CODEWITHSUNDEEP
javahibernateormone-to-many
k13i
k13i

Reputation: 4351

Hibernate Collection vs List as field type

I have two entities, Author and Book, connected with a one-to-many relationship. What's the difference between specifying field type as Collection<Book> and List<Book>? Aforementioned scenario is presented below:

@Entity
public class Author {

    @Id
    @GeneratedValue
    private Long id;

    private String name;

    @OneToMany(mappedBy = "author")
    private Collection<Book> books = new ArrayList<>(); // List<Book> instead?
}

The only difference I have already noticed is that when I want to use @OrderColumn annotation I need to use List, but are there any other differences I don't know about? Should I always use Collection if I don't need an order?

Upvotes: 1

Views: 694

Answers (2)

Alexander Petrov
Alexander Petrov

Reputation: 9492

  • Set - contains no duplicates no order
  • (Bag)Collection - duplicates no order
  • List - duplicates order

For Set you need to be carefull about hashcode and equals. And one interesting twist with Bags in relation to SQL generated:

If we are using List as a mapped attribute in hibernate without indexed column, hibernates treats it as a Bag. Since Hibernate handles List as a Bag (Unordered collection with non unique values. The best feature of a bag is that you can get the number of occurrences of an object through the API With a list, there is no way to do the same without iterating through the whole list.) as soon as we delete and add a element in this collection. Hibernate issues a SQL to delete all the elements first from join table which are no supposed to be deleted and then it re-insert all of them back from the Bag.

http://lkumarjain.blogspot.no/2013/07/why-hibernate-does-delete-all-entries.html

Upvotes: 2

Karol Dowbecki
Karol Dowbecki

Reputation: 44970

java.util.Collection is the most generic unordered collection of elements while the java.util.List implies existence of an iteration order.

Using @OrderColumn will give this iteration order however it might change the generated SQL query. Often it results in ORDER BY statement added to the SQL query. Without @OrderColumn the JPA provider has more flexibility but you should always measure the performance in your actual database instead of tuning it blindly.

Upvotes: 0

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