Collection Interview Questions |Garbage Collector - Java Interview Questions

2011-04-07T05:08:00.000-07:00

Q1) If an object becomes eligible for Garbage Collection and its finalize() method has been called and inside this method the object becomes accessible by a live thread of execution and is not garbage collected. Later at some point the same object becomes eligible for Garbage collection, will the finalize() method be called again?
Ans) No

Q2) How many times does the garbage collector calls the finalize() method for an object?
Ans) Only once.

Q3) What happens if an uncaught exception is thrown from during the execution of the finalize() method of an object?
Ans) The exception will be ignored and the garbage collection (finalization) of that object terminates.

Q4) What are different ways to call garbage collector?
Ans) Garbage collection can be invoked using System.gc() or Runtime.getRuntime().gc().

Q5) How to enable/disable call of finalize() method of exit of the application
Ans) Runtime.getRuntime().runFinalizersOnExit(boolean value) . Passing the boolean value will either disable or enable the finalize() call.

Java Garbage Collection Interview Questions

2011-04-07T05:04:00.000-07:00

Q1) Which part of the memory is involved in Garbage Collection? Stack or Heap?

Ans) Heap

Q2)What is responsiblity of Garbage Collector?

Ans) Garbage collector frees the memory occupied by the unreachable objects during the java program by deleting these unreachable objects.
It ensures that the available memory will be used efficiently, but does not guarantee that there will be sufficient memory for the program to run.

Q3) Is garbage collector a dameon thread?

Ans) Yes GC is a dameon thread. A dameon thread runs behind the application. It is started by JVM. The thread stops when all non-dameon threads stop.

Q4)Garbage Collector is controlled by whom?

Ans) The JVM controls the Garbage Collector; it decides when to run the Garbage Collector. JVM runs the Garbage Collector when it realizes that the memory is running low, but this behavior of jvm can not be guaranteed.
One can request the Garbage Collection to happen from within the java program but there is no guarantee that this request will be taken care of by jvm.

Q5) When does an object become eligible for garbage collection?

Ans) An object becomes eligible for Garbage Collection when no live thread can access it.

Q6) Can the Garbage Collection be forced by any means?
Ans) No. The Garbage Collection can not be forced, though there are few ways by which it can be requested there is no guarantee that these requests will be taken care of by JVM.

Q7) How can the Garbage Collection be requested?

Ans) There are two ways in which we can request the jvm to execute the Garbage Collection.

1) The methods to perform the garbage collections are present in the Runtime class provided by java. The Runtime class is a Singleton for each java main program.
The method getRuntime() returns a singleton instance of the Runtime class. The method gc() can be invoked using this instance of Runtime to request the garbage collection.
2) Call the System class System.gc() method which will request the jvm to perform GC.

Q8) What is the purpose of overriding finalize() method?

Ans) The finalize() method should be overridden for an object to include the clean up code or to dispose of the system resources that should to be done before the object is garbage collected.

What is HashMap and Map? |HashMap in java

2011-04-07T05:01:00.000-07:00

A map will traverse a tree to find a match. ie log complexity A hash map will create a hash of the string, and use that hash to index an array, giving constant lookup. note: collision detection is required because a hash of a string can produce the same index as the hash of another string. Therefore complexity is added to manage collisions.

map:

log traverse complexity
string comparison at each node

hashmap

hash creation (might be linear complexity to string size depending on algorithm to create hash)
constant lookup with hash
with consideration to collision. which might add linear complexity to number of collisions for the same hash.

