Tutorial :How do I guarantee the order of items in a collection



Question:

I have a list of objects and each and every object in the list have a position which may not change unless explicitly changed, in other words, the objects are in a queue. What collection should I use in my entity class to store these objects and how should that collection be annotated?

I currently have this

@Entity  class Foo {    ...    @OneToMany(mappedBy = "foo", cascade = CascadeType.ALL)    List<Bar> bars = new ArrayList<Bar>();    ...  }  

If this is not possible with JPA purely, I'm using EclipseLink as my JPA provider, so EclipseLink specific annotations will ok if nothing else helps.

EDIT: Note people, the problem is not that Java wouldn't preserv the order, I know most collections do, the problem is that I don't know a smart way for JPA to preserv the order. Having an order id and making the query order by it is possible, but in my case maintaining the order id is laborious (because the user interface allows reordering of items) and I'm looking for a smarter way to do it.


Solution:1

If you want this to be ordered after round-tripping to SQL, you should provide some sort of ordering ID within the entity itself - SQL is naturally set-oriented rather than list-oriented. You can then either sort after you fetch them back, or make sure your query specifies the ordering too.

If you give the entity an auto-generated integer ID this might work, but I wouldn't like to guarantee it.


Solution:2

Use a sort order id, as Jon suggested, then add an @OrderBy annotation below the @OneToMany. This will order any query by the specified field.

As an example, if you add a new field called "sortId" to Bar, Foo would look like this:

@Entity  class Foo {    ...    @OneToMany(mappedBy = "foo", cascade = CascadeType.ALL)    @OrderBy("sortId ASC")    List bars = new ArrayList();    ...  }  


Solution:3

You can

  1. Sort a List before creation
  2. Sort a List after creation
  3. Use a collection that performs a sort on insert. TreeMap, TreeSet


Solution:4

A linked list implements the Queue inteface in java and allows you to add things in the middle...

TBH most of the collections are ordered aren't they...

Check the docs, most say whether or not they are ordered.


Solution:5

It's worth trying LinkedList instead of ArrayList, however, as Jon said, you need to find a way of persisting the order information.

A solution will probably involve issuing an order number to each entry an storing it as a SortedMap, when converting into the List, if List is that you need.

However, ORM could potentially be clever enough to do all the conversions for you if you stored the collection as LinkedList.


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