“Illegal attempt to map a non collection as a @OneToMany, @ManyToMany or @CollectionOfElements”

Question

Good morning Stackoverflow,

I have the problem that it gives me the error:

Failed to create sessionFactory object.org.hibernate.AnnotationExcept

Answer 1

The Exception is straightforward and says : Illegal attempt to map a non collection as a @OneToMany, @ManyToMany or @CollectionOfElements, so the cause is obvious here and if we take a look at the Hibernate Collection mapping documentation it clearly states that:

As a requirement persistent collection-valued fields must be declared as an interface type (see Example 7.2, “Collection mapping using @OneToMany and @JoinColumn”). The actual interface might be java.util.Set, java.util.Collection, java.util.List, java.util.Map, java.util.SortedSet, java.util.SortedMap...

And you used TreeSet which is an implementation class for both Set and SortedSet interfaces. So your actual mapping won't work with TreeSet, you should use a Set instead of a TreeSet:

private Set coachGroups = new HashSet();