Question

Any reason why a such asSet<MyObject> objects = new HashSet<MyObject>(); shouldn’t work in the JSF Datatable? It works with List.

Answer 1

As to why a Set in general isn’t supported, this is because this data structure is never intented to hold a collection of objects which is ordered by an index. The List does that and this data structure is the most sensible data structure to represent the value of an UIData component. The DataModel interface, which represents the wrapped value of the UIData components and holds the row indexes and remembers the current row for iteration on render and form submit processing on postback, supports from the Java collection classes only the List interface in flavor of ListDataModel.

After a long decision process (especially pushed by Hibernate/JPA community who generally uses Set for n-m relationships), the JSF spec team has for the upcoming JSF 2.2 finally decided to let the DataModel interface support the Collection interface instead of alone the List, with help of the new CollectionDataModel implementation. This supports sets as well. See also JSF spec issue 479. You should only keep in mind to use LinkedHashSet instead of HashSet, certainly if your intention is to have an editable data table. A LinkedHashSet maintains the ordering of the elements.