Java Set retain order?

Question

Does a Java Set retain order? A method is returning a Set to me and supposedly the data is ordered but iterating over the Set, the data is unordered. Is there a better way to manage this? Does the method need to be changed to return something other than a Set?

"The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee)." is what the iterator method for a set says. found [here](http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Set.html) — keyser, May 25 '12 at 10:29

score 275 · Accepted Answer · edited Sep 23 '18 at 09:56

275

The Set interface does not provide any ordering guarantees.

Its sub-interface SortedSet represents a set that is sorted according to some criterion. In Java 6, there are two standard containers that implement SortedSet. They are TreeSet and ConcurrentSkipListSet.

In addition to the SortedSet interface, there is also the LinkedHashSet class. It remembers the order in which the elements were inserted into the set, and returns its elements in that order.

edited Sep 23 '18 at 09:56

Benny Bottema

9,605
10
59
80

answered May 25 '12 at 10:30

NPE

438,426
93
887
970

23

Furthermore, due to the [different String hashing](http://permalink.gmane.org/gmane.comp.java.openjdk.core-libs.devel/10361) in Java 8, the default (unsorted) ordering in Sets and Maps will change. If you rely on unsorted ordering, your code will behave differently under Java 8. – rustyx Nov 17 '14 at 16:15
I get that the class not ordering is normal, but the behavior I was expecting was to leave them as they were introduced and not mess with the order, instead it is just shuffling the elements each time one is aggregated. Your solution is not optimal wither because then I will have to implement a whole structure for them to be sorted THE SAME WAY they were introduced :S – White_King Feb 25 '19 at 08:39
@White_King: A set is a mathematical concept that does not contain the notion of "insertion order" so it makes sense for the Java interface to follow its conventions. There are ordered sets but the order is specified by a relation (comparator in Java), again matching the definition in set theory with the definition in Java. Your expectation of it keeping insertion order probably comes from lists but sets are not lists. – Konrad Höffner Aug 09 '19 at 10:53

score 118 · Answer 2 · edited May 13 '16 at 18:15

118

LinkedHashSet is what you need.

edited May 13 '16 at 18:15

Belphegor

3,711
11
34
57

answered May 25 '12 at 10:29

xiaofeng.li

7,284
2
20
29

43

A `List` is not a `Set` (it doesn't guarantee uniqueness of membership). – lmat - Reinstate Monica Jun 06 '13 at 22:03
11

In many business unique cases List can't be used just to retain order instead of Set. LinkedHashSet maintains order and stores unique. – gubs Jul 09 '15 at 15:34

score 18 · Answer 3 · edited Nov 24 '17 at 18:23

As many of the members suggested use LinkedHashSet to retain the order of the collection. U can wrap your set using this implementation.

SortedSet implementation can be used for sorted order but for your purpose use LinkedHashSet.

Also from the docs,

"This implementation spares its clients from the unspecified, generally chaotic ordering provided by HashSet, without incurring the increased cost associated with TreeSet. It can be used to produce a copy of a set that has the same order as the original, regardless of the original set's implementation:"

Source : http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashSet.html

score 9 · Answer 4 · answered May 25 '12 at 10:29

Set is just an interface. In order to retain order, you have to use a specific implementation of that interface and the sub-interface SortedSet, for example TreeSet or LinkedHashSet. You can wrap your Set this way:

Set myOrderedSet = new LinkedHashSet(mySet);

JHS · Answer 5 · 2012-05-25T10:53:31.827

7

To retain the order use List or a LinkedHashSet.

edited May 25 '12 at 10:53

answered May 25 '12 at 10:31

JHS

7,344
2
25
50

1

It's `LinkedHashSet`, not ...`Map`. – Marko Topolnik May 25 '12 at 10:52
I need a Set not a List, I need a Set that ALSO retains the order of injection of the objects I guess – White_King Aug 09 '19 at 14:21

assylias · Answer 6 · 2012-05-25T10:41:18.087

Here is a quick summary of the order characteristics of the standard Set implementations available in Java:

keep the insertion order: LinkedHashSet and CopyOnWriteArraySet (thread-safe)
keep the items sorted within the set: TreeSet, EnumSet (specific to enums) and ConcurrentSkipListSet (thread-safe)
does not keep the items in any specific order: HashSet (the one you tried)

For your specific case, you can either sort the items first and then use any of 1 or 2 (most likely LinkedHashSet or TreeSet). Or alternatively and more efficiently, you can just add unsorted data to a TreeSet which will take care of the sorting automatically for you.

score 6 · Answer 7 · answered Apr 04 '18 at 11:58

6

A LinkedHashSet is an ordered version of HashSet that maintains a doubly-linked List across all elements. Use this class instead of HashSet when you care about the iteration order.

answered Apr 04 '18 at 11:58

Danail Tsvetanov

353
4
6

score 3 · Answer 8 · edited May 23 '17 at 11:47

From the javadoc for Set.iterator():

Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee).

And, as already stated by shuuchan, a TreeSet is an implemention of Set that has a guaranteed order:

The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.

score 3 · Answer 9 · answered May 25 '12 at 10:30

3

Normally set does not keep the order, such as HashSet in order to quickly find a emelent, but you can try LinkedHashSet it will keep the order which you put in.

answered May 25 '12 at 10:30

user1335794

1,032
6
12

score 1 · Answer 10 · edited Oct 30 '19 at 19:38

1

There are 2 different things.

Sort the elements in a set. For which we have SortedSet and similar implementations.
Maintain insertion order in a set. For which LinkedHashSet and CopyOnWriteArraySet (thread-safe) can be used.

edited Oct 30 '19 at 19:38

marc_s

675,133
158
1,253
1,388

answered Oct 02 '19 at 15:44

Aruna

76
3

score 0 · Answer 11 · answered May 25 '12 at 10:30

0

The Set interface itself does not stipulate any particular order. The SortedSet does however.

answered May 25 '12 at 10:30

Jens Borgland

733
4
14

score 0 · Answer 12 · edited May 23 '17 at 12:10

0

Iterator returned by Set is not suppose to return data in Ordered way. See this Two java.util.Iterators to the same collection: do they have to return elements in the same order?

edited May 23 '17 at 12:10

Community

1
1

answered May 25 '12 at 10:31

ejb_guy

1,115
6
6

score -2 · Answer 13 · answered May 25 '12 at 10:33

-2

Only SortedSet can do the ordering of the Set

answered May 25 '12 at 10:33

The question is about retaining the insertion order (which happens to be sorted). – assylias May 25 '12 at 10:42

Java Set retain order?

13 Answers13

Linked

Related