Errata for OCP: Oracle Certified Professional Java SE 8 Programmer II Study Guide - Chapter 7

[Mike Constantin]

Hi,

In chapter 7, "Creating parallel streams":

On the other hand, flatMap() creates a new stream that is not parallel by default, regardless of whether the underlying elements were parallel.

Also in chapter 7, question 13 the explanation says:

flatMap() is a new stream that is not parallel by default, even though its elements are parallel streams.

But the truth is that flatMap preserves the parallel attribute of the stream going in.

public class ParallelStreams {
	public static void main(String[] args) {
		Stream<String> cats = Stream.of("leopard","lynx","ocelot","puma").parallel();
		Stream<String> bears = Stream.of("panda","grizzly","polar").parallel();
		Stream<Stream<String>> doubleStream = Stream.of(cats,bears);
		
		System.out.println("cats:" + cats.isParallel() // true
				+ " bears:" + bears.isParallel()   // true
				+ " doubles:" + doubleStream.isParallel()); // false
		
		Stream newStream = doubleStream.parallel().flatMap(s -> s);
		System.out.println("new stream: " + newStream.isParallel()); // true
          }
}

It's the Stream builder the one who sets the parallel attribute to false in the new Stream.

[Scott Selikoff]

Hi Mike,

Yes, if the stream being mapped to is parallel, it will preserve that parallel mapping. The text was referring to the underlying streams. For example, the following is not a parallel stream even though cats and bears are both parallel.

System.out.println(Stream.of(cats,bears).flatMap(s -> s).isParallel()); // false

You can contrast that with this behavior which confirms the underlying elements are still parallel:

Stream.of(cats, bears).forEach(s -> System.out.println(s.isParallel())); // true x 2

The idea in the text is to say that taking multiple parallel streams and applying a flatMap() operation does not preserve the parallel attribute on the underlying streams. As you pointed out, it does preserve it on the top-most Stream.of(...).parallel(). We'll add a note to be clearer about this in the text next time!