CODEWITHSUNDEEP
javaarraylist
Victor
Victor

Reputation: 17097

Array List retrieval order

I have a java ArrayList to which I add 5 objects.

If I iterate over the list and print them out, then iterate over the list and print them out again.

Will the retrieval order in these 2 cases be the same? (I know it may be different from the insertion order)

Upvotes: 19

Views: 35736

Answers (10)

advait
advait

Reputation: 21

Yes the retrieval order is guaranteed to be the same as long as list is not mutated and you use the same iterator, but having to need to rely on retrieval order indicated something fishy with the design. It is generally not a good idea to base business logic upon certain retrieval order.

Upvotes: -1

Óscar López
Óscar López

Reputation: 236004

Yes, an ArrayList guarantees iteration order over its elements - that is, they will come out in the same order you inserted them, provided that you don't make any insertions while iterating over the ArrayList.

Upvotes: 2

Mike Samuel
Mike Samuel

Reputation: 120516

(I know it may be different from the insertion order)

No it won't. The contract of List requires that the add order is the same as the iteration order, since add inserts at the end, and iterator produces an iterator that iterates from start to end in order.

Set doesn't require this, so you may be confusing the contract of Set and List regarding iteration order.

From the Javadoc:

Iterator<E> iterator()

Returns an iterator over the elements in this list in proper sequence.

Upvotes: 11

Andy
Andy

Reputation: 9048

The iteration order will be the same everytime you iterate over the same unmodified list.

Also, assuming you add the elements using the add() method, the iteration order will be the same as the insertion order since this method appends elements to the end of the list.

Upvotes: 1

Ted Hopp
Ted Hopp

Reputation: 234807

When you add an element to an ArrayList using add(E e), the element is appended to the end of the list. Consequently, if all you do is call the single-argument add method a number of times and then iterate, the iteration will be in exactly the same order as the calls to add.

Upvotes: 1

celopez3
celopez3

Reputation: 86

Retrieval does not vary unless you change the iterator you are using. As long as you are using the same method for retrieval and have not changed the list itself then the items will be returned in the same order.

Upvotes: 1

trutheality
trutheality

Reputation: 23465

It's in the specification of the List interface to preserve order.

It's the Set classes that don't preserve order.

Upvotes: 7

Amir Pashazadeh
Amir Pashazadeh

Reputation: 7302

Even Sets will return the same result, if you don't modify them (adding or removing items to them).

Upvotes: -2

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272517

Yes, assuming you haven't modified the list in-between. From http://docs.oracle.com/javase/6/docs/api/java/util/List.html:

iterator

Iterator<E> iterator()

Returns an iterator over the elements in this list in proper sequence.

A bit vague, perhaps, but in other portions of that page, this term is defined:

proper sequence (from first to last element)

Upvotes: 29

Louis Wasserman
Louis Wasserman

Reputation: 198103

If you're not mutating the list, then the iteration order will stay the same. Lists have a contractually specified ordering, and the iterator specification guarantees that it iterates over elements in that order.

Upvotes: 3

Related Questions