Learn Java-PART 1

History of java

In 1991, Sun Microsystems engineers led James Gosling decided to develop a language for consumer device like (Cable boxes etc). Consumers want to be hardware independent (means independent application, suppose you develop a application software in windows but it not capable to run on Unix) but hands on those person who free from these tenses. Since different manufactures would use different CPUs, different systems configurations but this language is ready to run on all platforms. The project as code name ‘GREEN’.

In this time, Sun microsystem uses UNIX for their project. We uses C++ language in this project because c++ language was used ‘object oriented’. The original name of the language was oak. And later  they changed the name(oak) to java in January 1995.

Finally, one big step was taken on 7 dec 1995 when Microsoft signed a letter of interest with sun for java technology source license.

Where it used?

Probably Java is used.

1.      Desktop Application such as acrobat reader, Antivirus etc.

2.      Web Application

3.      Enterprise application such as banking application

4.      Mobile such as android and java ME

5.      Embedded system

6.      Smart card

7.      Robotics

8.      Games etc.

Java is high level language. It is completely hardware  independent language means “write once, run anywhere and at anytime, forever”. Programs are run by an interpreter that converts the byte code to the appropriate native machine code.

Byte code: byte code consists of optimized set of instruction that are not specific to processor. We get byte code after compiling the java program using a compiler called javac.

Native code: native code is computer programming (code) that is compiled to run with a particular processor and its set of instruction.

JVM, JRE, JIT and JDK

JVM (java virtual machine):- the bytecode is to be executed by java runtime environment (JRE) which is called as java virtual machine. The program that are running on JVM must be compiled into a binary format which is denoted by .class files.

The  JVM execute .class or .jar files, by either interpreting it or using a just in time compiler (JITc).

Note: the JIT is used for compiling and not for interpreting the file. 

1.      JVM is like a specification (map) of documentation.

2.      JRE is like a implementation of document, it is not an open source.

3.      JDK (java development kit)- it is the collection of JRE and package file.

Note: every source code (.java) generate the .class file.

Firstly we are install the jdk in our computer.

Path set: there are two way  of run the java program.

1.      Computer icon right click -> property ->advanced system -> environment variable -> System variable -> new -> variable name is path and Variable value is address of the bin file.

C:/program files/java/jdk/bin

This the permanent method of path save of java program. You can be save anywhere in computer and run program.

2.      C drives -> program file -> java -> jdk -> bin -> save the program.

Simple program

Class A

{

Public static void main(String arr[])

{

System.out.println(“hello”);

}

}

Compile -> javac A.java

Execution -> java A

BY default java classes and packages are distributed in .jar format.

.jar file is the compress format of the file.

Note:  Java is open source. So you can check rt.java. it is the compress file of classes and package. This is provided by sun microsystem as part of JDK.

C:\program file\java\jdk\jre\lib\rt.jar

E:\jar -> describe all keyword

E:\jar –xf rt.jar

Void: No return type. If there is an error in the program then o.s check it and give the message.

Public : JRE is the outside part of the class. JRE call the main method and access specifier.

Static: In a class there are two type of members.

1.      Instance Member

2.      Class Member

Instance member: this member represent attributes

And behavior of individual object.

Class Member:- class member represent attribute & behavior of the whole class.

Note:  static keyword denotes class members.

By default all the member of a class are instance member.

Method: first letter of each word except the first word of the method is capitalized.

print();

getPrioprity();

getKeyMathodMap();       etc.

String arr[] : main method is to used command line argument. It is represent I/P provided by command prompt with name of command.

System: it is the final class in java.lang package. System class facility provided input, output, error, loading file & library.

Out: out is static member of the system class and is type printstream class.

Println: it is the method of printstream class.

Example:

 class printstream

{

Public void print(String s)

{

-------

}

Public void println(String s)

{

-------

}

Class system

{

Public static printstream out;

{------

}

                                                          Author-Ravi Kumar

Difference Between .ToString() and Convert.ToString()

This is a very important question which usually ask in the interviews.

Let us discuss-

Answer- This question have very simple answer that .Tostring() can’t handle the null value but Convert.ToString can.

