如何用 java swing打开或者保存文件

import javax.swing.*;
import javax.swing.filechooser.FileFilter;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.*;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * Created by asus on 2017/3/21.
 */
public class NotePad extends JFrame implements ActionListener, ThreadProc {
    private JTextArea text = null; // 定义文本区域
    private JScrollPane jScrollPane = null; // 定义滚动条面板
    private JMenuBar menuBar = null; // 定义JMenuBar
    private JMenu menuFile, menuSet = null; // 定义菜单对象
    private JMenuItem newItem, openItem, saveItem, exitItem = null; // 定义文件菜单各个菜单项组件
    private JMenuItem fontItem, colorItem = null; // 定义设置菜单的菜单项组件
    private JLabel timeLabel, numberLabel = null;  //状态栏显示系统时间
    private JButton openButton, saveButton;
    private JFileChooser jFileChooser;

    public NotePad() {
        initNotepade();
        init();
        setTitle("我的记事本");
        setSize(800, 600);
        setVisible(true);
        setLocationRelativeTo(null);
        setJMenuBar(menuBar); // 在窗体中加入JMenuBar组件
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        openButton = new JButton("打开");
        saveButton = new JButton("保存");
        add(openButton, BorderLayout.NORTH);
        add(saveButton, BorderLayout.SOUTH);
        saveButton.addActionListener(this);
        openButton.addActionListener(this);
    }

    public void init() {
        text = new JTextArea();
        text.setFont(new Font("宋体", Font.BOLD, 20));
        text.setEditable(true); // 设置文本域组件可编辑
        jScrollPane = new JScrollPane(text);
        add(jScrollPane, BorderLayout.CENTER);
        menuBar = new JMenuBar();
        menuBar.setLayout(new FlowLayout(FlowLayout.LEFT, 10, 2));
        menuFile = new JMenu("文件");
        menuFile.setIcon(new ImageIcon(this.getClass().getResource("/img/file.png")));
        menuSet = new JMenu("设置");
        menuSet.setIcon(new ImageIcon(this.getClass().getResource("/img/set.png")));
        menuBar.add(menuFile);
        menuBar.add(menuSet);
        newItem = new JMenuItem("新建", new ImageIcon(this.getClass().getResource("/img/new.png")));
        openItem = new JMenuItem("打开", new ImageIcon(this.getClass().getResource("/img/open.png")));
        saveItem = new JMenuItem("保存", new ImageIcon(this.getClass().getResource("/img/save.png")));
        exitItem = new JMenuItem("退出");
        newItem.setMnemonic('N');
        newItem.setAccelerator(KeyStroke.getKeyStroke('N', java.awt.Event.CTRL_MASK));//设置快捷键 Ctrl+N
        openItem.setMnemonic('O');
        openItem.setAccelerator(KeyStroke.getKeyStroke('O', java.awt.Event.CTRL_MASK));//设置快捷键 Ctrl+O
        saveItem.setMnemonic('S');
        saveItem.setAccelerator(KeyStroke.getKeyStroke('S', java.awt.Event.CTRL_MASK));// 设置快捷键Ctrl+S
        newItem.addActionListener(this);
        openItem.addActionListener(this);
        saveItem.addActionListener(this);
        exitItem.addActionListener(this);
        menuFile.add(newItem);
        menuFile.add(openItem);
        menuFile.add(saveItem);
        menuFile.addSeparator();
        menuFile.add(exitItem);
        fontItem = new JMenuItem("字体设置", new ImageIcon(this.getClass().getResource("/img/print.png")));
        menuSet.add(fontItem);
        fontItem.addActionListener(this);
        colorItem = new JMenuItem("颜色设置", new ImageIcon(this.getClass().getResource("/img/color.png")));
        menuSet.add(colorItem);
        colorItem.addActionListener(this);
        timeLabel = new JLabel();
        add(timeLabel, BorderLayout.SOUTH);
        numberLabel = new JLabel();
        numberLabel.setBounds(100, 100, 100, 100);
        numberLabel.setFont(new Font("宋体", Font.BOLD, 60));
        text.add(numberLabel);
        ClockThread clockThread = new ClockThread(this);  //创建时钟线程对象
        clockThread.start();
    }

    public static void main(String[] args) {
        new NotePad();
    }

    public void initNotepade() {
        jFileChooser = new JFileChooser();
        jFileChooser.addChoosableFileFilter(new FileFilter() {
            @Override
            public boolean accept(File f) {
                if (f.isDirectory()) {
                    return true;
                }
                if (f.getName().endsWith(".txt")) {
                    return true;
                }
                return false;
            }

            @Override
            public String getDescription() {
                return "*.txt";
            }
        });
    }

    InputStream inputStream = null;

    public void openFile(File file) {
        try {
            inputStream = new FileInputStream(file);
            byte[] b = new byte[(int) file.length()];
            inputStream.read(b);
            text.setText(new String(b, "GB2312"));
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                inputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    OutputStream outputStream = null;

    public void saveFile(File file) {
        try {
            outputStream = new FileOutputStream(file);
            String string = text.getText();
            outputStream.write(string.getBytes("UTF-8"));
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                outputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    @Override
    public void actionPerformed(ActionEvent e) {

        if (e.getSource() == newItem) {
            JOptionPane.showMessageDialog(null, "新建文件");
        }
        if (e.getSource() == openItem) {
            JOptionPane.showMessageDialog(null, "打开文件");
            jFileChooser.setDialogTitle("打开文件");
            int result = jFileChooser.showOpenDialog(this);
            if (result == JFileChooser.APPROVE_OPTION) {
                File file = jFileChooser.getSelectedFile();
                if (!file.exists()) {
                    JOptionPane.showMessageDialog(this, file + "指定的文件不存在！",
                            "文件不存在", JOptionPane.OK_OPTION);//显示一个对话框来实现是否覆盖源文件
                } else {
                    openFile(file);
                }
            }
        }
        if (e.getSource() == saveItem) {
            JOptionPane.showMessageDialog(null, "保存文件");
            jFileChooser.setDialogTitle("保存文件");
            int result = jFileChooser.showSaveDialog(this);
            if (result == JFileChooser.APPROVE_OPTION) {
                File file = jFileChooser.getSelectedFile();
                if (file.exists()) {
                    int opt = JOptionPane.showConfirmDialog(this, file + "文件已经存在,是否覆盖!",
                            "文件存在", JOptionPane.YES_NO_OPTION);//显示一个对话框来实现是否覆盖源文件
                    if (JOptionPane.YES_OPTION != opt)
                        return;
                }
                saveFile(file);
            }
        }
        if (e.getSource() == exitItem) {
            this.dispose();
        }
    }

    @Override
    public void time() {
        Date date = new Date();//创建日期对象
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        String timeString = sdf.format(date);
        timeLabel.setText(timeString);
    }

    @Override
    public void number() {
        int randomNumber = (int) (Math.random() * 47 + 1);
        String newRandom = String.valueOf(randomNumber);
        numberLabel.setText(newRandom);
    }
}