java collections interview questions

2011-04-07T04:58:00.002-07:00

What is an Iterator interface? Is Iterator a Class or Interface? What is its use?
The Iterator is an interface, used to traverse through the elements of a Collection. It is not advisable to modify the collection itself while traversing an Iterator.
What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.
What is the List interface?
The List interface provides support for ordered collections of objects.
How can we access elements of a collection?
We can access the elements of a collection using the following ways:
1.Every collection object has get(index) method to get the element of the object. This method will return Object.
2.Collection provide Enumeration or Iterator object so that we can get the objects of a collection one by one.
What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
What’s the difference between a queue and a stack?
Stack is a data structure that is based on last-in-first-out rule (LIFO), while queues are based on First-in-first-out (FIFO) rule.
What is the Map interface?
The Map interface is used associate keys with values.
What is the Properties class?
The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.
Which implementation of the List interface provides for the fastest insertion of a new element into the middle of the list?
a. Vector
b. ArrayList
c. LinkedList
d. None of the above
ArrayList and Vector both use an array to store the elements of the list. When an element is inserted into the middle of the list the elements that follow the insertion point must be shifted to make room for the new element. The LinkedList is implemented using a doubly linked list; an insertion requires only the updating of the links at the point of insertion. Therefore, the LinkedList allows for fast insertions and deletions.
How can we use hashset in collection interface?
This class implements the set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the Null element.
This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets.
What are differences between Enumeration, ArrayList, Hashtable and Collections and Collection?
Enumeration: It is series of elements. It can be use to enumerate through the elements of a vector, keys or values of a hashtable. You can not remove elements from Enumeration.
ArrayList: It is re-sizable array implementation. Belongs to 'List' group in collection. It permits all elements, including null. It is not thread -safe.
Hashtable: It maps key to value. You can use non-null value for key or value. It is part of group Map in collection.
Collections: It implements Polymorphic algorithms which operate on collections.
Collection: It is the root interface in the collection hierarchy.
What is difference between array & arraylist?
An ArrayList is resizable, where as, an array is not. ArrayList is a part of the Collection Framework. We can store any type of objects, and we can deal with only objects. It is growable. Array is collection of similar data items. We can have array of primitives or objects. It is of fixed size. We can have multi dimensional arrays.
Array: can store primitive            ArrayList: Stores object only
Array: fix size                            ArrayList: resizable
Array: can have multi dimensional
Array: lang                                ArrayList: Collection framework
Can you limit the initial capacity of vector in java?
Yes you can limit the initial capacity. We can construct an empty vector with specified initial capacity
public vector(int initialcapacity)
What method should the key class of Hashmap override?
The methods to override are equals() and hashCode().
What is the difference between Enumeration and Iterator?
The functionality of Enumeration interface is duplicated by the Iterator interface. Iterator has a remove() method while Enumeration doesn't. Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects, where as using Iterator we can manipulate the objects also like adding and removing the objects.

Java Collections API Interview Questions |Java Collections Interview Questions

2011-04-07T04:58:00.000-07:00

What is HashMap and Map?
Map is Interface and Hashmap is class that implements this interface.
What is the significance of ListIterator?
Or
What is the difference b/w Iterator and ListIterator?
Iterator : Enables you to cycle through a collection in the forward direction only, for obtaining or removing elements
ListIterator : It extends Iterator, allow bidirectional traversal of list and the modification of elements
Difference between HashMap and HashTable? Can we make hashmap synchronized?
1. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls).
2. HashMap does not guarantee that the order of the map will remain constant over time.
3. HashMap is non synchronized whereas Hashtable is synchronized.
4. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.
Note on Some Important Terms
1)Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.
2)Fail-safe is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke "set" method since it doesn’t modify the collection "structurally”. However, if prior to calling "set", the collection has been modified structurally, "IllegalArgumentException" will be thrown.
HashMap can be synchronized by
Map m = Collections.synchronizeMap(hashMap);
What is the difference between set and list?
A Set stores elements in an unordered way and does not contain duplicate elements, whereas a list stores elements in an ordered way but may contain duplicate elements.
Difference between Vector and ArrayList? What is the Vector class?
Vector is synchronized whereas ArrayList is not. The Vector class provides the capability to implement a growable array of objects. ArrayList and Vector class both implement the List interface. Both classes are implemented using dynamically resizable arrays, providing fast random access and fast traversal. In vector the data is retrieved using the elementAt() method while in ArrayList, it is done using the get() method. ArrayList has no default size while vector has a default size of 10. when you want programs to run in multithreading environment then use concept of vector because it is synchronized. But ArrayList is not synchronized so, avoid use of it in a multithreading environment.

java.net.ConnectException: Connection refused: connect; No available router to destination

2011-04-05T23:26:00.000-07:00

Question :
I am trying to read from a URL and display the contents in an Applet. I am using URL,HttpURLConnection classes. I am consistently getting Connection time out Exception. I am unable to connect. Could any one help me out with this. I have tried some TimeOut handler classes available on the net but still I am getting the same Exception. Please provide your comments if you have come across similar problems before.

