如何序列化和反序列化哈希表？

Question

我正在尝试 Serialize 和 Deserialize 和 Hashtable 但没有成功。

代码如下：

反序列化

    public static void read(File f) throws IOException, ClassNotFoundException
    {
        FileInputStream fos = new FileInputStream(f);
        ObjectInputStream oos = new ObjectInputStream(fos);

        list = new Hashtable<Date,String>((Hashtable<Date,String>)oos.readObject());
        oos.close();
    }

序列化

    public static void write(String s) throws FileNotFoundException, IOException
    {
        FileOutputStream fos = new FileOutputStream(s);
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(list);
    }

我写了 Class 日期，它不是 Java 的，但我写了 implemet Serializable。

在我 Deserialize Hashtable 并打印它之后，我只得到 {}.

我做错了什么？

编辑

主要Class:

public class Main implements Serializable
{
    public static void main(String[] args) throws ClassNotFoundException, IOException
    {
        String[] options = {"Existing file","New file"};

        int choice = JOptionPane.showOptionDialog(null,
                "Whould you like to use an existing file or make a new file?",
                "Reminders", JOptionPane.DEFAULT_OPTION, JOptionPane.QUESTION_MESSAGE, null, options, null);

        System.out.println(choice);

        if(choice == 0)  //existing file
        {
            JFileChooser fc = new JFileChooser();

            try
            {
                Thread.sleep(1);
            }
            catch(InterruptedException ex)
            {
                Thread.currentThread().interrupt();
            }

            int returnVal = fc.showOpenDialog(null);

            GUI.read(fc.getSelectedFile());
        }
        else if(choice == 1) //new file
        {
            String name;
            do { name = JOptionPane.showInputDialog("Please enter the name of the new file: "); }
            while((name == null) || (name.length() == 0));

            GUI.write(name);
        }
        else if(choice == -1)
        {
            JOptionPane.showMessageDialog(null, "Good Bye!");
            System.exit(0);
        }

GUI Class:

public class GUI extends JPanel implements Serializable
{
    private DateGUI date;
    private JButton save;
    private static JTextArea text;
    private JScrollPane scroll;

    private static Hashtable<Date,String> list = new Hashtable<Date,String>;

    public static void read(File f) throws IOException, ClassNotFoundException
    {
        FileInputStream fos = new FileInputStream(f);
        ObjectInputStream oos = new ObjectInputStream(fos);

        list = ((Hashtable<Date,String>)oos.readObject());
        oos.close();
    }

    public static void write(String s) throws FileNotFoundException, IOException
    {
        FileOutputStream fos = new FileOutputStream(s);
        ObjectOutputStream oos = new ObjectOutputStream(fos);

        oos.writeObject(list);
    }
    private class SaveListener implements ActionListener
    {
        @Override
        public void actionPerformed(ActionEvent e) 
        {
            if(e.getSource() == save)
            {
                list.put(new Date(DateGUI.getDay(),DateGUI.getMonth(),DateGUI.getYear()), text.getText());
                text.setText("");
            }

        }
    }
}

日期Class:有日、月、年3个String字段。我确实实现了 Serializable 并覆盖了 equals 和 HashCode.

DateGUI Class: 有 GUI "stuff" 和:

    public static String getDay()
    {
        return (String)day.getSelectedItem();
    }

    public static String getMonth()
    {
        return (String)month.getSelectedItem();
    }

    public static String getYear()
    {
        return (String)year.getSelectedItem();
    }

   private class ShowListener implements ActionListener
   {
       @Override
       public void actionPerformed(ActionEvent e) 
       {
           if(e.getSource() == show)
           {
               GUI.set(GUI.get(getDay(), getMonth(), getYear()));
           }
       }
   }

Answer 1

我相信你应该写完就冲水。

oos.writeObject(list); oos.close();

Answer 2

您是否正在向文件写入任何内容，因为 write 方法将 String 作为参数，但您尚未指定要写入哪个文件。尝试：

public static void write(File f) throws FileNotFoundException, IOException
{
    FileOutputStream fos = new FileOutputStream(f);
    ObjectOutputStream oos = new ObjectOutputStream(fos);

    Hashtable<Date, String> list = new Hashtable<>();
    list.put(YourDateObject, "String");

    oos.writeObject(list);
}

Answer 3

这是代码示例：

public class Main {

/**
 * File name
 */
private final static String FILENAME = "test.bin";

/**
 * Entry point
 * 
 * @param args
 * @throws Exception 
 */
public static void main(String[] args) throws Exception {

    Hashtable<Date, String> original = new Hashtable<>();

    // write some data to hashtable
    for (int i = 0; i < 3; i ++) {
        original.put(new Date(), String.valueOf(i));
        TimeUnit.MILLISECONDS.sleep(100);
    }

    // serialize
    write(FILENAME, original);

    // deserialize
    Hashtable<Date, String> restored = read(FILENAME);

    // compare results
    System.out.println(restored);
    System.out.println(restored);
}

/**
 * Deserialization
 * 
 * @param filename
 * @return
 * @throws IOException
 * @throws ClassNotFoundException 
 */
public static Hashtable<Date, String> read(String filename) throws IOException, ClassNotFoundException {
    try (ObjectInputStream oos = new ObjectInputStream(new FileInputStream(filename))) {
        return (Hashtable<Date, String>) oos.readObject();
    }
}

/**
 * Serialization
 * 
 * @param filename
 * @param list
 * @throws FileNotFoundException
 * @throws IOException 
 */
public static void write(String filename, Hashtable<Date, String> list) throws FileNotFoundException, IOException {
    try(ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(filename))) {
        oos.writeObject(list);
    }
}

}

Answer 4

After I deserialize the Hashtable and print it I am getting just {}.

What am I doing wrong?

此代码是从您的 MCVE 代码复制而来的。您已经将其编辑掉了，但我相信这个或类似的东西是您问题的真正原因。

public static void write(String s) throws FileNotFoundException, IOException
{
    list = new Hashtable<Date,String>();
    FileOutputStream fos = new FileOutputStream(s);
    ObjectOutputStream oos = new ObjectOutputStream(fos);

    oos.writeObject(list);
}

您应该在 oos 上调用 close()，但这不是真正的问题。

(缺失的close() 可能导致序列化文件为空或不完整/损坏，但这不会导致你读回一个空的Hashtable。你会得到一个异常。注意 new FileOutputStream(s) 将截断文件 ...)

真正的问题是方法的第一条语句。您正在无条件 分配一个新的空 Hashtable 给 list。然后你在写它。所以（自然地）当你读回你写入文件的内容时，你会再次得到一个空的 Hashtable 。这就是您看到的 {} 的意思。

简而言之，您的代码正在写出一个空 table 并再次读回。 Hashtable 序列化工作正常。

如何序列化和反序列化哈希表？

How to serialize and deserialize a Hashtable?

java

serialization

hashtable