@Id @GeneratedValue(strategy=GenerationType.AUTO)
  private int userId;

By using @GeneratedValue annotation we can create primary key in table.The annotations takes 4 parameter as strategy attribute.

@GeneratedValue(strategy=GenerationType.AUTO)
Auto tells the Hibernate to do the primary key addition from hibernate side

@GeneratedValue(strategy=GenerationType.IDENTITY)
IDENTITY tells the Hibernate to do the primary key addition from DB side when set to auto increment.

@GeneratedValue(strategy=GenerationType.SEQUENCE)
SEQUENCE tells the Hibernate to do the primary key addition as defined by sequence in DB.

@GeneratedValue(strategy=GenerationType.TABLE)
TABLE tells the Hibernate to create a seperate table in DB and maintain the last inserted values.
The table contains only one row with last inserted value.