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

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

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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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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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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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LinkedHashSet is what you need

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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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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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Only SortedSet can do the ordering of the Set

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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

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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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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

[java] Java Set retain order?

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