java – 序列化包含对象ArrayList的对象

public class User  implements  Serializable{

    public String name;
    public String surname;
    public List<Picked> pickedBooks = new ArrayList<>();

    // Code omitted.
}

然后去上课：

public class Picked {

    public Book book;
    public int period;
    public int cost;

    // Code omitted.
}

和班级书：

public class Book {

    public String name;
    public String bookTitle;
    public int howMany;

    // Code omitted.
}

所以在主要的我创建新用户并序列化它：

User user = new User();
user.setName("John");
user.setSurname("James");
String fileName = "data.bin";
try (ObjectOutputStream os = new ObjectOutputStream(new FileOutputStream(fileName))) {
    os.writeObject(user);
} catch (IOException ex) {
    Logger.getLogger(Library.class.getName()).log(Level.SEVERE,null,ex);
}

一切都很完美,但如果我这样做的话：

Book book = new Book("Dan Brown","Angels and Demons",1);
Picked pck = new Picked(book,20,2);
user.add(pck);

然后我想序列化对象用户,程序崩溃了.
我得到的输出是：

java.io.NotSerializableException:
at
java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1183)
at
java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347)
at java.util.ArrayList.writeObject(ArrayList.java:742) at
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606) at
java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:988)
at
java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1495)
at
java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431)
at
java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177)
at
java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1547)
at
java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1508)
at
java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431)
at
java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177)
at
java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347)