"Illegal attempt to map non-collection as a @OneToMany, @ManyToMany or @CollectionOfElements"

mh123hack：

good morning #1,

I have the error problem it gives me:

Cannot create SessionFactory object.org.hibernate.AnnotationException: Illegal attempt to map non-collection as @OneToMany, @ManyToMany or @CollectionOfElements: nl.scalda.pasimo.model.employeemanagement.EducationTeam.coachGroups

Do you know why?

@OneToMany(cascade=CascadeType.ALL, targetEntity=CoachGroup.class)
@JoinColumn(name="id")
private TreeSet<CoachGroup> coachGroups = new TreeSet<>();
private SessionFactory factory;

private void initialiseFactory() {
    try {
        factory = new Configuration().configure().buildSessionFactory();
    } catch (Throwable ex) {
        System.err.println("Failed to create sessionFactory object." + ex);
        throw new ExceptionInInitializerError(ex);
    }
}
cнŝdk ：

The only exception is straightforward: an illegal attempt to map a non-collection as a @OneToMany, @ManyToMany or @CollectionOfElements , so the reason is obvious here, if we take a look at the collection mapping documentation in Hibernate , it clearly states:

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

And what you use TreeSet is an implementation class and interface. So, your actual mapping won't work together , you should use instead :Set<E>SortedSet<E>TreeSetSet<CoachGroup>TreeSet<CoachGroup>

private Set<CoachGroup> coachGroups = new HashSet<CoachGroup>();

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