Here is the code Iam using

url = new URL((URL)null, str, new HttpTimeoutHandler(15000)); // timeout value in milliseconds
HttpURLConnection httpConnection = (HttpURLConnection) url.openConnection();
httpConnection.connect();
nputStream is = httpConnection.getInputStream();

Solution;
connection timeouts occur when:

the IP address for the requested server is successfully found connection establishment packets are dispatched to the IP address the destination address deliberately ignores or does not receive them
similar to connection timeout is Connection Refused, but in this case the destination system is actually sending packets back saying "go away, there is no service running on the port you are trying to connect to"
your packets are just being lost.. either the system or some intermediate system is firewalled. follow CMA's advice.. if you cant connect using telnet, a browser or some other app, its a network problem, not a java one

java.net.connectexception connection timed out connect Exception In java

2011-04-05T23:24:00.000-07:00

I'm getting a ConnectException: Connection timed out with some frequency from my code. The URL I am trying to hit is up. The same code works for some users, but not others. It seems like once one user starts to get this exception they continue to get the exception.

java.net.ConnectException: Connection timed outCaused by: java.net.ConnectException: Connection timed out
    at java.net.PlainSocketImpl.socketConnect(Native Method)
    at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
    at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
    at java.net.Socket.connect(Socket.java:516)
    at java.net.Socket.connect(Socket.java:466)
    at sun.net.NetworkClient.doConnect(NetworkClient.java:157)
    at sun.net.www.http.HttpClient.openServer(HttpClient.java:365)
    at sun.net.www.http.HttpClient.openServer(HttpClient.java:477)
    at sun.net.www.http.HttpClient.<init>(HttpClient.java:214)
    at sun.net.www.http.HttpClient.New(HttpClient.java:287)
    at sun.net.www.http.HttpClient.New(HttpClient.java:299)

Solution:

Connection Timeouts (assuming a local network and several client machines) typically result from
a) some kind of firewall on the way that simply eats the packets without telling the sender things like "No Route to host"
b) packet loss due to wrong network configuration or line overload
c) too many requests overloading the server
d) a small number of simultaneously available threads/processes on the server which leads to all of them being taken. This happens especially with requests that take a long time to run and may combine with c).

Segmentation fault (core dumped) in Unix

2011-04-05T23:21:00.000-07:00

A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access, or attempts to access a memory location in a way that is not allowed (for example, attempting to write to a read-only location, or to overwrite part of the operating system).
Segmentation fault can also occur under following circumstances:
a) A buggy program / command, which can be only fixed by applying patch.
b) It can also appear when you try to access an array beyond the end of an array under C programming.
c) Inside a chrooted jail this can occur when critical shared libs, config file or /dev/ entry missing.
d) Sometime hardware or faulty memory or driver can also create problem.
e) Maintain suggested environment for all computer equipment (overheating can also generate this problem).

Oracle PL/SQL Character / String Functions :ASCII Function

2011-04-03T12:27:00.000-07:00

In Oracle/PLSQL, the ascii function returns the NUMBER code that represents the specified character.
Syntax:

ascii( Character_name )

Character_name is the specified character to retrieve the NUMBER code for. If more than one character is entered, the function will return the value for the first character and ignore all of the characters after the first.

Example:

SELECT ASCII('t') FROM dual; Output :116
SELECT ASCII('T') FROM dual; Output :84
SELECT ASCII('Thanks') FROM dual; Output :84

Oracle PL/SQL Tutorial :LAST_DAY, TO_CHAR, TO_DATE

2011-04-01T20:53:00.000-07:00

Oracle PL/SQL Tutorial :LAST_DAY, TO_CHAR, TO_DATE

SQL>

SQL> SELECT TO_CHAR(LAST_DAY(TO_DATE('01APR2011','ddMONyyyy')),

 'Month dd, yyyy') FROM dual;

TO_CHAR(LAST_DAY(T
------------------
  April 01, 2011

SQL>
SQL>

Oracle PL/SQL Date Timestamp Functions :Date Functions

2011-04-01T20:48:00.000-07:00

We must format dates to see all of the information contained in a date column.

We use the TO_CHAR function to do the converting.

 SQL> SELECT SYSDATE FROM dual;

SYSDATE
---------
01-APR-11

SQL>

 SQL> SELECT TO_CHAR(SYSDATE, 'ddMONyyyy') Today
  2  FROM dual;

TODAY
---------
01APR2011

SQL>

Oracle PL/SQL DateTimestamp Functions :Time Interval Related Functions

2011-04-01T20:47:00.000-07:00

Function	Description
NUMTODSINTERVAL(x, interval_unit)	Converts the number x to an INTERVAL DAY TO SECOND with the interval for x supplied in interval_unit, which you may set to DAY, HOUR, MINUTE, or SECOND.
NUMTOYMINTERVAL(x, interval_unit)	Converts the number x to an INTERVAL YEAR TO MONTH with the interval for x supplied in interval_unit, which you may set to YEAR or MONTH.
TO_DSINTERVAL(x)	Converts the string x to an INTERVAL DAY TO SECOND.
TO_YMINTERVAL(x)	Converts the string x to an INTERVAL YEAR TO MONTH.

