Operation to connect multiple Streams @Java

By the way, I updated Java, so I will do it with REPL. Just type jshell on the console (java9 or above?)

jshell>

How to do it!

Concat when connecting two Streams

jshell> var streamA = Stream.of(1,2,3);
streamA ==> java.util.stream.ReferencePipeline$Head@1f36e637

jshell> var streamB = Stream.of(4,5);
streamB ==> java.util.stream.ReferencePipeline$Head@578486a3

jshell> Stream.concat(streamA, streamB).forEach(System.out::println);
1
2
3
4
5

As you can see, concat has only two arguments? https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#concat-java.util.stream.Stream-java.util.stream.Stream-

Three or more is the turn of flatMap

In the first place, there was no problem with the operation of connecting due to the nature of stream. Let's go honestly.

jshell> var streamA = Stream.of(1,2,3);
streamA ==> java.util.stream.ReferencePipeline$Head@536aaa8d

jshell> var streamB = Stream.of(4,5);
streamB ==> java.util.stream.ReferencePipeline$Head@e320068

jshell> var streamC = Stream.of(6,7,8,9);
streamC ==> java.util.stream.ReferencePipeline$Head@1f57539

jshell> Stream.of(streamA, streamB, streamC).flatMap(Function.identity()).forEach(System.out::println);
1
2
3
4
5
6
7
8
9

I don't know how to break the jshell in the middle ...

jshell> var streamA = Stream.of(1,2,3);
streamA ==> java.util.stream.ReferencePipeline$Head@27a8c74e

jshell> var streamB = Stream.of(4,5);
streamB ==> java.util.stream.ReferencePipeline$Head@2d8f65a4

jshell> var streamC = Stream.of(6,7,8,9);
streamC ==> java.util.stream.ReferencePipeline$Head@1b68ddbd

jshell> Stream.of(streamA, streamB, streamC).
   ...> flatMap(Function.identity()).
   ...> forEach(System.out::println);
1
2
3
4
5
6
7
8
9

This method was my limit. If you find out something, I will add it (maybe). I mean, the period is unpleasant at the end. No, in natural language this is normal driving.

Useless of reduce! I hate it anymore!

As you can see, reducing it makes it optional and it's a hassle twice. YO

jshell> var streamA = Stream.of(1,2,3);
streamA ==> java.util.stream.ReferencePipeline$Head@2781e022

jshell> var streamB = Stream.of(4,5);
streamB ==> java.util.stream.ReferencePipeline$Head@57e1b0c

jshell> var streamC = Stream.of(6,7,8,9);
streamC ==> java.util.stream.ReferencePipeline$Head@4232c52b

jshell> Stream.of(streamA, streamB, streamC).
   ...> reduce(Stream::concat)
$38 ==> Optional[java.util.stream.ReferencePipeline$Head@ea1a8d5]

That's all for today. Tell yourself that it is important to continue little by little.

Postscript: Pointed out by swordone

"If you don't want reduce to be Optional, you can use a 3-argument reduce."

Stream.of(streamA, streamB, streamC).reduce(Stream.empty(), Stream::concat, Stream::concat);

You're right. No, but then reduce argument 2 is fine, right?

Stream.of(streamA, streamB, streamC).reduce(Stream.empty(), Stream::concat)

The story of reduce is Beckenbauer.