package  mypackage;
 import  java.sql.Connection;
 import  java.sql.DriverManager;
 import  java.sql.ResultSet;
 import  java.sql.SQLException;
 import  java.sql.Statement;

 public   class  OracleDBOperation 
{

     private  Connection connection =  null ;
     private  String oracleURL = "jdbc:oracle:thin:@localhost:1521:EX";
     private  Statement statement;
     private  ResultSet resultSet;

     public   void  connect (String username, String userpassword) {
         try  {
             Class .forName("oracle.jdbc.driver.OracleDriver");
            connection = DriverManager.getConnection(oracleURL, username, userpassword);
        }
          catch  (ClassNotFoundException e) {
            System.err.println ("Error loading driver: " + e);
        }
         catch  (SQLException e) {
            System.err.println ("SQL Error: " + e);
        }
    }

     public   void  disconnect () {
         try  {
            connection.close();
        }
         catch  (SQLException e) {
            System.err.println ("SQL Error: " + e);
        }
    }
 }

db.connect("Alex","123");

SQL Error: java.sql.SQLException: Listener refused the connection with the following error:
ORA- 12505 , TNS:listener does not currently know of SID given in connect descriptor
The Connection descriptor used by the client was:
localhost: 1521 :EX

SQL Error: java.sql.SQLException: ORA- 00604 : error occurred at recursive SQL level  1 
ORA- 12705 : Cannot access NLS data files or invalid environment specified