Oracle PL/SQL Date Timestamp Functions :Timestamp-Related Functions

2011-04-01T20:46:00.000-07:00

Function	Description
CURRENT_TIMESTAMP()	Returns a TIMESTAMP WITH TIME ZONE containing the current session time along with the session time zone.
EXTRACT({ YEAR \| MONTH \| DAY \| HOUR \| MINUTE \| SECOND } \| { TIMEZONE_HOUR \| TIMEZONE_MINUTE } \| { TIMEZONE_REGION \| } TIMEZONE_ABBR ) FROM x)	Extracts and returns a year, month, day, hour, minute, second, or time zone from x; x may be one of the timestamp types or a DATE.
FROM_TZ(x, time_zone)	Converts the TIMESTAMP x and time zone specified by time_zone to a TIMESTAMP WITH TIMEZONE.
LOCALTIMESTAMP()	Returns a TIMESTAMP containing the current time in the session time zone.
SYSTIMESTAMP()	Returns a TIMESTAMP WITH TIME ZONE containing the current database time along with the database time zone.
SYS_EXTRACT_UTC(x)	Converts the TIMESTAMP WITH TIMEZONE x to a TIMESTAMP containing the date and time in UTC.
TO_TIMESTAMP(x, [format])	Converts the string x to a TIMESTAMP. You may also specify an optional format for x.
TO_TIMESTAMP_TZ(x, [format])	Converts the string x to a TIMESTAMP WITH TIMEZONE. You may also specify an optional format for x.

Oracle PL/SQL Date Timestamp Functions :Using Datetime Functions

2011-04-01T20:45:00.000-07:00

In the following table, x represents a datetime or a timestamp.

Function	Description
ADD_MONTHS(x, y)	Add y months to x. If y is negative, y months are subtracted from x.
LAST_DAY(x)	Get the last day of the month.
MONTHS_BETWEEN(x, y)	Returns the number of months between x and y. If x appears before y on the calendar, the number returned is positive.
NEXT_DAY(x, day)	Returns the datetime of the next day following x; day is specified as a literal string, for example SATURDAY.
NEW_TIME	Returns the time/day value from a time zone specified by the user.
ROUND(x [, unit])	Rounds x. By default, x is rounded to the beginning of the nearest day. You may supply an optional unit string to indicate the rounding unit.
SYSDATE()	Returns the current datetime set for the operating system.
TRUNC(x [, unit])	Truncates x. By default, x is truncated to the beginning of the day. You may supply an optional unit string that indicates the truncating unit.

Abstraction vs Encapsulation - Principles of Object Oriented Programming (OOPS)

2011-03-30T05:54:00.000-07:00

Abstraction
In order to process something from the real world we have to extract the essential characteristics of that object.
Data abstraction can be viewed as the process of refining away the unimportant details of an object, so that only the useful characteristics that define it remain. Evidently, this is task specific.

Encapsulation
Encapsulation is one step beyond abstraction. Whilst abstraction involves reducing a real world entity to its essential defining characteristics, encapsulation extends this idea by also modeling and linking the functionality of that entity. Encapsulation links the data to the operations that can be performed upon the data.

OOP makes use of encapsulation to enforce the integrity of a type (i.e. to make sure data is used in an appropriate manner) by preventing programmers from accessing data in a non-intended manner

Through encapsulation, only a predetermined group of functions can access the data. The collective term for datatypes and operations (methods) bundled together with access restrictions (public/private, etc.) is a class

Basic differences between Oracle and SQL Server

2011-03-29T20:56:00.000-07:00

ORACLE

Oracle provides password complexity enforcement rule.
Connect with one Schema and can work with other schema.
Oracle implemented the row versioning on the DB block level.
Oracle metadata is managed in the same manner as table data.So during data querying most of online DDL statements on the table can be performed simultaneously.
Starting from Oracle 9i the configuration of ROLLBACK SEGMENTS is automatic.
Oracle is well rich with Index Options.
Oracle provides Automatic Storage Management (ASM), which assists with RAC/Grid based cluster deployment.
Oracle provides proprietary cluster file system on Linux and Windows platforms that helps in avoiding the use of raw devices.
Oracle provides Materialized Views for performance improvements of Stored Data with multiple Tables.
Oracle has more recovery options for corrupted database, redo log or datafile than MSSQL.

