Cannot populate jcombobox depending on other combobox from postgres

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Java

PostgreSQL

netbeans

There are two ComboBoxes "jComboCins1 and jComboCinsM". I want to populate jComboCinsM depending on jComboCins1 from postgresql. Here is the code, no errors but still cannot populate. Thanks in advance for all replies.

public class MekleFrame extends javax.swing.JFrame {


        Connection conn = null;
        ResultSet rs = null;
        PreparedStatement pst = null;
        Statement st = null;

    /**
     * Creates new form MekleFrame
     */
    public MekleFrame() {
        initComponents();
        this.setLocationRelativeTo(null);
        this.setResizable(false);
        jTextFielddummy.setVisible(false);



       conn = dbConnection.ConnectDB();
       update_table();
       pop_combo();
    }
private void pop_combo(){

      try {    
        //String cnsCo=jComboCins1.getSelectedItem().toString();
        String sql = "SELECT * from cinsdb WHERE sinif= ?";
        pst=conn.prepareStatement(sql);
        //pst.setString(1,cnsCo);
        pst.setString(1,String.valueOf(jComboCins1.getSelectedItem()));
        rs=pst.executeQuery();

        while (rs.next())
        {
        String cinsad = rs.getString("cinsad");
        jComboCinsM.addItem(cinsad);
        }
    }
    catch(Exception e){

    JOptionPane.showMessageDialog(null, e);

    }

    }
}

What I have tried:

If i change the SQL Query without "where" and disable pst.setString() i can populate ComboBox. I guess there is someting wrong with the ResultSet or PreparedStatement. I tried the SQL Query as "SELECT cinsad from cinsdb WHERE sinif= ?", didnt work either. By the way SQL connection and Queries work fine, tested.