我想用输出语句一次性全部输出，结果出错，只能一次一次输出，为什么？

package com.imooc.name;

import java.io.Serializable;

public class Product implements Serializable{

private int id;

private String name;

private String categories;

private int price;

public Product() {

super();

}

public Product(int id, String name, String categories, int price) {

super();

this.id = id;

this.name = name;

this.categories = categories;

this.price = price;

}

public int getId() {

return id;

}

public void setId(int id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public String getCategories() {

return categories;

}

public void setCategories(String categories) {

this.categories = categories;

}

public int getPrice() {

return price;

}

public void setPrice(int price) {

this.price = price;

}

    public String toString() {

    return ("产品id:"+this.getId()+"\n产品名称:"+this.getName()+"\n产品属性:"+this.getCategories()+"\n产品价格："+this.getPrice());

    }

}

package com.imooc.name;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.IOException;

import java.io.ObjectInputStream;

import java.io.ObjectOutputStream;

public class Test {

public static void main(String[] args) {

// TODO Auto-generated method stub

        Product iphone=new Product(123,"ipone","telephone",4888);

        Product ipad=new Product(234,"ipad","computer",5088);

        Product macbook=new Product(345,"macbook","computer",10688);

        Product iwatch=new Product(256,"iwatch","watch",4799);

        try {

FileOutputStream fi=new FileOutputStream("imooc.txt");

ObjectOutputStream ob=new ObjectOutputStream(fi);

FileInputStream fi1=new FileInputStream("imooc.txt");

ObjectInputStream ob1=new ObjectInputStream(fi1);

ob.writeObject(iphone);

ob.writeObject(ipad);

ob.writeObject(macbook);

ob.writeObject(iwatch);

ob.flush();

try {

Product product=(Product)ob1.readObject();

System.out.println(product);

Product product1=(Product)ob1.readObject();

System.out.println("\n"+product1);

Product product2=(Product)ob1.readObject();

System.out.println("\n"+product2);

Product product3=(Product)ob1.readObject();

System.out.println("\n"+product3);

} catch (ClassNotFoundException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

} catch (FileNotFoundException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

}