MS SQL
Microsoft implemented row level lock based on each single row.
SQL Server

all DDL operations that are currently running on tables belong to database "Snapshot Isolation". "Snapshot Isolation" queries are prohibited.
SQL requires a complex setup of ROLLBACK Segments and Transaction Level use on it.
SQL has just BTREE Index while compare to Oracle.
SQL has limitations using Materialized Views.

What is the difference between DBMS and RDBMS?

2011-03-29T20:51:00.000-07:00

DBMS : Data Base Management System
1)A DBMS has to be persistent (it should be accessible when
the program created the data donot exist or even the
application that created the data restarted).
2) DBMS has to provide some uniform methods independent of a
specific application for accessing the information that is
stored.
3)DBMS does not impose any constraints or security with
regard to data manipulation. It is user or the programmer
responsibility to ensure the ACID PROPERTY of the database
4)In DBMS Normalization process will not be present

RDBMS: Relational Data Base Management System

1)RDBMS is based on relational model, in which data is
represented in the form of relations, with enforced
relationships between the tables.
2)RDBMS defines the integrity constraint for the purpose of
holding ACID PROPERTY.
3)In RDBMS, normalization process will be present to check
the database table cosistency
4)RDBMS helps in recovery of the database in case of loss of
database due to system failure or any other reason

A DBMS has to be persistent, that is it should be accessible when theprogram created the data ceases to exist or even the application thatcreated the data restarted. A DBMS also has to provide some uniformmethods independent of a specific application for accessing theinformation that is stored.
Many DBA's think that RDBMS is a Client Server Database system but thats not the case with RDBMS.

5 Common Errors in Setting Java Heap Size

2011-03-29T20:46:00.000-07:00

Two JVM options are often used to tune JVM heap size: -Xmx for maximum heap size, and -Xms for initial heap size. Here are some common mistakes I have seen when using them:

