How can I turn a List of Lists into a List in Java 8

Question

If I have a List lt List lt Object gt  gt   how can I turn that into a List lt Object gt  that contains all the objects in the same iteration order by using the features of Java 8

User · Accepted Answer

You can use flatMap to flatten the internal lists  after converting them to Streams  into a single Stream  and then collect the result into a list   List lt List lt Object gt  gt  list       List lt Object gt  flat        list stream            flatMap List  stream           collect Collectors toList

User · Answer

The flatMap method on Stream can certainly flatten those lists for you  but it must create Stream objects for element  then a Stream for the result   You don t need all those Stream objects   Here is the simple  concise code to perform the task      listOfLists is a List lt List lt Object gt  gt   List lt Object gt  result   new ArrayList lt  gt     listOfLists forEach result  addAll     Because a List is Iterable  this code calls the forEach method  Java 8 feature   which is inherited from Iterable        Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception   Actions are performed in the order of iteration  if that order is specified    And a List s Iterator returns items in sequential order   For the Consumer  this code passes in a method reference  Java 8 feature  to the pre-Java 8 method List addAll to add the inner list elements sequentially      Appends all of the elements in the specified collection to the end of this list  in the order that they are returned by the specified collection s iterator  optional operation

User · Answer

I just want to explain one more scenario like List lt Documents gt   this list contains a few more lists of other documents like List lt Excel gt   List lt Word gt   List lt PowerPoint gt   So the structure is   class A     List lt Documents gt  documentList     class Documents     List lt Excel gt  excels    List lt Word gt  words    List lt PowerPoint gt  ppt      Now if you want to iterate Excel only from documents then do something like below    So the code would be    List lt Documents gt  documentList   new A   getDocumentList        check documentList as not null   Optional lt Excel gt  excelOptional   documentList stream                             map doc - gt  doc getExcel                              flatMap List  stream  findFirst     if excelOptional isPresent        Excel exl   optionalExcel get          now get the value what you want       I hope this can solve someone s issue while coding

User · Answer

Method to convert a List lt List gt  to List   listOfLists stream   flatMap List  stream  collect Collectors toList      See this example  public class Example        public static void main String   args            List lt List lt String gt  gt  listOfLists   Collections singletonList Arrays asList  quot a quot    quot b quot    quot v quot             List lt String gt  list   listOfLists stream   flatMap List  stream  collect Collectors toList              System out println  quot listOfLists   gt   quot    listOfLists           System out println  quot list   gt   quot    list                    It prints  listOfLists   gt    a  b  c   list   gt   a  b  c   In Python this can be done using List Comprehension  list of lists      Roopa   Roopi   Tabu    Soudipta    180 0  1231  2112  3112    130    158 2    220 2    flatten    val for sublist in list of lists for val in sublist   print flatten     Roopa    Roopi    Tabu    Soudipta   180 0  1231  2112  3112  130  158 2  220 2

User · Answer

An expansion on Eran s answer that was the top answer  if you have a bunch of layers of lists  you can keep flatmapping them     This also comes with a handy way of filtering as you go down the layers if needed as well     So for example   List lt List lt List lt List lt List lt List lt Object gt  gt  gt  gt  gt  gt  multiLayeredList        List lt Object gt  objectList   multiLayeredList      stream        flatmap someList1 - gt  someList1          stream            filter    Optional           flatmap someList2 - gt  someList2          stream            filter    Optional           flatmap someList3 - gt  someList3          stream            filter    Optional                   collect Collectors toList      This is would be similar in SQL to having SELECT statements within SELECT statements

User · Answer

We can use flatmap for this  please refer below code      List lt Integer gt  i1  Arrays asList 1  2  3  4    List lt Integer gt  i2  Arrays asList 5  6  7  8     List lt List lt Integer gt  gt  ii  Arrays asList i1  i2    System out println  List lt List lt Integer gt  gt   ii    List lt Integer gt  flat ii stream   flatMap l- gt  l stream    collect Collectors toList      System out println  Flattened to List lt Integer gt   flat

User · Answer

Just as  Saravana mentioned      flatmap is better but there are other ways to achieve the same    listStream reduce new ArrayList lt  gt      l1  l2  - gt            l1 addAll l2           return l1         To sum up  there are several ways to achieve the same as follows   private  lt T gt  List lt T gt  mergeOne Stream lt List lt T gt  gt  listStream        return listStream flatMap List  stream  collect toList        private  lt T gt  List lt T gt  mergeTwo Stream lt List lt T gt  gt  listStream        List lt T gt  result   new ArrayList lt  gt         listStream forEach result  addAll       return result     private  lt T gt  List lt T gt  mergeThree Stream lt List lt T gt  gt  listStream        return listStream reduce new ArrayList lt  gt      l1  l2  - gt            l1 addAll l2           return l1             private  lt T gt  List lt T gt  mergeFour Stream lt List lt T gt  gt  listStream        return listStream reduce  l1  l2  - gt            List lt T gt  l   new ArrayList lt  gt  l1           l addAll l2           return l         orElse new ArrayList lt  gt         private  lt T gt  List lt T gt  mergeFive Stream lt List lt T gt  gt  listStream        return listStream collect ArrayList  new  List  addAll  List  addAll

User · Answer

You can use the flatCollect   pattern from Eclipse Collections   MutableList lt List lt Object gt  gt  list   Lists mutable empty    MutableList lt Object gt  flat   list flatCollect each - gt  each     If you can t change list from List   List lt List lt Object gt  gt  list   new ArrayList lt  gt     List lt Object gt  flat   ListAdapter adapt list  flatCollect each - gt  each     Note  I am a contributor to Eclipse Collections

User · Answer

List lt List gt  list   map values   stream   collect Collectors toList         List lt Employee gt  employees2   new ArrayList lt  gt               list stream   forEach                             n- gt  employees2 addAll n

User · Answer

flatmap is better but there are other ways to achieve the same  List lt List lt Object gt  gt  listOfList          fill  List lt Object gt  collect          listOfList stream                    collect ArrayList  new  List  addAll  List  addAll

[java] How can I turn a List of Lists into a List in Java 8?

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