addbutton.addActionListener (new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                String sid = new String(idtf.getText()) ;
                String n = new String(nametf.getText()) ;
                String t = new String(teltf.getText()) ;
                System.out.println(sid+" "+n+" "+t);
                Str2DB ();
    }
}

public void Str2DB (){
        try {
            st = c.createStatement();
            System.out.println(sid+" "+n+" "+t);  // null null null
            st.execute("INSERT INTO peoples (id,name,telephone) VALUES ('" +sid+ "','" +n+ "','" +t+ "')");
        } catch (SQLException ae) {
            ae.printStackTrace();  
        }
    }