1. Only setting -Xms JVM option and its value is greater than the default maximum heap size, which is 64m. The default minimum heap size seems to be 0. For example,
      java -Xms128m BigApp
      Error occurred during initialization of VM
      Incompatible initial and maximum heap sizes specified
      The correct command should be java -Xms128m -Xmx128m BigApp. It’s a good idea to set the minimum and maximum heap size to the same value. In any case, don’t let the minimum heap size exceed the maximum heap size.
Heap size is larger than your computer’s physical memory. For example,
      java -Xmx2g BigApp
      Error occurred during initialization of VM
      Could not reserve enough space for object heap
      Could not create the Java virtual machine.
      The fix is to make it lower than the physical memory: java -Xmx1g BigApp
    2.Incorrectly use mb as the unit, where m or M should be used instead.
      java -Xms256mb -Xmx256mb BigApp
      Invalid initial heap size: -Xms256mb
      Could not create the Java virtual machine.
    3 The heap size is larger than JVM thinks you would ever need. For example,
      java -Xmx256g BigApp
      Invalid maximum heap size: -Xmx256g
      The specified size exceeds the maximum representable size.
      Could not create the Java virtual machine.
      The fix is to lower it to a reasonable value: java -Xmx256m BigApp
    4.The value is not expressed in whole number. For example,
      java -Xmx0.9g BigApp
      Invalid maximum heap size: -Xmx0.9g
      Could not create the Java virtual machine.
      The correct command should be java -Xmx928m BigApp
NOTE:
How to set java heap size in Tomcat?
Stop Tomcat server, set environment variable CATALINA_OPTS, and then restart Tomcat. Look at the file tomcat-install/bin/catalina.sh or catalina.bat for how this variable is used. For example,
set CATALINA_OPTS=”-Xms512m -Xmx512m” (Windows)
export CATALINA_OPTS=”-Xms512m -Xmx512m” (ksh/bash)
setenv CATALINA_OPTS “-Xms512m -Xmx512m” (tcsh/csh)
In catalina.bat or catallina.sh, you may have noticed CATALINA_OPTS, JAVA_OPTS, or both can be used to specify Tomcat JVM options. What is the difference between CATALINA_OPTS and JAVA_OPTS? The name CATALINA_OPTS is specific for Tomcat servlet container, whereas JAVA_OPTS may be used by other java applications (e.g., JBoss). Since environment variables are shared by all applications, we don’t want Tomcat to inadvertently pick up the JVM options intended for other apps. I prefer to use CATALINA_OPTS.
How to set java heap size in JBoss?
Stop JBoss server, edit $JBOSS_HOME/bin/run.conf, and then restart JBoss server. You can change the line with JAVA_OPTS to something like:
JAVA_OPTS=”-server -Xms128m -Xmx128m”
How to set java heap size in Eclipse?
You have 2 options:
1. Edit eclipse-home/eclipse.ini to be something like the following and restart Eclipse.
-vmargs
-Xms64m
-Xmx256m
2. Or, you can just run eclipse command with additional options at the very end. Anything after -vmargs will be treated as JVM options and passed directly to the JVM. JVM options specified in the command line this way will always override those in eclipse.ini. For example,
eclipse -vmargs -Xms64m -Xmx256m
How to set java heap size in NetBeans?
Exit NetBeans, edit the file netbeans-install/etc/netbeans.conf. For example,
netbeans_default_options=”-J-Xms512m -J-Xmx512m -J-XX:PermSize=32m -J-XX:MaxPermSize=128m -J-Xverify:none
How to set java heap size in Apache Ant?
Set environment variable ANT_OPTS. Look at the file $ANT_HOME/bin/ant or %ANT_HOME%\bin\ant.bat, for how this variable is used by Ant runtime.
set ANT_OPTS=”-Xms512m -Xmx512m” (Windows)
export ANT_OPTS=”-Xms512m -Xmx512m” (ksh/bash)
setenv ANT_OPTS “-Xms512m -Xmx512m” (tcsh/csh)
How to set java heap size in jEdit?
jEdit is a java application, and basically you need to set minimum/maximum heap size JVM options when you run java command. jEdit by default runs with a default maximum heap size 64m. When you work on large files, you are likely to get these errors:
java.lang.OutOfMemoryError: Java heap space
at java.lang.String.concat(String.java:2001)
at org.gjt.sp.jedit.buffer.UndoManager.contentInserted(UndoManager.java:160)
at org.gjt.sp.jedit.Buffer.insert(Buffer.java:1139)
at org.gjt.sp.jedit.textarea.JEditTextArea.setSelectedText(JEditTextArea.java:2052)
at org.gjt.sp.jedit.textarea.JEditTextArea.setSelectedText(JEditTextArea.java:2028)
at org.gjt.sp.jedit.Registers.paste(Registers.java:263)
How to fix it? If you click a desktop icon, or Start menu item to start jEdit: right-click the icon or menu item, view its property, and you can see its target is something like:
C:\jdk6\bin\javaw.exe -jar “C:\jedit\jedit.jar”
You can change that line to:
C:\jdk6\bin\javaw.exe -Xmx128m -Xms128m -jar “C:\jedit\jedit.jar”
If you run a script to start jEdit: just add these JVM options to the java line inside the script file:
java -Xmx128m -Xms128m -jar jedit.jar
If you start jEdit by running java command: just add these JVM options to your java command:
java -Xmx128m -Xms128m -jar jedit.jar
Note that when you run java with -jar option, anything after -jar jar-file will be treated as application arguments. So you should always put JVM options before -jar. Otherwise, you will get error:
C:\jedit>java -jar jedit.jar -Xmx128m
Unknown option: -Xmx128m
Usage: jedit [] []
How to set java heap size in JavaEE SDK/J2EE SDK/Glassfish/Sun Java System Application Server?
Stop the application server, edit
$GLASSFISH_HOME/domains/domain1/config/domain.xml, search for XML element name java-config and jvm-options. For example,

-Xmx512m

-XX:NewRatio=2

-XX:MaxPermSize=128m

…

You can also change these settings in the web-based admin console, typically at http://localhost:4848/, or https://localhost:4848/. Go to Application Server near the top of the left panel, and then on the right panel, click JVM Settings -> JVM Options, and you will see a list of existing JVM options. You can add new ones and modify existing ones there.
Yet another option is to use its Command Line Interface (CLI) tool command, such as:
./asadmin help create-jvm-options
./asadmin help delete-jvm-options
They may be a bit hard to use manually, but are well suited for automated scripts.