Message

import java.io.Serializable;
import java.util.Date;

public class Message implements Serializable {
    private int id;
    private Date date;
    private String content;

    public Message(int id, Date date, String content) {
        this.id = id;
        this.date = date;
        this.content = content;
    }

    @Override
    public String toString() {
        return "Message{" +
                "id=" + id +
                ", date=" + date +
                ", content='" + content + '\'' +
                '}';
    }
}

MessageStore

import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.Serializable;
import java.util.Collection;
import java.util.List;

public interface MessageStore {

    void persist(Message message) throws IOException;
    void persist(Collection<Message> list) throws IOException;
    Message load() throws IOException, ClassNotFoundException;
    List<Message> load(int count) throws IOException, ClassNotFoundException;
}

TestForMessage

import java.io.*;
import java.sql.Date;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

public class TestForMessage implements MessageStore{

    @Override
    public void persist(Message message) throws IOException {
        FileOutputStream fos = new FileOutputStream("out_file.txt");
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(message);
        oos.flush();
        oos.close();
    }

    @Override
    public void persist(Collection<Message> list) throws IOException {
        FileOutputStream fos = new FileOutputStream("out_file.txt");
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        for (Message message : list) {
            oos.writeObject(message);
        }
        oos.flush();
        oos.close();
    }

    @Override
    public Message load() throws IOException, ClassNotFoundException {
        FileInputStream fis = new FileInputStream("out_file.txt");
        ObjectInputStream oin = new ObjectInputStream(fis);

        return (Message) oin.readObject();
    }

    @Override
    public List<Message> load(int count) throws IOException, ClassNotFoundException {
        FileInputStream fis = new FileInputStream("out_file.txt");
        ObjectInputStream oin = new ObjectInputStream(fis);

        List<Message> msgs = new ArrayList<>();
        for (int i = 0; i < count; i++) {
            msgs.add((Message) oin.readObject());
        }
        return msgs;
    }

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        //System.out.println(Date.valueOf("2018-03-21"));
        Message newMessage = new Message(1, Date.valueOf("2018-03-21"),"qwertry");
        TestForMessage testForMessage = new TestForMessage();
        testForMessage.persist(newMessage);

        Message msg = testForMessage.load();
        System.out.println(msg.toString());

        ArrayList<Message> newMessages = new ArrayList<>();
        newMessages.add(new Message(1, Date.valueOf("2018-03-21"),"qwertry"));
        newMessages.add(new Message(2, Date.valueOf("2018-03-22"),"zxcvbn"));
        testForMessage.persist(newMessages);

        List<Message> msgs = testForMessage.load(2);
        for (Message message : msgs) {
            System.out.println(message);
        }
    }
}