net.sf.hibernate.collection.Map
//...line 202, code trimmed for clarity
   public Object readFrom(....){...
      Object index = persister.readIndex( rs, getSession() );
// the index (key) is hollow, and so map.put uses a zero hashcode!
      map.put(index, element);
      return element;
   }

public class Catalog {
    // ....
    @ManyToMany
    @MapKey(name = "type")
    @JoinTable(name = "catalog_group_rel",
               joinColumns = {@JoinColumn(name = "catalog_id")},
               inverseJoinColumns = {@JoinColumn(name = "group_id")})
    public Map<Type, ProductGroup> getProductGroups() {
        return productGroups;
    }
}


public class ProductGroup {
    // .....
    @ManyToOne
    @JoinColumn(name = "type", nullable = false)
    public Type getType() {
        return type;
    }
}

public class Type {
    // .....
    @Column (name = "name", nullable = false, unique = true)
    public String getName() {
        return name;
    }

    // .....
    @Override
    public int hashCode() {
        return (getName() != null) ? getName().hashCode() : 0;
    }
}