Jdbc Faqs-->2

JDBC interview questions and answers

6:  How to establish a Database connection between java application and Database?

If we want to establish a connection between java application and the database we will the following piece of code.

Connection con= DriverManager.getConnection(“jdbc:odbc:nag”,”nag”,”system”,”manager”);

Where getConnectin() is a static method from DriverManager class, which can be used to return connection object.

7:  Basically Connection is an interface, how getConnection() will create an object for
Connection interface?

Ans:   Connection is an interface from java.sql package, for which getConnection(_)  was return an anonymous inner class object of the Connection interface.

Note:-  Anonymous inner class is a nameless inner class, which can be sued to provide an implementation either for the interfaces or for abstract classes.

Eg:   interface I
         {
                void m1();
          }
         Class Outer
         {
                     I  i = new I(){
                                             public void m1()
                                             {
                                            
                                             }
                                             public void m2()
                                             {

}
                                           }
            }
            Outer o = new Outer();
            o.i.m1();   à  correct
            o.i.m2();   à  wrong

                        getConnection(_) is a static method from DriverManager class, which will call internally  connect() method, this connect() will establish a virtual socket connection in between the java application and the database.

8: What is the requirement to use Statement object?

• After establishing a connection between java application and the database we need to write the sql queries and we need to execute them.
• To execute the sql queries we will use some pre-defined library, which was defined in the form of Statement object,  PreparedStattement object and CallableStatement object.
• As per the application requirements we need to create either Statement object or CallableStatement object and PreparedStatement object.
• To create Statement object dwe will use the following method from connection object.

public  Statement createStatement()
Eg:     Statement st = con.createStatement();

9:  How to  execute SQL Queries from a java application?

To execute the sql queries we will use the following methods from Statement object.

• st.executeQuery(…)
• st.executeUpdate(…)
• st.execute(…)

10:  What are the differences between executeQuery(…), executeUpdate(…) and execute(…)
      methods?

Ans:    where executeQuery() can be used to execute selection group sql queries to fetch the data from database.
When we use selection group sql query with executeQuery() then JVM will send that sql query to the database engine, database engine will execute it, by this database engine(DBE) will fetch the data from database and send back to the java application.
Java is a purely object oriented technology. That’s why the jdbc application will maintain the fetched data from database, in the form of an object at heap memory, called as ResultSet object.

            public  ResultSet executeQuery(String sqlquery)

            where executeUpdate() can be used to execute updation group sql query to update the database. When we provide updation group sql query as a parameter to executeUpdate(), then JVM will send that sql query to DBE, here DBE will execute it and perform updations on the database, by this DBE will identify the number of records got updated value called as “records updated count” and return back to the java application.

            public int executeUpdate(String sqlquery)

            where execute() can be used to execute either selection group sql queries or updation group queries.
            When we use selection group sql query with the execute() then we will get ResultSet object at heap memory with the fetched data. But execute() will return “true” as a Boolean value.
            When we use updation group sql query with execute() then we will get “ records updated count value” at jdbc application. But execute() will return “false” as a Boolean value.

            public  boolean execute(String sqlquery)

11: How to create a table dynamically from a jdbc application?

      //import section
       import java.sql.*;
      import java.io.*;

      public class CreateTableEx
      {
               public static void main(String[] args)throws Exception
               {
                           //create buffered reader object
                  BufferedReader br = new BufferedReader(new       InputStreamReader(System.in));

                  //load and register the driver
                  Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);

                  //establish connection
                  Connection con = DriverManager.getConnection(“jdbc:odbc:nag”,”system”,”durga”);

                  //create statement object
                  Statement st = con.createStatement();

                  //take table name as dynamic input
                  System.out.println(“Enter table name”);
                  String tname = br.readLine();

                  //execute sql query
                  St.executeUpdate(“create table”+tname+”(eno number,ename varchar2(10),esal number,eaddr varchar2(10))”);

                  System.out.println(“table created successfully”);

                  //closing the connection
                  con.close();
      }
}

12: How to insert records into a table from a JDBC application?

import java.sql.*;
import java.io.*;
public class InsertTableEx
{
   public static void main(String[] args) throws Exception
   {
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            Class.forName(“oracle.jdbc.driver.OracleDriver”);
Connection con  =               DriverManager.getConnection(“jdbc:oracle:thin:@localhost:1521:xe”,”system”,”durga”);
Statement st = con.createStatement();
while(true)
{
            System.out.println(“Enter emp number”);
            Int eno = Integer.parseInt(br.readLine());
            System.out.println(“Enter emp name”);
            String ename = br.readLine();
            System.out.println(“Enter emp sal”);
            Float esal = Float.parseFloat(br.readLine());
            System.out.println(“Enter emp address”);
            String eaddr = br.readLine();
st.executeUpdate(“insert into emp1 values(“+eno+”,’”+ename+”’,”+esal+”,’”+eaddr+”’)”);
            System.out.println(“read successfully inserted”);
            System.out.println(“one more record[y/n]);
            String option = br.readLine();
            If(option.equals(“n”))
                        break;
}
      }
}