issue on jcombobox

Here is an example that shows the selected value from the jcombobox to textfield.

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class  SelectComboboxValue{

    public static void main(String[] args) throws Exception{
        JFrame f=new JFrame();
        f.setLayout(null);
        JLabel lab=new JLabel("Select Programming Language: ");
        final JComboBox combo=new JComboBox();
        final JTextField text=new JTextField(20);
        combo.addItem("--Select--");
           combo.addItem("Java");
           combo.addItem("C/C++");
           combo.addItem(".NET");
           combo.addItem("Perl");


        ActionListener actionListener = new ActionListener(){
        public void actionPerformed(ActionEvent actionEvent) {
        ItemSelectable is = (ItemSelectable)actionEvent.getSource();
        String st=selectedString(is);
        text.setText(st);
        }
    };
       combo.addActionListener(actionListener);
        lab.setBounds(20,20,200,20);
        combo.setBounds(200,20,80,20);
        text.setBounds(280,20,100,20);
        f.add(lab);
        f.add(combo);
        f.add(text);

        f.setVisible(true);
        f.setSize(400,120);
    }
    static private String selectedString(ItemSelectable is) {
    Object selected[] = is.getSelectedObjects();
    return ((selected.length == 0) ? "null" : (String)selected[0]);
  } 
}