Java 8 Streams FlatMap method example

Question

I have been checking the upcoming Java update  namely  Java 8 or JDK 8  Yes  I am impatient  there s a lot of new stuff  but  there is something I don t understand  some simple code   final Stream lt Integer gt stream   Stream of 1 2 3 4 5 6 7 8 9 10   stream flatMap      the javadocs are     public  lt R gt  Stream lt R gt  flatMap Function lt   super T   extends Stream lt   extends R gt  gt  mapper       Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element  Each mapped stream is closed after its contents have been placed into this stream   If a mapped stream is null an empty stream is used  instead         This is an intermediate operation      I would appreciate if somebody created some simple real-life examples about flatMap  how you could code it in previous java versions Java 6 7  and how you can code the same routines using Java 8

User · Answer

Extract unique words sorted ASC from a list of phrases:

List<String> phrases = Arrays.asList(
        "sporadic perjury",
        "confounded skimming",
        "incumbent jailer",
        "confounded jailer");

List<String> uniqueWords = phrases
        .stream()
        .flatMap(phrase -> Stream.of(phrase.split("\\s+")))
        .distinct()
        .sorted()
        .collect(Collectors.toList());
System.out.println("Unique words: " + uniqueWords);

... and the output:

Unique words: [confounded, incumbent, jailer, perjury, skimming, sporadic]

User · Answer

Made up example  Imagine that you want to create the following sequence  1  2  2  3  3  3  4  4  4  4 etc   in other words  1x1  2x2  3x3 etc    With flatMap it could look like   IntStream sequence   IntStream rangeClosed 1  4                             flatMap i - gt  IntStream iterate i  identity    limit i    sequence forEach System out  println     where    IntStream rangeClosed 1  4  creates a stream of int from 1 to 4  inclusive IntStream iterate i  identity    limit i  creates a stream of length i of int i - so applied to i   4 it creates a stream  4  4  4  4 flatMap  flattens  the stream and  concatenates  it to the original stream   With Java  lt  8 you would need two nested loops   List lt Integer gt  list   new ArrayList lt  gt     for  int i   1  i  lt   4  i          for  int j   0  j  lt  i  j              list add i             Real world example  Let s say I have a List lt TimeSeries gt  where each TimeSeries is essentially a Map lt LocalDate  Double gt   I want to get a list of all dates for which at least one of the time series has a value  flatMap to the rescue   list stream   parallel        flatMap ts - gt  ts dates   stream       for each TS  stream dates and flatmap      distinct                              remove duplicates      sorted                                sort ascending      collect toList       Not only is it readable  but if you suddenly need to process 100k elements  simply adding parallel   will improve performance without you writing any concurrent code

User · Answer

Am I the only one who finds unwinding lists boring   -   Let s try with objects  Real world example by the way    Given  Object representing repetitive task  About important task fields  reminders are starting to ring at start and repeat every repeatPeriod repeatUnit e g  5 HOURS  and there will be repeatCount reminders in total including starting one    Goal  achieve a list of task copies  one for each task reminder invocation   List lt Task gt  tasks               Arrays asList                      new Task                              false   completed sign                              My important task    task name  text                              LocalDateTime now   plus 2  ChronoUnit DAYS    first reminder start                              true   is task repetitive                              1   reminder interval                             ChronoUnit DAYS   interval unit                             5  total number of reminders                                       tasks stream   flatMap          x - gt  LongStream iterate                  x getStart   toEpochSecond ZoneOffset UTC                   p - gt   p   x getRepeatPeriod   x getRepeatUnit   getDuration   getSeconds              limit x getRepeatCount    boxed            map  y - gt  new Task x LocalDateTime ofEpochSecond y 0 ZoneOffset UTC      forEach System out  println     Output   Task completed false  text  My important task   start 2014-10-01T21 35 24  repeat false  repeatCount 0  repeatPeriod 0  repeatUnit null  Task completed false  text  My important task   start 2014-10-02T21 35 24  repeat false  repeatCount 0  repeatPeriod 0  repeatUnit null  Task completed false  text  My important task   start 2014-10-03T21 35 24  repeat false  repeatCount 0  repeatPeriod 0  repeatUnit null  Task completed false  text  My important task   start 2014-10-04T21 35 24  repeat false  repeatCount 0  repeatPeriod 0  repeatUnit null  Task completed false  text  My important task   start 2014-10-05T21 35 24  repeat false  repeatCount 0  repeatPeriod 0  repeatUnit null    P S   I would appreciate if someone suggested a simpler solution  I m not a pro after all   UPDATE   RBz asked for detailed explanation so here it is  Basically flatMap puts all elements from streams inside another stream into output stream  A lot of streams here     So  for each Task in initial stream lambda expression x - gt  LongStream iterate    creates a stream of long values that represent task start moments  This stream is limited to x getRepeatCount   instances  It s values start from x getStart   toEpochSecond ZoneOffset UTC  and each next value is calculated using lambda p - gt   p   x getRepeatPeriod   x getRepeatUnit   getDuration   getSeconds    boxed   returns the stream with each long value as a Long wrapper instance  Then each Long in that stream is mapped to new Task instance that is not repetitive anymore and contains exact execution time  This sample contains only one Task in input list  But imagine that you have a thousand  You will have then a stream of 1000 streams of Task objects  And what flatMap does here is putting all Tasks from all streams onto the same output stream  That s all as I understand it  Thank you for your question

