Adding NULL in database

Adding NULL in database

I am new to hibernate.
I have two classes UserDetails and Address.The relation between these two
is One-To-Many.The details are as follows(Skiping getter and setters)
UserDetails.java

@Entity
@Table(name="UserDetails")
public class UserDetails {
@Id @GeneratedValue(strategy=GenerationType.AUTO)
@Column(name="userId")
private int userId;
@Column(name="UserName")
private String userName;
@OneToMany
@JoinColumn(name="address")
private Collection<Address> address=new ArrayList<Address>();
}
Address.java
@Entity @Table(name="address")
public class Address {
@Id @GeneratedValue
@Column(name="id")
private int id;
@Column(name="city")
private String city;
}
App.java

UserDetails ud=new UserDetails();
ud.setUserName("User 1");
Address ad1=new Address();
ad1.setCity("Mumbai");
Address ad2=new Address();
ad1.setCity("Pune");
ud.getAddress().add(ad1);
ud.getAddress().add(ad2);
Session session=factory.openSession();
session.beginTransaction();
session.save(ud);
session.save(ad1);
session.save(ad2);
session.getTransaction().commit();
session.close();
In hibernate.cfg.xml property name="hbm2ddl.auto" is set to update After
Running above code the entry in database is
UserDetails Table
Id UserName Address 1 User 1 NULL
Address Table
Id City Address 1 Pune 1 2 NULL 1
My Question is why hibernate is inserting in UserDetails Address=NULL and
City=NULL in address table instead of creating new table.