How can I count occurrences with groupBy

Question

I want to collect the items in a stream into a map which groups equal objects together  and maps to the number of occurrences  List lt String gt  list   Arrays asList  quot Hello quot    quot Hello quot    quot World quot    Map lt String  Long gt  wordToFrequency      what goes here   So in this case  I would like the map to consist of these entries  Hello - gt  2 World - gt  1  How can I do that

User · Answer

Here are slightly different options to accomplish the task at hand.

using toMap:

list.stream()
    .collect(Collectors.toMap(Function.identity(), e -> 1, Math::addExact));

using Map::merge:

Map<String, Integer> accumulator = new HashMap<>();
list.forEach(s -> accumulator.merge(s, 1, Math::addExact));

User · Answer

I think you re just looking for the overload which takes another Collector to specify what to do with each group    and then Collectors counting   to do the counting   import java util    import java util stream     class Test       public static void main String   args            List lt String gt  list   new ArrayList lt  gt              list add  Hello            list add  Hello            list add  World             Map lt String  Long gt  counted   list stream                collect Collectors groupingBy Function identity    Collectors counting               System out println counted             Result    Hello 2  World 1     There s also the possibility of using groupingByConcurrent for more efficiency  Something to bear in mind for your real code  if it would be safe in your context

User · Answer

If you re open to using a third-party library  you can use the Collectors2 class in Eclipse Collections to convert the List to a Bag using a Stream  A Bag is a data structure that is built for counting   Bag lt String gt  counted           list stream   collect Collectors2 countBy each - gt  each     Assert assertEquals 1  counted occurrencesOf  World     Assert assertEquals 2  counted occurrencesOf  Hello      System out println counted toStringOfItemToCount       Output    World 1  Hello 2    In this particular case  you can simply collect the List directly into a Bag   Bag lt String gt  counted            list stream   collect Collectors2 toBag       You can also create the Bag without using a Stream by adapting the List with the Eclipse Collections protocols   Bag lt String gt  counted   Lists adapt list  countBy each - gt  each     or in this particular case   Bag lt String gt  counted   Lists adapt list  toBag      You could also just create the Bag directly   Bag lt String gt  counted   Bags mutable with  Hello    Hello    World      A Bag lt String gt  is like a Map lt String  Integer gt  in that it internally keeps track of keys and their counts   But  if you ask a Map for a key it doesn t contain  it will return null   If you ask a Bag for a key it doesn t contain using occurrencesOf  it will return 0   Note  I am a committer for Eclipse Collections

User · Answer

Here is example for list of Objects  Map lt String  Long gt  requirementCountMap   requirements stream   collect Collectors groupingBy Requirement  getRequirementType  Collectors counting

User · Answer

Here is the simple solution by StreamEx  StreamEx of list  groupingBy Function identity    MoreCollectors countingInt      This has the advantage of reducing the Java stream boilerplate code  collect Collectors

User · Answer

List lt String gt  list   new ArrayList lt  gt      list add  Hello    list add  Hello    list add  World     Map lt String  List lt String gt  gt  collect   list stream                                            collect Collectors groupingBy o - gt  o    collect entrySet           forEach e - gt  System out println e getKey       -     e getValue   size

[java] How can I count occurrences with groupBy?

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