Can you split a stream into two streams

Question

I have a data set represented by a Java 8 stream   Stream lt T gt  stream          I can see how to filter it to get a random subset - for example  Random r   new Random    PrimitiveIterator OfInt coin   r ints 0  2  iterator       Stream lt T gt  heads   stream filter  x  - gt   coin nextInt      0      I can also see how I could reduce this stream to get  for example  two lists representing two random halves of the data set  and then turn those back into streams  But  is there a direct way to generate two streams from the initial one  Something like   heads  tails    stream  some kind of split based on filter    Thanks for any insight

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not exactly  but you may be able to accomplish what you need by invoking Collectors groupingBy    you create a new Collection  and can then instantiate streams on that new collection

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This is against the general mechanism of Stream  Say you can split Stream S0 to Sa and Sb like you wanted  Performing any terminal operation  say count    on Sa will necessarily  consume  all elements in S0  Therefore Sb lost its data source   Previously  Stream had a tee   method  I think  which duplicate a stream to two  It s removed now   Stream has a peek   method though  you might be able to use it to achieve your requirements

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Shorter version that uses Lombok   import java util function Consumer  import java util function Predicate   import lombok RequiredArgsConstructor          Forks a Stream using a Predicate into postive and negative outcomes       RequiredArgsConstructor  FieldDefaults makeFinal   true  level   AccessLevel PROTECTED  public class StreamForkerUtil lt T gt  implements Consumer lt T gt        Predicate lt T gt  predicate      Consumer lt T gt  positiveConsumer      Consumer lt T gt  negativeConsumer        Override     public void accept T t             predicate test t    positiveConsumer   negativeConsumer  accept t

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How about   Supplier lt Stream lt Integer gt  gt  randomIntsStreamSupplier          - gt   new Random    ints 0  2  boxed     Stream lt Integer gt  tails       randomIntsStreamSupplier get   filter x- gt x equals 0    Stream lt Integer gt  heads       randomIntsStreamSupplier get   filter x- gt x equals 1

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This was the least bad answer I could come up with    import org apache commons lang3 tuple ImmutablePair  import org apache commons lang3 tuple Pair   public class Test        public static  lt T  L  R gt  Pair lt L  R gt  splitStream Stream lt T gt  inputStream  Predicate lt T gt  predicate              Function lt Stream lt T gt   L gt  trueStreamProcessor  Function lt Stream lt T gt   R gt  falseStreamProcessor             Map lt Boolean  List lt T gt  gt  partitioned   inputStream collect Collectors partitioningBy predicate            L trueResult   trueStreamProcessor apply partitioned get Boolean TRUE  stream             R falseResult   falseStreamProcessor apply partitioned get Boolean FALSE  stream              return new ImmutablePair lt L  R gt  trueResult  falseResult              public static void main String   args             Stream lt Integer gt  stream   Stream iterate 0  n - gt  n   1  limit 10            Pair lt List lt Integer gt   String gt  results   splitStream stream                  n - gt  n  gt  5                  s - gt  s filter n - gt  n   2    0  collect Collectors toList                     s - gt  s map n - gt  n toString    collect Collectors joining                  System out println results              This takes a stream of integers and splits them at 5  For those greater than 5 it filters only even numbers and puts them in a list  For the rest it joins them with     outputs      6  8  0 1 2 3 4 5    Its not ideal as it collects everything into intermediary collections breaking the stream  and has too many arguments

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I stumbled across this question while looking for a way to filter certain elements out of a stream and log them as errors  So I did not really need to split the stream so much as attach a premature terminating action to a predicate with unobtrusive syntax  This is what I came up with   public class MyProcess          Return a Predicate that performs a bail-out action on non-matching items         private static  lt T gt  Predicate lt T gt  withAltAction Predicate lt T gt  pred  Consumer lt T gt  altAction        return x - gt            if  pred test x                 return true                    altAction accept x           return false                 Example usage in non-trivial pipeline        public void processItems Stream lt Item gt  stream            stream filter Objects  nonNull                 peek this  logItem                 map Item  getSubItems                 filter withAltAction SubItem  isValid                                      i - gt  logError i   Invalid                    peek this  logSubItem                 filter withAltAction i - gt  i size    gt  10                                      i - gt  logError i   Too large                    map SubItem  toDisplayItem                 forEach this  display

