Java how can I split an ArrayList in multiple small ArrayLists

Question

How can I split an ArrayList  size 1000  in multiple ArrayLists of the same size   10     ArrayList lt Integer gt  results

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You can add the Guava library to your project and use the Lists partition method  e g   List lt Integer gt  bigList       List lt List lt Integer gt  gt  smallerLists   Lists partition bigList  10

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Apache Commons Collections 4 has a partition method in the ListUtils class  Here   s how it works   import org apache commons collections4 ListUtils       int targetSize   100  List lt Integer gt  largeList       List lt List lt Integer gt  gt  output   ListUtils partition largeList  targetSize

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Create a new list and add a sublist view of the source list using the addAll   method to create a new sublist List lt T gt  newList   new ArrayList lt T gt     newList addAll sourceList subList startIndex  endIndex

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Just to be clear  This still have to be tested more     public class Splitter    public static  lt T gt  List lt List lt T gt  gt  splitList List lt T gt  listTobeSplit  int size        List lt List lt T gt  gt  sublists  new LinkedList lt  gt         if listTobeSplit size   gt size        int counter 0      boolean lastListadded false       List lt T gt  subList new LinkedList lt  gt          for T t  listTobeSplit                        if  counter  0                                subList  new LinkedList lt  gt                  subList add t                counter                 lastListadded false                      else if counter gt 0  amp  amp  counter lt size-1                 subList add t                counter                        else                lastListadded true               subList add t                sublists add subList                counter 0                                     if lastListadded  false          sublists add subList                   else           sublists add listTobeSplit             log debug  sublists    sublists       return sublists

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You can use the chunk method from Eclipse Collections   ArrayList lt Integer gt  list   new ArrayList lt  gt  Interval oneTo 1000    RichIterable lt RichIterable lt Integer gt  gt  chunks   Iterate chunk list  10   Verify assertSize 100  chunks     A few examples of the chunk method were included in this DZone article as well   Note  I am a committer for Eclipse Collections

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The answer provided by polygenelubricants splits an array based on given size  I was looking for code that would split an array into a given number of parts  Here is the modification I did to the code   public static  lt T gt List lt List lt T gt  gt  chopIntoParts  final List lt T gt  ls  final int iParts         final List lt List lt T gt  gt  lsParts   new ArrayList lt List lt T gt  gt         final int iChunkSize   ls size     iParts      int iLeftOver   ls size     iParts      int iTake   iChunkSize       for  int i   0  iT   ls size    i  lt  iT  i    iTake                 if  iLeftOver  gt  0                         iLeftOver--               iTake   iChunkSize   1                    else                       iTake   iChunkSize                     lsParts add  new ArrayList lt T gt   ls subList  i  Math min  iT  i   iTake                     return lsParts      Hope it helps someone

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Divide a list to lists of n size        import java util AbstractList      import java util ArrayList      import java util List       public final class PartitionUtil lt T gt  extends AbstractList lt List lt T gt  gt             private final List lt T gt  list          private final int chunkSize           private PartitionUtil List lt T gt  list  int chunkSize                this list   new ArrayList lt  gt  list               this chunkSize   chunkSize                     public static  lt T gt  PartitionUtil lt T gt  ofSize List lt T gt  list  int chunkSize                return new PartitionUtil lt  gt  list  chunkSize                       Override         public List lt T gt  get int index                int start   index   chunkSize              int end   Math min start   chunkSize  list size                  if  start  gt  end                    throw new IndexOutOfBoundsException  Index     index     is out of the list range  lt 0      size   - 1      gt                                return new ArrayList lt  gt  list subList start  end                        Override         public int size                 return  int  Math ceil  double  list size      double  chunkSize                        Function call                  List lt List lt String gt  gt  containerNumChunks   PartitionUtil ofSize list  999    more details  https   e printstacktrace blog divide-a-list-to-lists-of-n-size-in-Java-8

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A similar question was discussed here  Java  split a List into two sub-Lists   Mainly you can use sublist  More details here   subList     Returns a view of the portion of this list between fromIndex  inclusive  and toIndex  exclusive   If fromIndex and toIndex are equal  the returned list is empty   The returned list is backed by this list  so changes in the returned list are reflected in this list  and vice-versa  The returned list supports all of the optional list operations supported by this list

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This works for me        Returns List of the List argument passed to this function with size   chunkSize       param largeList input list to be portioned    param chunkSize maximum size of each partition    param  lt T gt  Generic type of the List    return A list of Lists which is portioned from the original list     public static   lt T gt  List lt List lt T gt  gt  chunkList List lt T gt  list  int chunkSize        if  chunkSize  lt   0            throw new IllegalArgumentException  Invalid chunk size      chunkSize             List lt List lt T gt  gt  chunkList   new ArrayList lt  gt  list size     chunkSize       for  int i   0  i  lt  list size    i    chunkSize            chunkList add list subList i  i   chunkSize  gt   list size     list size  -1   i   chunkSize              return chunkList      Eg     List lt Integer gt  stringList   new ArrayList lt  gt     stringList add 0   stringList add 1   stringList add 2   stringList add 3   stringList add 4   stringList add 5   stringList add 6   stringList add 7   stringList add 8   stringList add 9    List lt List lt Integer gt  gt  chunkList   getChunkList1 stringList  2

