java swing 下拉框與文本框

import java.awt.*; 
import javax.swing.*;
import javax.swing.border.*;

import java.awt.event.*;
public class Test extends JFrame {
    private JPanel jPane;//大的面板
    private JTextField textField;//文本框
    public JComboBox comboBox;//下拉選擇框
    public static String a =null;
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                	Test frame = new Test();
                    frame.setVisible(true);
//             	設置窗口可見
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
 
    /**
     * Create the frame.
     */
    public Test() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//      EXIT_ON_CLOSE相當於執行System.exit(0);的動作方法退出應用程序，關閉進程
//      如果不寫這句話點擊X我們也能關閉窗口，但是在后台進程里面我們會發現我們的進程數並不減少
        setBounds(200, 200, 400, 400);//首次點開，發現距離屏幕左邊,上方200，長寬均為400
        jPane = new JPanel();
        jPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(jPane);
        jPane.setLayout(null);
//按照規定的位置顯示，否則按照默認的FlowLayout（每個JPanel對象初始化時都使用FlowLayout布局，）布局顯示的
         
        comboBox = new JComboBox();
        String[] select = {"選項一","選項二","選項三"};
//        添加選項值
        comboBox.setModel(new DefaultComboBoxModel(select));
        comboBox.setBounds(15, 15, 100, 25);
        jPane.add(comboBox);
     
        textField = new JTextField();
        textField.setBounds(150, 15, 76, 22);
        jPane.add(textField);
        textField.setText("選項一");
   
        comboBox.addItemListener(new ItemListener() { 
            @Override
            public void itemStateChanged(ItemEvent e) {
                if(e.getStateChange() == ItemEvent.SELECTED) {
                	 textField.setText(comboBox.getSelectedItem().toString());
                	 a = textField.getText();
                	 System.out.print(a);
                }
            }   
        });
    }
}