Colloection choice, which one is best for my problem

I have been studing the collection material I could find on the webulator, My problem is as follows;

1, Class a (customers) containd a Cust_ID String and a customer name.
2, Class b (SalesOrder) contains details of sales orders including Cust_ID which has a one to many relationship with class a

I would like to create a data structure which contains both sets of data with class a as the key and class b as the
value(s).

I would like to be able to retrieve multiple instances of class b dependant on a value of class a.  I also want that class b is retrieved in a particular sequence (Document number)

I have looked at MAPs   but they want unique values.

I do not want anyone to write the code ,just a nudge in the right direction, I'm thinking TreeSets. Examples of code pertaining to nudging however will not be frowned upon.