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You can also use FunctionalJava library - there is partition method for List  This lib has its own collection types  you can convert them to java collections back and forth   import fj data List   java util List lt String gt  javaList   Arrays asList  a    b    c    d      List lt String gt  fList   Java  lt String gt Collection List   f javaList    List lt List lt String gt  partitions   fList partition 2

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if you don t want to import the apache commons library try this simple code   final static int MAX ELEMENT   20   public static void main final String   args         final List lt String gt  list   new ArrayList lt String gt          for  int i   1  i  lt   161  i              list add String valueOf i            System out print       String valueOf i              System out println          System out println       gt  gt  gt          final List lt List lt String gt  gt  result   splitList list  MAX ELEMENT        for  final List lt String gt  entry   result            System out println  ------------------------            for  final String elm   entry                System out println elm                     System out println  ------------------------              private static List lt List lt String gt  gt  splitList final List lt String gt  list  final int maxElement         final List lt List lt String gt  gt  result   new ArrayList lt List lt String gt  gt          final int div   list size     maxElement       System out println div        for  int i   0  i  lt   div  i               final int startIndex   i   maxElement           if  startIndex  gt   list size                  return result                     final int endIndex    i   1    maxElement           if  endIndex  lt  list size                  result add list subList startIndex  endIndex              else               result add list subList startIndex  list size                            return result

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import org apache commons collections4 ListUtils  ArrayList lt Integer gt  mainList                  List lt List lt Integer gt  gt  multipleLists   ListUtils partition mainList 100   int i 1  for  List lt Integer gt  indexedList   multipleLists     System out println  Values in List   i     for  Integer value   indexedList      System out println value   i

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Let s suppose you want the considere the class that split the list into multiple chuncks as a library class  So let s say the class is called  shared  and in should be final to be sure it won t be extended     import java util ArrayList     import java util Arrays     import java util List   public final class Shared   List lt Integer gt  input  int portion   public Shared int portion  Integer    input        this setPortion portion       this setInput input      public List lt List lt Integer gt  gt  listToChunks         List lt List lt Integer gt  gt  result   new ArrayList lt List lt Integer gt  gt         int size   this size        int startAt   0      int endAt   this portion       while  endAt  lt   size             result add this input subList startAt  endAt            startAt   endAt          endAt    size - endAt  lt  this portion  amp  amp  size - endAt  gt  0     this size       endAt   this portion              return result     public int size         return this input size       public void setInput Integer    input        if  input    null  amp  amp  input length  gt  0          this input   Arrays asList input       else         System out println  quot Error 001   please enter a valid array of integers  quot       public void setPortion int portion        if  portion  gt  0          this portion   portion      else         System out println  quot Error 002   please enter a valid positive number  quot         Next  let s try to execute it from another class that hold the public static void main String    args  public class exercise    public static void main String   args        Integer   numbers    1  2  3  4  5  6  7       int portion   2      Shared share   new Shared portion  numbers       System out println share listToChunks             Now  if you enter an array of integer  1  2  3  4  5  6  7  with a partition of 2  the result will be   1  2    3  4    5  6    7

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You can use subList int fromIndex  int toIndex  to get a view of a portion of the original list   From the API      Returns a view of the portion of this list between the specified fromIndex  inclusive  and toIndex  exclusive   If fromIndex and toIndex are equal  the returned list is empty   The returned list is backed by this list  so non-structural changes in the returned list are reflected in this list  and vice-versa  The returned list supports all of the optional list operations supported by this list    Example   List lt Integer gt  numbers   new ArrayList lt Integer gt       Arrays asList 5 3 1 2 9 5 0 7      List lt Integer gt  head   numbers subList 0  4   List lt Integer gt  tail   numbers subList 4  8   System out println head      prints   5  3  1  2   System out println tail      prints   9  5  0  7    Collections sort head   System out println numbers      prints   1  2  3  5  9  5  0  7    tail add -1   System out println numbers      prints   1  2  3  5  9  5  0  7  -1     If you need these chopped lists to be NOT a view  then simply create a new List from the subList  Here s an example of putting a few of these things together      chops a list into non-view sublists of length L static  lt T gt  List lt List lt T gt  gt  chopped List lt T gt  list  final int L        List lt List lt T gt  gt  parts   new ArrayList lt List lt T gt  gt         final int N   list size        for  int i   0  i  lt  N  i    L            parts add new ArrayList lt T gt               list subList i  Math min N  i   L                         return parts      List lt Integer gt  numbers   Collections unmodifiableList      Arrays asList 5 3 1 2 9 5 0 7     List lt List lt Integer gt  gt  parts   chopped numbers  3   System out println parts      prints    5  3  1    2  9  5    0  7    parts get 0  add -1   System out println parts      prints    5  3  1  -1    2  9  5    0  7    System out println numbers      prints   5  3  1  2  9  5  0  7    unmodified