User · Answer

It doesn t make sense to flatMap a Stream that s already flat  like the Stream lt Integer gt  you ve shown in your question     However  if you had a Stream lt List lt Integer gt  gt  then it would make sense and you could do this   Stream lt List lt Integer gt  gt  integerListStream   Stream of      Arrays asList 1  2        Arrays asList 3  4        Arrays asList 5      Stream lt Integer gt  integerStream   integerListStream  flatMap Collection  stream   integerStream forEach System out  println     Which would print   1 2 3 4 5   To do this pre-Java 8 you just need a loops   List lt List lt Integer gt  gt  integerLists   Arrays asList      Arrays asList 1  2        Arrays asList 3  4        Arrays asList 5     List lt Integer gt  flattened   new ArrayList lt  gt      for  List lt Integer gt  integerList   integerLists        flattened addAll integerList      for  Integer i   flattened        System out println i

User · Answer

A very simple example  Split a list of full names to get a list of names  regardless of first or last   List lt String gt  fullNames   Arrays asList  Barry Allen    Bruce Wayne    Clark Kent      fullNames stream                flatMap fullName - gt  Pattern compile      splitAsStream fullName                forEach System out  println     This prints out   Barry Allen Bruce Wayne Clark Kent

User · Answer

Given this     public class SalesTerritory               private String territoryName          private Set lt String gt  geographicExtents           public SalesTerritory  String territoryName  Set lt String gt  zipCodes                         this territoryName   territoryName              this geographicExtents   zipCodes                     public String getTerritoryName                         return territoryName                     public void setTerritoryName  String territoryName                         this territoryName   territoryName                     public Set lt String gt  getGeographicExtents                         return geographicExtents    null   Collections unmodifiableSet  geographicExtents     Collections emptySet                       public void setGeographicExtents  Set lt String gt  geographicExtents                         this geographicExtents   new HashSet lt  gt   geographicExtents                        Override         public int hashCode                         int hash   7              hash   53   hash   Objects hashCode  this territoryName                return hash                      Override         public boolean equals  Object obj                         if   this    obj                     return true                            if   obj    null                     return false                            if   getClass      obj getClass                       return false                            final SalesTerritory other    SalesTerritory  obj              if    Objects equals  this territoryName  other territoryName                       return false                            return true                      Override         public String toString                         return  SalesTerritory      territoryName     territoryName      geographicExtents     geographicExtents                           and this   public class SalesTerritories       private static final Set lt SalesTerritory gt  territories           new HashSet lt  gt               Arrays asList                  new SalesTerritory                        new SalesTerritory   North-East  USA                                           new HashSet lt  gt   Arrays asList  new String     Maine    New Hampshire    Vermont                                                                                        Rhode Island    Massachusetts    Connecticut                                                                                        New York    New Jersey    Delaware    Maryland                                                                                        Eastern Pennsylvania    District of Columbia                               new SalesTerritory   Appalachia  USA                                           new HashSet lt  gt   Arrays asList  new String     West-Virgina    Kentucky                                                                                        Western Pennsylvania                               new SalesTerritory   South-East  USA                                           new HashSet lt  gt   Arrays asList  new String     Virginia    North Carolina    South Carolina                                                                                        Georgia    Florida    Alabama    Tennessee                                                                                        Mississippi    Arkansas    Louisiana                               new SalesTerritory   Mid-West  USA                                           new HashSet lt  gt   Arrays asList  new String     Ohio    Michigan    Wisconsin    Minnesota                                                                                        Iowa    Missouri    Illinois    Indiana                               new SalesTerritory   Great Plains  USA                                           new HashSet lt  gt   Arrays asList  new String     Oklahoma    Kansas    Nebraska                                                                                        South Dakota    North Dakota                                                                                        Eastern Montana                                                                                        Wyoming    Colorada                               new SalesTerritory   Rocky Mountain  USA                                           new HashSet lt  gt   Arrays asList  new String     Western Montana    Idaho    Utah    Nevada                               new SalesTerritory   South-West  USA                                           new HashSet lt  gt   Arrays asList  new String     Arizona    New Mexico    Texas                               new SalesTerritory   Pacific North-West  USA                                           new HashSet lt  gt   Arrays asList  new String     Washington    Oregon    Alaska                               new SalesTerritory   Pacific South-West  USA                                           new HashSet lt  gt   Arrays asList  new String     California    Hawaii                                                          public static Set lt SalesTerritory gt  getAllTerritories                 return Collections unmodifiableSet  territories               private SalesTerritories                    We can then do this   System out println    System out      println   We can use  flatMap  in combination with the  AbstractMap SimpleEntry  class to flatten a hierarchical data-structure to a set of Key Value pairs        SalesTerritories getAllTerritories        stream        flatMap  t - gt  t getGeographicExtents            stream            map  ge - gt  new SimpleEntry lt  gt   t getTerritoryName    ge                map  e - gt  String format    -30s    s                                 e getKey                                  e getValue            forEach  System out  println

User · Answer

This method takes one Function as an argument  this function accepts one parameter T as an input argument and return one stream of parameter R as a return value  When this function is applied on each element of this stream  it produces a stream of new values  All the elements of these new streams generated by each element are then copied to a new stream  which will be a return value of this method    http   codedestine com java-8-stream-flatmap-method

[java] Java 8 Streams FlatMap method example

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