Convert Set to List without creating new List

Question

I am using this code to convert a Set to a List   Map lt String  List lt String gt  gt  mainMap   new HashMap lt  gt      for  int i 0  i  lt  something size    i        Set lt String gt  set   getSet         returns different result each time   List lt String gt  listOfNames   new ArrayList lt  gt  set     mainMap put differentKeyName  listOfNames       I want to avoid creating a new list in each iteration of the loop  Is that possible

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Map lt String  List gt  mainMap   new HashMap lt String  List gt      for int i 0  i lt something size    i       Set set   getSet         return different result each time   mainMap put differentKeyName  new ArrayList set

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I create simple static method   public static  lt U gt  List lt U gt  convertSetToList Set lt U gt  set        return new ArrayList lt U gt  set           or if you want to set type of List you can use   public static  lt U  L extends List lt U gt  gt  List lt U gt  convertSetToList Set lt U gt  set  Class lt L gt  clazz  throws InstantiationException  IllegalAccessException       L list   clazz newInstance        list addAll set       return list

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For the sake of completeness     Say that you really do want to treat the Map values as Lists  but you want to avoid copying the Set into a List each time   For instance  maybe you are calling one library function that creates a Set  but you are passing your Map lt String  List lt String gt  gt  result to a  poorly-designed but out of your hands  library function that only takes Map lt String  List lt String gt  gt   even though somehow you know that the operations it does with the Lists are equally applicable to any Collection  and thus any Set   And for some reason you need to avoid the speed memory overhead of copying each Set to a List   In this super niche case  depending on the  maybe unknowable  behavior the library function needs out of your Lists  you may be able to create a List view over each Set  Note that this is inherently unsafe  because the library function s requirements from each List could presumably change without you knowing   so another solution should be preferred  But here s how you d do it   You d create a class that implements the List interface  takes a Set in the constructor and assigns that Set to a field  and then uses that internal Set to implement the List API  to the extent possible  and desired    Note that some List behavior you simply will not be able to imitate without storing the elements as a List  and some behavior you will only partially be able to imitate  Again  this class is not a safe drop-in replacement for Lists in general  In particular  if you know that the use case requires index-related operations or MUTATING the List  this approach would go south very fast   public class ListViewOfSet lt U gt  implements List lt U gt        private final Set lt U gt  wrappedSet      public ListViewOfSet Set lt U gt  setToWrap    this wrappedSet   setToWrap          Override public int size     return this wrappedSet size           Override public boolean isEmpty     return this wrappedSet isEmpty           Override public boolean contains Object o    return this wrappedSet contains o          Override public java util Iterator lt U gt  iterator     return this wrappedSet iterator           Override public Object   toArray     return this wrappedSet toArray           Override public  lt T gt  T   toArray T   ts    return this wrappedSet toArray ts          Override public boolean add U e    return this wrappedSet add e          Override public boolean remove Object o    return this wrappedSet remove o          Override public boolean containsAll Collection lt   gt  clctn    return this wrappedSet containsAll clctn          Override public boolean addAll Collection lt   extends U gt  clctn    return this wrappedSet addAll clctn          Override public boolean addAll int i  Collection lt   extends U gt  clctn    throw new UnsupportedOperationException           Override public boolean removeAll Collection lt   gt  clctn    return this wrappedSet removeAll clctn          Override public boolean retainAll Collection lt   gt  clctn    return this wrappedSet retainAll clctn          Override public void clear     this wrappedSet clear           Override public U get int i    throw new UnsupportedOperationException           Override public U set int i  U e    throw new UnsupportedOperationException           Override public void add int i  U e    throw new UnsupportedOperationException           Override public U remove int i    throw new UnsupportedOperationException           Override public int indexOf Object o    throw new UnsupportedOperationException           Override public int lastIndexOf Object o    throw new UnsupportedOperationException           Override public ListIterator lt U gt  listIterator     throw new UnsupportedOperationException           Override public ListIterator lt U gt  listIterator int i    throw new UnsupportedOperationException           Override public List lt U gt  subList int i  int i1    throw new UnsupportedOperationException             Set lt String gt  set   getSet       ListViewOfSet lt String gt  listOfNames   new ListViewOfSet lt  gt  set