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Java 8  We can split a list based on some size or based on a condition    static Collection lt List lt Integer gt  gt  partitionIntegerListBasedOnSize List lt Integer gt  inputList  int size            return inputList stream                    collect Collectors groupingBy s - gt   s-1  size                    values      static  lt T gt  Collection lt List lt T gt  gt  partitionBasedOnSize List lt T gt  inputList  int size            final AtomicInteger counter   new AtomicInteger 0           return inputList stream                        collect Collectors groupingBy s - gt  counter getAndIncrement   size                        values      static  lt T gt  Collection lt List lt T gt  gt  partitionBasedOnCondition List lt T gt  inputList  Predicate lt T gt  condition            return inputList stream   collect Collectors partitioningBy s- gt   condition test s     values        Then we can use them as   final List lt Integer gt  list   Arrays asList 1 2 3 4 5 6 7 8 9 10   System out println partitionIntegerListBasedOnSize list  4          1  2  3  4    5  6  7  8    9  10   System out println partitionBasedOnSize list  4          1  2  3  4    5  6  7  8    9  10   System out println partitionBasedOnSize list  3          1  2  3    4  5  6    7  8  9    10   System out println partitionBasedOnCondition list  i - gt  i lt 6          6  7  8  9  10    1  2  3  4  5

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You need to know the chunk size by which you re dividing your list  Say you have a list of 108 entries and you need a chunk size of 25  Thus you will end up with 5 lists     4 having 25 entries each  1  the fifth  having 8 elements     Code   public static void main String   args             List lt Integer gt  list   new ArrayList lt Integer gt             for  int i 0  i lt 108  i                 list add i                     int size  list size            int j 0                  List lt  List lt Integer gt   gt  splittedList   new ArrayList lt List lt Integer gt  gt                       List lt Integer gt  tempList   new ArrayList lt Integer gt             for j 0 j lt size j                 tempList add list get j            if  j 1  25  0                  chunk of 25 created and clearing tempList             splittedList add tempList               tempList   null                intializing it again for new chunk              tempList   new ArrayList lt Integer gt                                 if size 25  0                 adding the remaining enteries              splittedList add tempList                     for  int k 0 k lt splittedList size    k                    k 1  because we started from k 0             System out println  Chunk number     k 1    has elements     splittedList get k  size

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Using the StreamEx library  you can use the StreamEx ofSubLists List lt T gt  source  int length  method   Returns a new StreamEx which consists of non-overlapping sublists of given source list having the specified length  the last sublist may be shorter       Assuming you don t actually care that the lists are of type ArrayList List lt List lt Integer gt  gt  sublists   StreamEx ofSubLists result  10  toList        If you actually want them to be of type ArrayList  per your question List lt List lt Integer gt  gt  sublists   StreamEx ofSubLists result  10  toCollection ArrayList  new

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I m guessing that the issue you re having is with naming 100 ArrayLists and populating them   You can create an array of ArrayLists and populate each of those using a loop   The simplest  read stupidest  way to do this is like this   ArrayList results   new ArrayList 1000          populate results here     for  int i   0  i  lt  1000  i              results add i             ArrayList   resultGroups   new ArrayList 100          initialize all your small ArrayList groups     for  int i   0  i  lt  100  i                  resultGroups i    new ArrayList                 put your results into those arrays     for  int i   0  i  lt  1000  i             resultGroups i 10  add results get i

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private ArrayList lt List lt String gt  gt  chunkArrayList ArrayList lt String gt  arrayToChunk  int chunkSize        ArrayList lt List lt String gt  gt  chunkList   new ArrayList lt  gt         int guide   arrayToChunk size        int index   0      int tale   chunkSize      while  tale  lt  arrayToChunk size                 chunkList add arrayToChunk subList index  tale                guide   guide - chunkSize              index   index   chunkSize              tale   tale   chunkSize            if  guide  gt 0           chunkList add arrayToChunk subList index  index   guide              Log i  Chunked Array      chunkList toString         return chunkList      Example       ArrayList lt String gt  test   new ArrayList lt  gt         for  int i 1  i lt  1000  i             test add String valueOf i               chunkArrayList test 10     Output  CHUNKED     1  2  3  4  5  6  7  8  9  10    11  12  13  14  15  16  17  18  19  20    21  22  23  24  25  26  27  28  29  30    31  32  33  34  35  36  37  38  39  40    41  42  43  44  45  46  47  48  49  50    51  52  53  54  55  56  57  58  59  60    61  62  63  64  65  66  67  68  69  70    71  72  73  74  75  76  77  78  79  80    81  82  83  84  85  86  87  88  89  90    91  92  93  94  95  96  97  98  99  100               you will see in your log

[java] Java: how can I split an ArrayList in multiple small ArrayLists?

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