How to get first item from a java util Set

Question

I have a Set instance  Set lt String gt  siteIdSet    Set lt String gt   pContext getParent   getPropertyValue getCatalogProperties   getSitesPropertyName      The pContext getParent   getPropertyValue   is out-of-the-box code upon which I don t have any control to modify  Requirement  I wanted to get the first default element out of it  always    However  I couldn t find a method get index  like in an ArrayList  Hence  right now  I am doing like this  for  Iterator lt String gt  it   siteIdSet iterator    it hasNext           siteId   it next        break     Is there any  other  efficient way  short and better  of achieving this

User · Answer

This is a difficult question I came up against the other day myself. java.util.LinkedHashSet maintains a linked list of its contents (addition-ordered by default) but does not provide any accessors. Other structure types will fail to provide O(1) on add(), remove(), and contains().

You can use a LinkedHashSet and get its iterator(), grab one element, and discard it. If you don't care too much about speed or memory when doing this frequently to numerous different sets, that is probably your solution... but that seemed wasteful to me. Plus I had a little extra desired functionality.

I ended up writing my own class, dubbed RandomAccessLinkedHashSet, which concurrently maintains a hashtable, a doubly linked list, and an order-irrelevant array. I wrote it to comply with both Set and Deque, though the Deque implementation is a little sketchy since it will fail to push() elements it already contains, a little bit of a stretch for the interface's contract. Maintaining the third structure, the array, is not necessary at all for what you're doing, but it also allows access to a random element in the set in whatever capacity you can actually provide a random value.

If you're interested I can provide this source. I haven't Serialized it yet but it works great in runtime.

If you cannot guarantee the type of Set provided in any way, then you'll have to stick with the Iterator thing.

User · Answer

As  you mentioned pContext getParent   getPropertyValue return Set  You can convert Set to List to get the first element  Just change your code like    Set lt String gt  siteIdSet    Set lt String gt   pContext getParent   getPropertyValue       List lt String gt  siteIdList new ArrayList lt  gt  siteIdSet     String firstItem siteIdList get 0

User · Answer

This works   Object firstElement   set toArray   0

User · Answer

This will return the first element  set iterator   next

User · Answer

I just had the same problem and found your question here      This was my solution   Set lt Integer gt  mySetOfIntegers   new HashSet lt Integer gt            there s at least one integer in the set         Integer iFirstItemInSet   new ArrayList lt Integer gt  mySetOfIntegers  get 0

User · Answer

Set does not enforce ordering  There is no guarantee that you will always get the  first  element even if you use an iterator over a HashSet like you have done in the question   If you need to have predictable ordering  you need to use the LinkedHashSet implementation  When you iterate over a LinkedHashSet  you will get the elements in the order you inserted  You still need to use an iterator  because having a get method in LinkedHashSet would need you to use the concrete class everywhere

User · Answer

Vector has some handy features   Vector lt String gt  siteIdVector   new Vector lt  gt  siteIdSet   String first   siteIdVector firstElement    String last   siteIdVector lastElement      But I do agree - this may have unintended consequences  since the underling set is not guaranteed to be ordered

User · Answer

tl dr  Call SortedSet  first  Move elements  and call first     new TreeSet lt String gt        pContext getParent   getPropertyValue             Transfer elements from your  Set  to this new  TreeSet   an implementation of the  SortedSet  interface      first     Set Has No Order  As others have said  a Set by definition has no order  Therefore asking for the    first    element has no meaning   Some implementations of Set have an order such as the order in which items were added  That unofficial order may be available via the Iterator  But that order is accidental and not guaranteed  If you are lucky  the implementation backing your Set may indeed be a SortedSet   CAVEAT  If order is critical  do not rely on such behavior  If reliability is not critical  such undocumented behavior might be handy  If given a Set you have no other viable alternative  so trying this may be better than nothing   Object firstElement   mySet iterator   next      To directly address the Question    No  not really any shorter way to get first element from iterator while handling the possible case of an empty Set  However  I would prefer an if test for isEmpty rather than the Question   s for loop   if     mySet isEmpty           Object firstElement   mySet iterator   next        Use SortedSet  If you care about maintaining a sort order in a Set  use a SortedSet implementation  Such implementations include    TreeSet  ConcurrentSkipListSet  In Google Guava  the SortedSetMultimap class returns a SortedSet from its asMap method    Use LinkedHashSet For Insertion-Order  If all you need is to remember elements in the order they were added to the Set use a LinkedHashSet   To quote the doc  this class        maintains a doubly-linked list running through all of its entries  This linked list defines the iteration ordering  which is the order in which elements were inserted into the set  insertion-order

User · Answer

From the Oracle docs      As implied by its name  this interface models the mathematical set abstraction    In Set Theory   a  set  is a collection of distinct objects  considered as an object in its own right   -  Wikipedia - Set    Mathematically  elements in sets are not individualised  Their only identity is derived from their presence in the set  Therefore  there is no point in getting the  first  element in a set  as conceptually such a task is illogical   There may be no point to getting the  first  element from a set  but if all you need is to get one single object from a set  with no guarantees as to which object that is  you can do the following   for String aSiteId  siteIdSet        siteId   aSiteId      break      This is a slightly shorter way  than the method you posted  to get the  first  object of a Set  however since an Iterator is still being created  under the hood  it does not grant any performance benefit

User · Answer

Set by definition is not ordered   You probably use wrong collection

User · Answer

Or  using Java8   Object firstElement   set stream   findFirst   get      And then you can do stuff with it straight away   set stream   findFirst   ifPresent  lt doStuffHere gt      Or  if you want to provide an alternative in case the element is missing  my example returns new default string    set stream   findFirst   orElse  Empty string      You can even throw an exception if the first element is missing   set stream   findFirst   orElseThrow    - gt  new MyElementMissingException  Ah  blip  nothing here        Kudos to Alex Vulaj for prompting me to provide more examples beyond the initial grabbing of the first element

User · Answer

There is no point in retrieving first element from a Set  If you have such kind of requirement use ArrayList instead of sets  Sets do not allow duplicates  They contain distinct elements

User · Answer

To Access the element you need to get an iterator   But Iterator does not guarantee in a particular order unless it is some Exceptional case  so it is not sure to get the first Element

User · Answer

Set is a unique collection of items  So there is no notion of first element  If you want items in the sorted order  you can use TreeSet from which you can retrieve the first element using TreeSet first

[java] How to get first item from a java.util.Set?

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