Conexion con jdbc

/*
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* and open the template in the editor.
*/

package planestmaterias;
import java.sql.*;
import javax.swing.JOptionPane;

/**
*
* @author montes
*/
public class Conexion {

String url,usuario,password;
Connection c;
Statement s;
ResultSet rs;

public Conexion()//throws ClassNotFoundException, SQLException,InstantiationException, IllegalAccessException
{//LLave de Conexion

try{
Class.forName("com.mysql.jdbc.Driver");//.newInstance(); Hace una nueva instancia
//conectar();

}catch (Exception e){
JOptionPane.showMessageDialog(null, "No se realizo la conexion a la Base De Datos" );
}
url = "jdbc:mysql://10.5.0.72:3306/plataforma";//Puerto/BaseDatosusuario ="Administrador";
password="root";
c=null;
s = null;
rs = null;

}//Llave que cierra la conexion

////////////////////////////////////////////////////////////////////////////

public String retornaCarrera()//RETORNA TODA LA INFO DE INSTITUCIONES
{
String cadena=null;

try
{c=DriverManager.getConnection(url,usuario,password);
s=c.createStatement();
cadena="SELECT * FROM carreras";
rs=s.executeQuery(cadena);
cadena="";
//while(rs.next()){

while (rs.next()){
cadena+="" +
rs.getString("nombre_carrera")+"";

}
//}
rs.close();
s.close();
}
catch (Exception e)
{JOptionPane.showMessageDialog(null, e.getMessage());
}
finally
{
if(rs!=null){
try{
rs.close();
}catch(SQLException e){}
}
if(s!=null){
try{
s.close();
}catch(SQLException e){}
}
if(c!=null){
try{
c.close();
} catch(SQLException e ){}
}
}
return cadena;

}

////////////////////////////////////////////////////////////////////////////

public String retornaMateria(String carrera)//RETORNA TODA LA INFO DE INSTITUCIONES
{
String cadena=null;

try
{c=DriverManager.getConnection(url,usuario,password);
s=c.createStatement();
String cadena="SELECT * FROM carreras where clave_carrera = "+carrera;
rs=s.executeQuery(cadena);
cadena="";

while (rs.next()){
cadena+="" +
rs.getString("nombre_materia")+"";

}
rs.close();
s.close();
}
catch (Exception e){JOptionPane.showMessageDialog(null, e.getMessage());
}
finally
{
if(rs!=null)
{
try{
rs.close();
}catch(SQLException e){}
}
if(s!=null){
try{
s.close();
}catch(SQLException e){}
}
if(c!=null){
try{
c.close();
} catch(SQLException e ){}
}
}
return cadena;

}

/////////////////////////////777
public String retornaNombremat1(String cadena, String cadena2,String cad)//RETORNA TODA LA INFO DEINSTITUCIONES
{
cadena=null;
cadena2=null;
cad=null;
try
{
c=DriverManager.getConnection(url,usuario,password);
s=c.createStatement();
cadena="select * from materias";
cadena2="select nombre_materia from materias, carreras where materias.clave_carrera=carreras.clave_carrera where carreras.nombre_carrera="+cad;
rs=s.executeQuery(cadena);
cadena="";

//while(rs.next()){while (rs.next()){
cadena+="" +
rs.getString("nombre_materia")+"";

}

//}
rs.close();
s.close();
}
catch (Exception e)
{JOptionPane.showMessageDialog(null, e.getMessage());
}
finally
{
if(rs!=null)
{
try{
rs.close();
}catch(SQLException e){}
}
if(s!=null){
try{
s.close();
}catch(SQLException e){}
}
if(c!=null){
try{
c.close();} catch(SQLException e ){}
}
}
return cadena;

}

//////////////////////////////////77
public String retornaNombremat()//RETORNA TODA LA INFO DE INSTITUCIONES
{
String cadena=null;
try
{
c=DriverManager.getConnection(url,usuario,password);
s=c.createStatement();
cadena="SELECT * FROM materias";
rs=s.executeQuery(cadena);
cadena="";
//while(rs.next()){

while...