In the chapter 6, in the section Using a List, the author said: "The primary key consists of the ITEM_ID and POSITION columns".
This is an obligation? Can I use an Id as a primary key of List elements?
If I have a list in my java class, as "private List customers", must I use the <list> in the mapping files?

Uh, this is completely explained in the chapter you are reading. Maybe re-read.

Ok, sorry, you're right. The name of the section is "Mapping collections of value types". I already understand that I should use id for entity types and not use for value types.

But, I don't understand the question about use of lists or sets yet. It's true that the association semantics of my collections is set, because the table structure doesn't support duplicates. But I want to use List object in my java code. In this case, what should I do? Use <set> in the mapping files and List in my java code? Can I do this? Or should I use <list> or <bag> in my mapping files?

The use of <bag> when I need a List in java code is an obligation?

Why not read page 212 again.

Thank you for attention.
You wrote that bag permits duplicated elements. I don't need this behavior. The only thing I need is to use List in java.
In all my cases, the real semantic is set (but the database table provide this). But in java code I need the List, because I will show the collections in JTable, that need a index for search the collection element.
So, the solution still be the use of the <bag> ?

Why not?