每天一点儿Java--ComboBox

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
import java.util.*;
import java.text.SimpleDateFormat;
/**
 * <p>Title: ComboBox下拉域演示</p>
 * <p>Description: 通过选择或这输入一种日期格式来格式化今天的日期</p>
 * <p>Copyright: Copyright (c) 2014</p>
 * <p>Filename: ComboBoxDemo.java</p>
 * @author 王海涛
 * @version 0.1
 */

public class ComboBoxDemo extends JPanel
                           implements ActionListener {
    static JFrame frame;
    JLabel result;
    String currentPattern;
/**
 *<br>方法说明：构造器。初始化窗体构件
 *<br>输入参数：
 *<br>返回类型：
 */
    public ComboBoxDemo() {
        setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
        String[] patternExamples = {
                 "dd MMMMM yyyy",
                 "dd.MM.yy",
                 "MM/dd/yy",
                 "yyyy.MM.dd G 'at' hh:mm:ss z",
                 "EEE, MMM d, ''yy",
                 "h:mm a",
                 "H:mm:ss:SSS",
                 "K:mm a,z",
                 "yyyy.MMMMM.dd GGG hh:mm aaa"
                 };

        currentPattern = patternExamples[0];

        //设置一个规范的用户界面
        JLabel patternLabel2 = new JLabel("从下拉列表中选择一种：");
        JComboBox patternList = new JComboBox(patternExamples);
        patternList.addActionListener(this);//patternList的监视器是这个面板
        patternList.setForeground(Color.yellow);

        //创建一个显示结果用户界面
        JLabel resultLabel = new JLabel("当前 日期/时间",
                                        JLabel.LEADING);//相当于LEFT
        result = new JLabel();
        result.setForeground(Color.black);
        result.setBorder(BorderFactory.createCompoundBorder(
             BorderFactory.createLineBorder(Color.cyan),
             BorderFactory.createEmptyBorder(7,7,7,7)
        ));

        //布置构件
        JPanel patternPanel = new JPanel();
        patternPanel.setLayout(new BoxLayout(patternPanel,
                               BoxLayout.PAGE_AXIS));
        patternPanel.add(patternLabel2);
        patternList.setAlignmentX(Component.LEFT_ALIGNMENT);
        patternPanel.add(patternList);

        JPanel resultPanel = new JPanel(new GridLayout(2, 1));//新建一个网格视图的面板
        resultPanel.add(resultLabel);
        resultPanel.add(result);

        patternPanel.setAlignmentX(Component.LEFT_ALIGNMENT);
        resultPanel.setAlignmentX(Component.LEFT_ALIGNMENT);

        add(patternPanel);
        add(Box.createRigidArea(new Dimension(0, 10)));
        add(resultPanel);

        setBorder(BorderFactory.createEmptyBorder(20,20,20,20));

        reformat();
    } 
/**
 *<br>方法说明：事件处理
 *<br>输入参数：
 *<br>返回类型：
 */
    public void actionPerformed(ActionEvent e) { //patternList监视器
        JComboBox cb = (JComboBox)e.getSource();
        String newSelection = (String)cb.getSelectedItem();
        currentPattern = newSelection;
        reformat();
    }
/**
 *<br>方法说明：格式和显示今天的日期
 *<br>输入参数：
 *<br>返回类型：
 */
    public void reformat() {
        Date today = new Date();
        SimpleDateFormat formatter =
           new SimpleDateFormat(currentPattern);
        try {
            String dateString = formatter.format(today);
            result.setForeground(Color.red);
            result.setText(dateString);
        } catch (IllegalArgumentException iae) {
            result.setForeground(Color.red);
            result.setText("Error: " + iae.getMessage());
        }
    }
/**
 *<br>方法说明：主方法
 *<br>输入参数：
 *<br>返回类型：
 */
    public static void main(String[] args) {
        JFrame.setDefaultLookAndFeelDecorated(true);

        //创建一个窗体
        frame = new JFrame("ComboBox");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //创建一个面版容器
        JComponent newContentPane = new ComboBoxDemo();
        newContentPane.setOpaque(true);
        frame.setContentPane(newContentPane);
        frame.setForeground(Color.cyan);

        //显示窗体
        frame.pack();
        frame.setVisible(true);
    }
}