Why doesn t java util Set have get int index

Question

I m sure there s a good reason  but could someone please explain why the java util Set interface lacks get int Index   or any similar get   method   It seems that sets are great for putting things into  but I can t find an elegant way of retrieving a single item from it   If I know I want the first item  I can use set iterator   next    but otherwise it seems I have to cast to an Array to retrieve an item at a specific index   What are the appropriate ways of retrieving data from a set   other than using an iterator   I m sure the fact that it s excluded from the API means there s a good reason for not doing this -- could someone please enlighten me   EDIT  Some extremely great answers here  and a few saying  more context    The specific scenario was a dbUnit test  where I could reasonably assert that the returned set from a query had only 1 item  and I was trying to access that item   However  the question is more valid without the scenario  as it remains more focussed   What s the difference between set and list   Thanks to all for the fantastic answers below

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The only reason I can think of for using a numerical index in a set would be for iteration  For that  use   for A a   set        visit a

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The reason why the Set interface doesn t have a get index-type call or even something even more basic  such as first   or last    is because it is an ambiguous operation  and therefore a potentially dangerous operation  If a method returns a Set  and you call  say first   method on it  what is the expected result  given that the a generic Set makes no guarantees on the ordering  The resultant object could very well vary between each call of the method  or it might not and lull you into a false sense of security  until the library you re using changes changes the implementation underneath and now you find that all your code breaks for no particular reason   The suggestions about workarounds listed here are good  If you need indexed access  use a list  Be careful with using iterators or toArray with a generic Set  because a  there is no guarantee on the ordering and b  there is no guarantee that the ordering will not change with subsequent invocations or with different underlying implementations  If you need something in between  a SortedSet or a LinkedHashSet is what you want      I do wish the Set interface had a get-random-element though

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I m not sure if anybody has spelled it out exactly this way  but you need to understand the following   There is no  first  element in a set   Because  as others have said  sets have no ordering   A set is a mathematical concept that specifically does not include ordering   Of course  your computer can t really keep a list of stuff that s not ordered in memory   It has to have some ordering   Internally it s an array or a linked list or something   But you don t really know what it is  and it doesn t really have a first element  the element that comes out  first  comes out that way by chance  and might not be first next time   Even if you took steps to  guarantee  a particular first element  it s still coming out by chance  because you just happened to get it right for one particular implementation of a Set  a different implementation might not work that way with what you did   And  in fact  you may not know the implementation you re using as well as you think you do   People run into this ALL  THE  TIME  with RDBMS systems and don t understand   An RDBMS query returns a set of records   This is the same type of set from mathematics  an unordered collection of items  only in this case the items are records   An RDBMS query result has no guaranteed order at all unless you use the ORDER BY clause  but all the time people assume it does and then trip themselves up some day when the shape of their data or code changes slightly and triggers the query optimizer to work a different way and suddenly the results don t come out in the order they expect   These are typically the people who didn t pay attention in database class  or when reading the documentation or tutorials  when it was explained to them  up front  that query results do not have a guaranteed ordering

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Because sets have no ordering  Some implementations do  particularly those implementing the java util SortedSet interface   but that is not a general property of sets   If you re trying to use sets this way  you should consider using a list instead

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To get element in a Set  i use to following one   public T getElement Set lt T gt  set  T element    T result   null  if  set instanceof TreeSet lt   gt         T floor     TreeSet lt T gt   set  floor element       if  floor    null  amp  amp  floor equals element       result   floor    else       boolean found   false      for  Iterator lt T gt  it   set iterator     found  amp  amp  it hasNext           if  true            T current   it next            if  current equals element             result   current          found   true                          return result

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That s true  element in Set are not ordered  by definition of the Set Collection  So they can t be access by an index   But why don t we have a get object  method  not by providing the index as parameter  but an object that is equal to the one we are looking for   By this way  we can access the data of the element inside the Set  just by knowing its attributes used by the equal method

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You can do new ArrayList lt T gt  set  get index

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If you are going to do lots of random accesses by index in a set  you can get an array view of its elements   Object   arrayView   mySet toArray      do whatever you need with arrayView i    There are two main drawbacks though    It s not memory efficient  as an array for the whole set needs to be created  If the set is modified  the view becomes obsolete

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If you don t mind the set to be sorted then you may be interested to take a look at the indexed-tree-map project    The enhanced TreeSet TreeMap provides access to elements by index or getting the index of an element  And the implementation is based on updating node weights in the RB tree  So no iteration or backing up by a list here

