如何将文件中的对象添加到 ArrayList？ 获得 EOFException

Question

我正在从事一个基本上允许用户创建、编辑和显示学生的学校项目。 我有一个createStudent() ，它使用Scanner和ObjectOutputStream将信息写入文件。 displayStudent()使用ObjectInputStream从文件中读取数据并显示出来。 editStudent()的想法是要求用户输入他们想要编辑的学生的 ID，然后更改日期并将其写回文件，我一直在尝试做的是使用从文件中读取数据ObjectInputStream然后将该数据分配给ArrayList或HashMap ，我想我将使用 ArrayList 因为 Z063A5ZBC470661C7C179 当我尝试将文件中的数据添加到 ArrayList 中时，出现以下错误：

java.io.EOFException at java.base/java.io.ObjectInputStream$BlockDataInputStream.peekByte(ObjectInputStream.java:3231) at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1663) at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:519) at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:477) at MidTermProject.editStudent(MidTermProject.java:194) at MidTermProject.main(MidTermProject.java:381)这是我的editStudent()代码：

public static void editStudent() throws IOException {
        
        int editID;
        String student;
        ArrayList<String> studentEdit = new ArrayList<String>();
        
        Scanner keyboard = new Scanner(System.in);
        
        FileInputStream fstream = new FileInputStream("studentInfo.dat");
        ObjectInputStream inputFile = new ObjectInputStream(fstream);
        
        System.out.print("Enter the ID of the student you would like to edit: ");
        editID = keyboard.nextInt();
        
        try {
            student = (String) inputFile.readObject();
            studentEdit.add(student);
            System.out.print(studentEdit);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
///Added create student method
public static void createStudent() throws IOException {
        
        File file = new File("studentInfo.dat");
        boolean append = file.exists();
        
        Scanner keyboard = new Scanner(System.in);
            
        try (
                FileOutputStream fout = new FileOutputStream(file, append);
                MidTermProject oout = new MidTermProject(fout, append);
            ) {
            id = idGenerator.getAndIncrement();
            String convertedId = Integer.toString(getId());
            oout.writeObject(convertedId);
                
            System.out.print("\nPlease enter your information bellow.\n" + "\nFull Name: ");
            FullName = keyboard.nextLine();
            oout.writeObject(FullName);
                
            System.out.print("Address: ");
            address = keyboard.nextLine();
            oout.writeObject(address);
                
            System.out.print("City: ");
            city = keyboard.nextLine();
            oout.writeObject(city);
                
            System.out.print("State: ");
            state = keyboard.nextLine();
            oout.writeObject(state);
            
            oout.close();
            System.out.println("Done!\n");
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        
    }

这是 MidTermProject 的 class 代码

public class MidTermProject  extends ObjectOutputStream {

    private boolean append;
    private boolean initialized;
    private DataOutputStream dout;
    static AtomicInteger idGenerator = new AtomicInteger(0001);
    static int id;
    public static String FullName;
    public static String address;
    public static String city;
    public static String state;
    public static String className;
    public static String instructor;
    public static String department;
    public static String classNumber;
    public static String courseNumber;
    public static String year;
    public static String semester;
    public static String grade;
    public static String studentID;
    public static String courseID;
    public static String enrollmentID;
    
    public static HashMap<String, Integer> map = new HashMap<>();
    
    Scanner keyboard = new Scanner(System.in);
    
    protected MidTermProject(boolean append) throws IOException, SecurityException {
        super();
        this.append = append;
        this.initialized = true;
    }
    
    public MidTermProject(OutputStream out, boolean append) throws IOException {
        super(out);
        this.append = append;
        this.initialized = true;
        this.dout = new DataOutputStream(out);
        this.writeStreamHeader();
    }
    
    @Override
    protected void writeStreamHeader() throws IOException {
        if (!this.initialized || this.append) return;
        if (dout != null) {
            dout.writeShort(STREAM_MAGIC);
            dout.writeShort(STREAM_VERSION);
        }
    }

Answer 1

如果认为您在滥用序列化：序列化是好是坏是另一回事，但这应该是这样的：

List<Student> students = ... ; 
try (OuputStream os = Files.newOutputStream(Paths.get("out"));
     ObjectOutputStream oos = new ObjectOutputStream(os)) {
  oos.writeObject(students);
}

阅读它应该很简单：

try (InputStream is = Files.newInputStream(Paths.get("out"));
     ObjectInputStream iis = new ObjectInputStream(is)) {
  List<Student> students = iis.readObject(students);
}

您必须确保Student是可Serializable的并且只有可Serializable或瞬态字段：

class Student implements Serializable {
  private static final long serialVersionUID  = 1L;
  private long id;
  private String fullName;
  private String address;
  private transient String wontBeExported;
  ...
}

请注意，这些字段不是static：序列化是关于序列化 object 及其字段。 Static 字段不属于任何实例。

您也不必扩展ObjectOutputStream class，或者如果您这样做，您必须确保您阅读由您的ObjectOutputStream实现编写的 object 与您正在使用的ObjectInputStream对称：

• 如果你写一个 header，你的ObjectInputStream必须读到 header。
• 如果你写一个 object，你的ObjectInputStream必须读到 object。

顺序很重要：在读取 header 之前，您无法读取 object。

You should also read the Java tutorial: https://docs.oracle.com/javase/tutorial/jndi/objects/serial.html