通过选择JComboBox中的项目来创建组件

Question

如何在JComboBox中选择项目后创建组件。下面是我编码的方式，但它似乎没有做任何事情（即使我将它添加到ActionListener或ItemListener）。我是新来的，如果有什么不对的，请告诉我。如果它已经有答案，请链接我。

import java.awt.FlowLayout;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;

import javax.swing.DefaultComboBoxModel;
import javax.swing.JComboBox;
import javax.swing.import javax.swing.JTextField;

public class Tester extends JFrame {
    private static final long serialVersionUID = 1L;
    private JComboBox<String> cbOne;
    private JTextField textfield;

    public Tester() {
        super("Create jtextfield throught combobox's item");
        setLayout(new FlowLayout());
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setAlwaysOnTop(true);
        setLocationRelativeTo(null);
        setSize(500, 300);

        cbOne = new JComboBox<String>();
        cbOne.setModel(new DefaultComboBoxModel<String>(new String[] { "A", "B", "C" }));
        cbOne.setSelectedItem(null);

// If you choose an item in combobox, it will create textfield
        cbOne.addItemListener(new ItemListener() {
            public void itemStateChanged(ItemEvent arg0) {
                if (cbOne.getSelectedItem() != null) {
                    textfield = new JTextField();
                    add(textfield);
                }
            }
        });
        add(cbOne);
    }
    public static void main(String[] args) {
        new Tester().setVisible(true);
    }
}