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?
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4"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
14 Answers
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.

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25Furthermore, 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
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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
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1@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
LinkedHashSet is what you need.

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46A `List` is not a `Set` (it doesn't guarantee uniqueness of membership). – lmat - Reinstate Monica Jun 06 '13 at 22:03
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13In 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
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
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);

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To retain the order use List
or a LinkedHashSet
.

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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
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.

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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.

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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.
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.

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

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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?
The Set
interface does not require a well-defined encounter order, a fact which is explicitly called out in the Javadocs for its iterator
method:
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).
Java 21 is scheduled to introduced a new SequencedSet
interface, which is a subinterface of Set
that also requires that the implementation have a well-defined encounter order. Previously existing Set
class LinkedHashSet
and interface SortedSet
implement this new interface.
If both encounter order and Set
semantics are important, an implementation of SequencedSet
can be used to achieve the desired behavior.

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Only SortedSet
can do the ordering of the Set
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The question is about retaining the insertion order (which happens to be sorted). – assylias May 25 '12 at 10:42