JDBC NullPointerException - help, I'm stuck!

Question

import java.sql.*;
import java.io.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;
public class capito extends JFrame implements ItemListener{
          public Vector comb(String tab) throws SQLException, IOException, NullPointerException{
              try
                  { Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");  }
             catch (ClassNotFoundException e)
                    System.out.println ("Could not load the driver.");
          String url = "jdbc:odbc:dizionarioJDBC";
              Connection conn = DriverManager.getConnection(url,"", "");
          Vector vettoreLemmi = new Vector();
              Statement stmt = conn.createStatement();
              ResultSet rset = stmt.executeQuery ("select * from "+tab);
          int me;
          //this.addItem("seleziona il lemma");
          while(rset.next()){
            String ueo = rset.getString("lemma");
            if (ueo!=null){
            if (ueo.length()<20) {
                 me = ueo.length()-1;
            } else {
                   me = 20;
            vettoreLemmi.add(ueo.substring(0,me));
          return vettoreLemmi;
     Font carattere = new Font("Verdana", Font.BOLD, 10);
     JTextField oeo = new JTextField(20);
     String tab = "A";
     Vector vs = comb(tab);
     String[] tabelle ={"A","BaBe","C","D","E","F","G","H","IeJ","K","L","M","N","O","P","Q","R","S","T","U","V","X","Y","Z"};
     JComboBox lettere = new JComboBox(tabelle);
     JComboBox lemmi = new JComboBox(vs);
     public void itemStateChanged(ItemEvent evt){
          Object src = evt.getItem();
          String h = src.toString();
          this.tab = h;
          oeo.setText(h);
          try{
               Vector vetto = comb(this.tab);
               lemmi.removeAllItems();
               for(int k=0;k<vetto.size();k++){
               lemmi.addItem(vetto.elementAt(k).toString());
          } catch (SQLException e){} catch(IOException g){}catch(NullPointerException u){}
     capito(String title,int w,int h) throws SQLException, IOException{
          super(title);
          Toolkit kit = Toolkit.getDefaultToolkit();
          Dimension scrw = kit.getScreenSize();
          setSize(scrw.width,scrw.height);
          setResizable(false);
          setUndecorated( true );
          oeo.setFont(carattere);
          cornice pannello = new cornice();
          //JPanel pannello = new JPanel();
          setContentPane(pannello);
          pannello.setLayout(new FlowLayout());
          this.lettere.setFont(carattere);
          pannello.add(this.lettere);
          lettere.addItemListener(this);
          pannello.add(lemmi);
          oeo.setEditable(false);
          pannello.add(oeo);
          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          setVisible(true);
     public class cornice extends JPanel{
          Toolkit kit = Toolkit.getDefaultToolkit();
          Dimension scrw = kit.getScreenSize();
          public void paintComponent(Graphics comp){
          Color culo = new Color(88,131,180);
          Graphics2D gra = (Graphics2D)comp;
          gra.setColor(culo);
          //gra.drawRoundRect(5,5,530,360,10,10);
          gra.fillRoundRect(5,5,(scrw.width-15),(scrw.height-15),10,10);
public static void main(String[] args) throws SQLException, IOException{
capito fede = new capito("ricerca",550,400);
}above is the code..
I'm a real newB and it'll probably look like a sloppy, buggy code, but still:
On some of the tables in my DB (tables BaBe,C,O,M,R) i keep gettin a NullPointerException. Other tables work fine.
I read about this kind of exception but I can't figure out where the problem is! the tables r all filled with data, so the String field "lemma" ain't null..
Any clue?

Answer 1

btw I'm learning Java with this book and most of the
examples extend swing classes, so I thought it was
ok..
I started coding 4 days ago and your advice is really
useful at this point.
I'll try and use swing classes as they are unless I
really mean to add functionalities.
The matter is that I really HAVE to find a way of
queryin a huge database (this ain't a school project)
in few seconds and I wanted to check Java speed on
doin it.
Plus I WANNA LEARN! :-)You don't have to be perfect right away, but think about what I said and try to figure out why it's beneficial as you learn. At first it porbably won't make sense but then one day it will click.
For now, pull out the capito method and create a new class. What you will be doing is creating a DAO (dabase access object) that you will use from other parts of the code. This means that if next week, you need to make a web version of this program, you won't need to rewrite that part of the code.
public class YourClassNameHere
   public Vector comb(String tab) throws SQLException, IOException, NullPointerException
      try {
         Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
      } catch (ClassNotFoundException e) {
         System.out.println ("Could not load the driver.");
      String url = "jdbc:odbc:dizionarioJDBC";
      Connection conn = DriverManager.getConnection(url,"", "");
      List vettoreLemmi = new ArrayList();
      Statement stmt = conn.createStatement();
      ResultSet rset = stmt.executeQuery ("select * from "+tab);
      int me;
      while(rset.next()){
         String ueo = rset.getString("lemma");
         if (ueo!=null){
            if (ueo.length()<20) {
               me = ueo.length()-1;
            } else {
               me = 20;
            vettoreLemmi.add(ueo.substring(0,me));
      return vettoreLemmi;          
}