Example 1- understand it with the help of program.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplication2
{
    public partial class Test : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
          
            Session["Name"] = null;
            string Name=Session["Name"].ToString();

          }
    }
}



Explanation- We can see in above example that there is a session which have null value, when we apply .ToString() it will throw Exception ‘object reference not set to an instance of an object’

Example 2-

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplication2
{
    public partial class Test : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
          
            Session["Name"] = null;
            string Name= Convert.ToString( Session["Name"]);

          }
    }
}

Explanation- We can see in above example that there is a session which also have null value, when we apply Convert.ToString it will not  throw Exception, it will handle it and sting ‘Name’ have  empty string.

                                                      Author- Er. Rahul Kr. Yadav

How we can find that – How many spaces and characters are there in a String

There is number of ways through which  we can achieve the above task, But I am going to tell you that How we can achieve it in optimize manner.

Example – There is a string-    "Rahul Yadav Dot Net Developer" , We need to find out how many spaces and characters are there ?

Program in C#-

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication6
{
    class Program
    {
        static void Main(string[] args)
        {

            Rahul obj = new Rahul();
            obj.Find_Char();

        }
    }

   class Rahul
    {

        public void Find_Char()
        {

            int k = 0;
            int j = 0;

           string Name = "Rahul Yadav Dot Net Developer";
           foreach (Char g in Name)
            {

                if (char.IsWhiteSpace(g))
                {

                    k = k + 1;

                }


               else
                {

                    j = j + 1;
               
               
                }
           
            }
            Console.WriteLine("Number Of Spaces :" + k);
            Console.Write("Number Of Characters :" + j);
            Console.ReadLine();

            }
      
    }
}


Output:

                                   
                                             Author- Er. Rahul Kr. Yadav

SQL Interview Questions and Answers Part -1

Question- What provider ADO.NET use by default?

Answer- There’s no default provider as such but there are generic providers (OLE DB and ODBC) which are not limited to a specific database such as SQL Server or Oracle.

Question - What is reference cursor in SQL?

Answer - A Ref cursor is just a pointer to the result set of the cursor with which it is associated.

Question - How to Display duplicate rows in a table?

Answer - This query for finding duplicates in a table. Suppose you want to find all Names in a table that exist more than once:

SELECT UserName,
 COUNT(UserName) AS TotalOccurrences
FROM users
GROUP BY UserName
HAVING ( COUNT(UserName) > 1 )

OR

SELECT *
  FROM EMP A
 WHERE EXISTS (SELECT 1
                 FROM EMP
                WHERE empno = A.empno AND ROWID < A.ROWID)

Question - How to select last N records from a Table ?

Answer - SELECT TOP N *  FROM EMP ORDER BY EMPNO DESC

Question - What is use of CASCADE CONSTRAINTS in SQL?

Answer - Cascade Constraints, are usually used when there exists a parent-child relationship between tables using foreign key or referential constraints defined.

For Example, In case of deletion a record from the Parent Table, where there exists a foreign key to the child table,
If you do not specify cascading deletes, the default behavior of the database prevents us from deleting data in the Parent table if the child table has a reference to the Parent.

Question – What is the use of in command in SQL.

Answer - The IN operator allows you to specify multiple values in a WHERE clause.

SQL IN Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name IN (value1,value2,...)

IN Operator Example

The "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

Now we want to select the persons with a last name equal to "Hansen" or "Pettersen" from the table above.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE LastName IN ('Hansen','Pettersen')

The result-set will look like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

Please refer the below link

http://www.w3schools.com/sql/sql_in.asp

Question - How to change database name in sql server.

Answer – “sp_renamedb”

You open that sp and find logic from there. Try this sp_helptext sp_renamedb

Question - How to find specify row value in SQL

Answer - Suppose

I have a table it's name is table1 , in it 3 column is specified , column name is id, name, date if in name column three are three same name like abc, abc, abc add id is a primary key, how to get second last name and date  in this table. table may be contain lot of records.

This way you will get the second last row (name, date). Incase if you need 3rd last you need to increase the 'red' number to 2 and so on......

 SELECT TOP 1 * FROM [Table1] 

WHERE ID NOT IN (SELECT TOP 1 ID FROM [Table1] ORDER BY ID DESC)

ORDER BY ID DESC