How to convert an iterator to a stream

Question

I am looking for a concise way to convert an Iterator to a Stream or more specifically to  view  the iterator as a stream   For performance reason  I would like to avoid a copy of the iterator in a new list   Iterator lt String gt  sourceIterator   Arrays asList  A    B    C   iterator    Collection lt String gt  copyList   new ArrayList lt String gt     sourceIterator forEachRemaining copyList  add   Stream lt String gt  targetStream   copyList stream        Based on the some suggestions in the comments  I have also tried to use Stream generate   public static void main String   args  throws Exception       Iterator lt String gt  sourceIterator   Arrays asList  A    B    C   iterator        Stream lt String gt  targetStream   Stream generate sourceIterator  next       targetStream forEach System out  println       However  I get a NoSuchElementException  since there is no invocation of hasNext   Exception in thread  main  java util NoSuchElementException     at java util AbstractList Itr next AbstractList java 364      at Main  Lambda 1 1175962212 get Unknown Source      at java util stream StreamSpliterators InfiniteSupplyingSpliterator OfRef tryAdvance StreamSpliterators java 1351      at java util Spliterator forEachRemaining Spliterator java 326      at java util stream ReferencePipeline Head forEach ReferencePipeline java 580      at Main main Main java 20    I have looked at StreamSupport and Collections but I didn t find anything

User · Answer

This is possible in Java 9   Stream generate    - gt  null       takeWhile x - gt  iterator hasNext         map n - gt  iterator next         forEach System out  println

User · Answer

One way is to create a Spliterator from the Iterator and use that as a basis for your stream  Iterator lt String gt  sourceIterator   Arrays asList  quot A quot    quot B quot    quot C quot   iterator    Stream lt String gt  targetStream   StreamSupport stream            Spliterators spliteratorUnknownSize sourceIterator  Spliterator ORDERED             false    An alternative which is maybe more readable is to use an Iterable - and creating an Iterable from an Iterator is very easy with lambdas because Iterable is a functional interface  Iterator lt String gt  sourceIterator   Arrays asList  quot A quot    quot B quot    quot C quot   iterator     Iterable lt String gt  iterable      - gt  sourceIterator  Stream lt String gt  targetStream   StreamSupport stream iterable spliterator    false

User · Answer

Great suggestion   Here s my reusable take on it   public class StreamUtils        public static  lt T gt  Stream lt T gt  asStream Iterator lt T gt  sourceIterator            return asStream sourceIterator  false              public static  lt T gt  Stream lt T gt  asStream Iterator lt T gt  sourceIterator  boolean parallel            Iterable lt T gt  iterable      - gt  sourceIterator          return StreamSupport stream iterable spliterator    parallel             And usage  make sure to statically import asStream    List lt String gt  aPrefixedStrings   asStream sourceIterator                   filter t - gt  t startsWith  A                     collect toList

User · Answer

Since version 21  Guava library provides Streams stream iterator   It does what  assylias s answer shows

User · Answer

Use Collections list iterator  stream

User · Answer

import com google common collect Streams    and use Streams stream iterator     Streams stream iterator          map v- gt  function v           collect Collectors toList

User · Answer

Another way to do this on Java 9  using Stream  iterate T  Predicate  UnaryOperator   Stream iterate iterator  Iterator  hasNext  UnaryOperator identity             map Iterator  next           forEach System out  println

User · Answer

Create Spliterator from Iterator using Spliterators class contains more than one function for creating spliterator  for example here am using spliteratorUnknownSize which is getting iterator as parameter  then create Stream using StreamSupport  Spliterator lt Model gt  spliterator   Spliterators spliteratorUnknownSize          iterator  Spliterator NONNULL   Stream lt Model gt  stream   StreamSupport stream spliterator  false

[java] How to convert an iterator to a stream?

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