Then I have a class called MyDBConn.java, inside this class there's a
method called GetResultFromPazienti():

MyDBConn.java:
==================================

package cc;

import java.sql.*;

public class MyDBConn {
   private Connection myConnection;
   private java.sql.Statement stmt;

// ...

public ResultSet getResultFromPazienti2(String query) {
       ResultSet rs=null;
       try{
           rs=stmt.executeQuery(query);
       }
       catch(Exception e){
           alerts.showErr(e.getMessage());
       }
       return rs;
   }

// ...

==================================

After this, I have a class called Paziente.java, from this class I
would call method: getResultFromPazienti2(), class is something like
this:

File: Paziente.java
==================================
package cc;
import java.sql.*;

public class Paziente extends javax.swing.JFrame {
       private MyDBConn mdbc;
       private java.sql.Statement stmt;

// ...
// ...

   private void formWindowOpened(java.awt.event.WindowEvent evt)
{
       ResultSet rs=mdbc.getResultFromPazienti2("select ... from ...
where ...");

       try {
           rs.next();
                // ...
                txtNome.setText(rs.getString("NOME"));
                // ...
       } catch (SQLException ex) {
           ex.printStackTrace();
       }
   }
}

==================================

At first moment seem that are not problems, infact there's no problem
in compile time, but when I execute project and method
formWindowOpened() is started an "Exception in thread "AWT-
EventQueue-0" java.lang.NullPointerException", where are the
problems???
Thank you to all....