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Java Set retain order?

Posted by: admin November 29, 2017 Leave a comment

Questions:

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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The Set interface does not provide any ordering guarantees.

It’s 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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LinkedHashSet is what you need.

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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

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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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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.

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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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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.

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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?

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Only SortedSet can do the ordering of the Set