Java Hibernate -> ArrayList 不能转换为 Set
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Hibernate -> ArrayList cannot be cast to Set
提问by Tim
I have a Java EE application and I use Hibernate. The domain objects, I changed the List / ArrayList to Set / HashSet, because it is better to use Sets.
我有一个 Java EE 应用程序，我使用 Hibernate。域对象，我把List/ArrayList改成了Set/HashSet，因为用Sets比较好。
But in my Dao implementation I run into a problem:
但是在我的 Dao 实现中，我遇到了一个问题：
public Set<Person> getAllPersons() {
SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
Session sess = sessionFactory.getCurrentSession();
Transaction tx = sess.beginTransaction();
@SuppressWarnings("unchecked")
Set<Item> items = (Set<Item>) sess.createQuery("from Item").list();
tx.commit();
return items;
}
Here I get an error:
在这里我得到一个错误：
java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.util.Set
What can I do to avoid this error?
我该怎么做才能避免这个错误？
Thank you in advance & Best Regards.
提前致谢并致以最诚挚的问候。
采纳答案by Jigar Joshi
List<Item> list = sess.createQuery("from Item").list();
Set<Item> items = new HashSet<Item>(list);
sess.createQuery("from Item").list();will return a array list of resulting items, if you need it in Set you can make it as shown in code
sess.createQuery("from Item").list();将返回结果项的数组列表，如果您在 Set 中需要它，您可以按照代码所示进行设置
回答by Pascal Thivent
The domain objects, I changed the List / ArrayList to Set / HashSet, because it is better to use Sets.
域对象，我把List/ArrayList改成了Set/HashSet，因为用Sets比较好。
It's not "better" to use Set, it depends entirely on what you need. But anyway, the collection type you use in your domain objects and the return type of Query#list()are two unrelated things.
使用并不是“更好” Set，这完全取决于您的需要。但无论如何，你在域对象中使用的集合类型和返回类型Query#list()是两个不相关的东西。
Personally, I can't say that I see much value in converting the result of a query into a Set(apart from eating more memory), unless you explicitly want to remove duplicates from query resultsof course (but in that case, I would challenge the mappings).
就我个人而言，我不能说我认为将查询结果转换为 a Set（除了占用更多内存）有多大价值，除非您当然想从查询结果中删除重复项（但在这种情况下，我会挑战映射）。
Now, if you insist, the various Setimplementations have a constructor that takes a Collection(and the question is essentially a duplicate of Easiest way to convert a List to a Set? - Java).
现在，如果您坚持，各种Set实现都有一个采用 a 的构造函数Collection（问题本质上是将 List 转换为 Set的最简单方法的重复？ - Java）。
