java11-stream-flatMap-fix

Reference: https://blog.jooq.org/2017/07/03/are-java-8-streams-truly-lazy-not-completely/
Reference: https://stackoverflow.com/questions/46288915/is-flatmap-guaranteed-to-be-lazy#comment79550832_46288915
Reference: https://bugs.openjdk.java.net/browse/JDK-8075939

project description

We provide two tests in FlatMapLaziness:

• flat map produces unlimited stream

  @Test
  public void flatMap_laziness_unlimitedStream() {
      Stream.of(1)
              .flatMap(x -> Stream.generate(Math::random))
              .findFirst()
              .orElseThrow(IllegalStateException::new);
  }    
  

• flat map produces limited stream

  @Test
  public void flatMap_laziness_limitedStream() {
      Integer d = Stream.of(1)
              .flatMap(x -> Stream.iterate(0, i -> ++i).peek(System.out::println).limit(10))
              .findFirst()
              .orElseThrow(IllegalStateException::new);
  
      assertThat(d, is(0));
  }    
  

And when using different JDKs, we get different results:

• openjdk 8:
  • flatMap_laziness_unlimitedStream never ends
  • flatMap_laziness_limitedStream prints (peek(System.out::println)) numbers from 0..9
• openjdk 11:
  • flatMap_laziness_unlimitedStream ends after first generated number
  • flatMap_laziness_limitedStream prints (peek(System.out::println)) only 0