为什么我的对象读取器不读取文件中的所有对象

Question

当我从主目录调用“读取文件”时，它仅给我对象，但同一事物在其他应用程序上也起作用，而我的写操作不带参数？ 请建议对此代码进行必要的更改。 另外，请解释为什么这不起作用。 谢谢您的时间：）

 public class FileIo {
    String location = "D:\\Swabhav\\Swabhav\\Practice\\Contact.txt";

    public void writeToFile(String fName, String lName, long phoneNo, String emailId) 
            throws IOException {
        Vector<Contact> val = new Vector<Contact>();
        val.add(new Contact(fName, lName, phoneNo, emailId));
        FileOutputStream fos = new FileOutputStream(location, true);
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(val);
        oos.close();
    }

    public void displayFile() throws Exception {
        FileInputStream fis = new FileInputStream(location);
        ObjectInputStream ois = new ObjectInputStream(fis);
        Vector<Contact> deserialize = (Vector<Contact>) ois.readObject();
        System.out.println(deserialize);
        Iterator<Contact> iter = deserialize.iterator();
        while(iter.hasNext()) {
            Contact v = iter.next();
            System.out.println(v.getfName() + " " +
            v.getlName() + " " +
            v.getEmailId() + " " +
            v.getPhoneNo());
        }
        ois.close();    
    }
}

public class MainMenu {
    Scanner sc = new Scanner(System.in);
    FileIo fileObj = new FileIo();


    public static void main(String[] args) {
        MainMenu mm = new MainMenu();
        try {
            mm.openMenu();
        } catch (Exception e) {
            System.out.println(e.getStackTrace());
            System.out.println(e.getMessage());
        }
    }

    public void openMenu() throws Exception {
        MainMenu mm = new MainMenu();
        System.out.println("1. Add Contact.\n2. Display Contacts."
                + "\n3 Search Contact.\n4 Exit");
        //System.out.println("Total Number of contacts: " +fileObj.readFromFile());
        System.out.println("Enter your choice(1-4): ");
        int choice = sc.nextInt();
        switch(choice) {
        case 1 : 
            System.out.println("Enter First Name: ");
            String fName = sc.next();
            System.out.println("Enter Last Name: ");
            String lName = sc.next();
            System.out.println("Enter mobile number: ");
            long phoneNo = sc.nextLong();
            System.out.println("Enter e-mail ID: ");
            String emailId = sc.next();
            fileObj.writeToFile(fName, lName, phoneNo, emailId);
            System.out.println("Contact Added");
            openMenu();
        case 2 :
            fileObj.displayFile();
            openMenu();
        case 4 :
            System.out.println("Thank You");
            System.exit(0);
        }       
    }       
}
public class Contact implements Serializable, Comparable {

    private String fName;
    private String lName;
    private long phoneNo;
    private String emailId;

    public String getfName() {
        return fName;
    }

    public String getlName() {
        return lName;
    }

    public long getPhoneNo() {
        return phoneNo;
    }

    public String getEmailId() {
        return emailId;
    }

    public Contact(String fName, String lName, long phoneNo, String emailID) {
        this.fName = fName;
        this.emailId = emailID;
        this.lName = lName;
        this.phoneNo = phoneNo;
    }

    @Override
    public int compareTo(Object o) {
        Contact val = (Contact) o;
        return this.fName.compareTo(val.getfName());
    }
}