CODEWITHSUNDEEP
javasortingset
user840930
user840930

Reputation: 5578

Java Set retain order?

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?

Upvotes: 253

Views: 314386

Answers (13)

NPE
NPE

Reputation: 500495

The Set interface does not provide any ordering guarantees.

Its sub-interface SortedSet, and later NavigableSet, 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/NavigableSet interfaces, 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.

In Java 21+, with sequenced collections added, we have the super-interface of SequencedSet to cover all three of those mentioned classes: ConcurrentSkipListSet, LinkedHashSet, TreeSet.

Upvotes: 370

M. Justin
M. Justin

Reputation: 21162

An implementation of interface SequencedSet (introduced in Java 21) can be used when both encounter order and Set semantics (e.g. element uniqueness) are important.

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).

The SequencedSet interface is a subinterface of Set that also requires that the implementation have a well-defined encounter order. Both LinkedHashSet and SortedSet implement/extend this new interface.

Upvotes: 1

xiaofeng.li
xiaofeng.li

Reputation: 8587

LinkedHashSet is what you need, as it is a Set with a well-defined encounter order.

If the item uniqueness constraint isn't necessary, you could alternatively use a List instead of a Set.

Upvotes: 154

Aruna
Aruna

Reputation: 76

There are 2 different things.

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

Upvotes: 1

Lakshman
Lakshman

Reputation: 300

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

Upvotes: 19

JHS
JHS

Reputation: 7871

To retain the order use List or a LinkedHashSet.

Upvotes: 8

assylias
assylias

Reputation: 328669

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

  1. keep the insertion order: LinkedHashSet and CopyOnWriteArraySet (thread-safe)
  2. keep the items sorted within the set: TreeSet, EnumSet (specific to enums) and ConcurrentSkipListSet (thread-safe)
  3. 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.

Upvotes: 7

user1417163
user1417163

Reputation:

Only SortedSet can do the ordering of the Set

Upvotes: -2

ejb_guy
ejb_guy

Reputation: 1125

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?

Upvotes: 0

Jens Borgland
Jens Borgland

Reputation: 773

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

Upvotes: 0

user1335794
user1335794

Reputation: 1092

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.

Upvotes: 3

hmjd
hmjd

Reputation: 121981

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.

Upvotes: 3

javatutorial
javatutorial

Reputation: 1944

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);

Upvotes: 18

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