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Use constructor to convert it   List lt   gt  list   new ArrayList lt   gt  set

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Java 8 provides the option of using streams and you can get a list from Set lt String gt  setString as   List lt String gt  stringList   setString stream   collect Collectors toList       Though the internal implementation as of now provides an instance of ArrayList   public static  lt T gt      Collector lt T     List lt T gt  gt  toList             return new CollectorImpl lt  gt   Supplier lt List lt T gt  gt   ArrayList  new  List  add                                      left  right  - gt    left addAll right   return left                                        CH ID           but JDK does not guarantee it  As mentioned here      There are no guarantees on the type  mutability  serializability  or   thread-safety of the List returned  if more control over the returned   List is required  use toCollection Supplier     In case you want to be sure always then you can request for an instance specifically as   List lt String gt  stringArrayList   setString stream                         collect Collectors toCollection ArrayList  new

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We can use following one liner in Java 8     List lt String gt  list   set stream   collect Collectors toList         Here is one small example   public static void main String   args            Set lt String gt  set   new TreeSet lt  gt             set add  A            set add  B            set add  C            List lt String gt  list   set stream   collect Collectors toList

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Recently I found this   ArrayList lt T gt  yourList   Collections list Collections enumeration yourSet lt T gt

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I would do    Map lt String  Collection gt  mainMap   new HashMap lt String  Collection gt      for int i 0  i lt something size    i       Set set   getSet         return different result each time   mainMap put differentKeyName set

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I found this working fine and useful to create a List from a Set   ArrayList  lt  String  gt  L1   new ArrayList  lt  String  gt      L1 addAll ActualMap keySet     for  String x  L1        System out println x toString

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Also from Guava Collect library  you can use newArrayList Collection    Lists newArrayList  your set     This would be very similar to the previous answer from amit  except that you do not need to declare  or instanciate  any list object

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You can use the List addAll   method  It accepts a Collection as an argument  and your set is a Collection   List lt String gt  mainList   new ArrayList lt String gt     mainList addAll set     EDIT  as respond to the edit of the question  It is easy to see that if you want to have a Map with Lists as values  in order to have k different values  you need to create k different lists  Thus  You cannot avoid creating these lists at all  the lists will have to be created   Possible work around  Declare your Map as a Map lt String Set gt  or Map lt String Collection gt  instead  and just insert your set

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Since it hasn t been mentioned so far  as of Java 10 you can use the new copyOf factory method  List copyOf set    From the Javadoc   Returns an unmodifiable List containing the elements of the given Collection  in its iteration order   Note that this creates a new list  ImmutableCollections ListN to be precise  under the hood by  calling Collection toArray   on the given set and then putting these objects into a new array

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You could use this one line change  Arrays asList set toArray new Object set size       Map lt String  List gt  mainMap   new HashMap lt String  List gt      for int i 0  i lt something size    i       Set set   getSet          mainMap put differentKeyName  Arrays asList set toArray new Object set size

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the simplest solution   I wanted a very quick way to convert my set to List and return it  so in one line I did    return new ArrayList lt Long gt  mySetVariable

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You convert Set to List without adding ordering information  like sorting  just to store it in the map  Because Set is unordered and no ordering information is added  List should not be used  as it will contain randomly ordered data and all it s methods that are related to ordered data will be ambiguous  You should use Collection interface instead  that accepts both Set and List in the map  This way  no additional memory is required as you use polymorphism instead of copying data  Map lt String  Collection lt String gt  gt  mainMap   new HashMap lt  gt      for  int i 0  i  lt  something size    i        Set lt String gt  set   getSet         returns different result each time   mainMap put differentKeyName  set       Disclaimer  my edit to a similar answer was rejected so I added my own answer with additional information

[java] Convert Set to List without creating new List

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