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I stumbled across this question to my self and I feel that a forked stream has some use cases that could prove valid  I wrote the code below as a consumer so that it does not do anything but you could apply it to functions and anything else you might come across   class PredicateSplitterConsumer lt T gt  implements Consumer lt T gt      private Predicate lt T gt  predicate    private Consumer lt T gt   positiveConsumer    private Consumer lt T gt   negativeConsumer     public PredicateSplitterConsumer Predicate lt T gt  predicate  Consumer lt T gt  positive  Consumer lt T gt  negative          this predicate   predicate      this positiveConsumer   positive      this negativeConsumer   negative          Override   public void accept T t          if  predicate test t               positiveConsumer accept t             else             negativeConsumer accept t                 Now your code implementation could be something like this   personsArray forEach          new PredicateSplitterConsumer lt  gt               person - gt  person getDateOfBirth   isPresent                person - gt  System out println person getName                 person - gt  System out println person getName       does not have Date of birth

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A collector can be used for this    For two categories  use Collectors partitioningBy   factory    This will create a Map from Boolean to List  and put items in one or the other list based on a Predicate   Note  Since the stream needs to be consumed whole  this can t work on infinite streams  And because the stream is consumed anyway  this method simply puts them in Lists instead of making a new stream-with-memory  You can always stream those lists if you require streams as output   Also  no need for the iterator  not even in the heads-only example you provided    Binary splitting looks like this      Random r   new Random     Map lt Boolean  List lt String gt  gt  groups   stream      collect Collectors partitioningBy x - gt  r nextBoolean       System out println groups get false  size     System out println groups get true  size        For more categories  use a Collectors groupingBy   factory      Map lt Object  List lt String gt  gt  groups   stream      collect Collectors groupingBy x - gt  r nextInt 3     System out println groups get 0  size     System out println groups get 1  size     System out println groups get 2  size       In case the streams are not Stream  but one of the primitive streams like IntStream  then this  collect Collectors  method is not available  You ll have to do it the manual way without a collector factory  It s implementation looks like this    Example 2 0 since 2020-04-16       IntStream    intStream   IntStream iterate 0  i - gt  i   1  limit 100000  parallel        IntPredicate predicate   ignored - gt  r nextBoolean         Map lt Boolean  List lt Integer gt  gt  groups   intStream collect                 - gt  Map of false  new ArrayList lt  gt  100000                            true   new ArrayList lt  gt  100000                 map  value  - gt  map get predicate test value   add value                map1  map2  - gt                    map1 get false  addAll map2 get false                    map1 get true   addAll map2 get true                       In this example I initialize the ArrayLists with the full size of the initial collection  if this is known at all   This prevents resize events even in the worst-case scenario  but can potentially gobble up 2 N T space  N   initial number of elements  T   number of threads   To trade-off space for speed  you can leave it out or use your best educated guess  like the expected highest number of elements in one partition  typically just over N 2 for a balanced split    I hope I don t offend anyone by using a Java 9 method  For the Java 8 version  look at the edit history

User · Answer

Unfortunately  what you ask for is directly frowned upon in the JavaDoc of Stream      A stream should be operated on  invoking an intermediate or terminal   stream operation  only once  This rules out  for example   forked    streams  where the same source feeds two or more pipelines  or   multiple traversals of the same stream    You can work around this using peek or other methods should you truly desire that type of behaviour   In this case  what you should do is instead of trying to back two streams from the same original Stream source with a forking filter  you would duplicate your stream and filter each of the duplicates appropriately   However  you may wish to reconsider if a Stream is the appropriate structure for your use case

User · Answer

Not exactly   You can t get two Streams out of one  this doesn t make sense -- how would you iterate over one without needing to generate the other at the same time   A stream can only be operated over once   However  if you want to dump them into a list or something  you could do  stream forEach  x  - gt    x    0    heads   tails  add x

[java] Can you split a stream into two streams?

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