I think that I already know the response but I also want to know your opinion. According with the specs :
The persistent fields or properties of an entity may be of the following types: Java primitive types; java.lang.String; other Java serializable types (including wrappers of the primitive types, java.math.BigInteger, java.math.BigDecimal, java.util.Date, java.util.Calendar, java.sql.Date, java.sql.Time, java.sql.Timestamp, user-defined serializable types, byte, Byte, char, and Character); enums; entity types and/or collections of entity types; and embeddable classes (see section 2.1.5).
Only this, that's means I can not have a field like this :
private Map<String, String> myMap.
Is used to specify the map key for associations of type Map.
If a persistent field or property other than the primary key is used as a map key then it is expected to have a uniqueness constraint associated with it.
that means the only collection what I can use without to envolve any relational fields. So basically I can not have non relational collections fields.