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Actually this is a recurring question when writing JavaEE applications which use Object-Relational Mapping  for example with Hibernate   and from all the people who replied here  Andreas Petersson is the only one who understood the real issue and offered the correct answer to it  Java is missing a UniqueList   or you can also call it OrderedSet  or IndexedSet    Maxwing mentioned this use-case  in which you need ordered AND unique data  and he suggested the SortedSet  but this is not what Marty Pitt really needed   This  IndexedSet  is NOT the same as a SortedSet - in a SortedSet the elements are sorted by using a Comparator  or using their  natural  ordering    But instead it is closer to a LinkedHashSet  which others also suggested   or even more so to an  also inexistent   ArrayListSet   because it guarantees that the elements are returned in the same order as they were inserted   But the LinkedHashSet is an implementation  not an interface  What is needed is an IndexedSet  or ListSet  or OrderedSet  or UniqueList  interface  This will allow the programmer to specify that he needs a collection of elements that have a specific order and without duplicates  and then instantiate it with any implementation  for example an implementation provided by Hibernate    Since JDK is open-source  maybe this interface will be finally included in Java 7

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I ran into situations where I actually wanted a SortedSet with access via index  I concur with other posters that accessing an unsorted Set with an index makes no sense    An example would be a tree where I wanted the children to be sorted and duplicate children were not allowed    I needed the access via index to display them and the set attributes came in handy to efficiently eliminate duplicates   Finding no suitable collection in java util or google collections  I found it straightforward to implement it myself  The basic idea is to wrap a SortedSet and create a List when access via index is required  and forget the list when the SortedSet is changed   This does of course only work efficiently when changing the wrapped SortedSet and accessing the list is separated in the lifetime of the Collection  Otherwise it behaves like a list which is sorted often  i e  too slow   With large numbers of children  this improved performance a lot over a list I kept sorted via Collections sort

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This kind of leads to the question when you should use a set and when you should use a list  Usually  the advice goes    If you need ordered data  use a List If you need unique data  use a Set If you need both  use either  a SortedSet  for data ordered by comparator  or an OrderedSet UniqueList  for data ordered by insertion   Unfortunately the Java API does not yet have OrderedSet UniqueList    A fourth case that appears often is that you need neither  In this case you see some programmers go with lists and some with sets  Personally I find it very harmful to see set as a list without ordering - because it is really a whole other beast  Unless you need stuff like set uniqueness or set equality  always favor lists

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Just adding one point that was not mentioned in mmyers  answer      If I know I want the first item  I can   use set iterator   next    but   otherwise it seems I have to cast to   an Array to retrieve an item at a   specific index       What are the appropriate ways of   retrieving data from a set   other   than using an iterator    You should also familiarise yourself with the SortedSet interface  whose most common implementation is TreeSet     A SortedSet is a Set  i e  elements are unique  that is kept ordered by the natural ordering of the elements or using some Comparator  You can easily access the first and last items using first   and last   methods  A SortedSet comes in handy every once in a while  when you need to keep your collection both duplicate-free and ordered in a certain way   Edit  If you need a Set whose elements are kept in insertion-order  much like a List   take a look at LinkedHashSet

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That is because Set only guarantees uniqueness  but says nothing about the optimal access or usage patterns   Ie  a Set can be a List or a Map  each of which have very different retrieval characteristics

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some data structures are missing from the standard java collections    Bag  like set but can contain elements multiple times   UniqueList  ordered list  can contain each element only once   seems you would need a uniquelist in this case  if you need flexible data structures  you might be interested in Google Collections

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Set is an interface and some of its implementation classes are HashSet  TreeSet and LinkedHashSet  It uses HashMap under the hood to store values  Because HashMap does not preserve the order  it is not possible to get value by index   You now must be thinking how Set is using HashMap since HashMap stores a key  value pair but the Set does not  valid question  when you add an element in Set  internally  it maintains a HashMap where the key is the element you want to enter in Set and the value is the dummy constant  Below is an internal implementation of add function  Hence  all the keys in the HashMap will have the same constant value      Dummy value to associate with an Object in the backing Map private static final Object PRESENT   new Object     public boolean add E e        return map put e  PRESENT   null

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java util Set is a collection of un-ordered items  It doesn t make any sense if the Set has a get int index   because  Set doesn t has an index and also you only can guess the value    If you really want this  code a method to get random element from Set

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Please note only 2 basic data structure can be accessed via index    Array data structure can be accessed via index with O 1  time complexity to achieve get int index  operation  LinkedList data structure can also be accessed via index  but with O n  time complexity to achieve get int index  operation    In Java  ArrayList is implemented using Array data structure   While Set data structure usually can be implemented via HashTable HashMap or BalancedTree data structure  for fast detecting whether an element exists and add non-existing element  usually a well implemented Set can achieve O 1  time complexity contains operation  In Java  HashSet is the most common used implementation of Set  it is implemented by calling HashMap API  and HashMap is implemented using separate chaining with linked lists  a combination of Array and LinkedList    Since Set can be implemented via different data structure  there is no get int index  method for it

[java] Why doesn't java.util.Set have get(int index)?

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