How can we implement Flow API using publish-subscribe pattern in Java 9?

Flow API (java.util.concurrent.Flow) was introduced in Java 9. It helps to understand the different ways in which the Publisher and SSubscriber interfaces interact to perform the required actions.

Flow API consists of publisher, subscriber, subscription and processor interfaces, which can be based on reactive flow specifications.

In the following example, we can use the publisher-subscriber interface to implement the Flow API.

Example

import java.util.concurrent.Flow.Publisher;
import java.util.concurrent.Flow.Subscriber;
import java.util.concurrent.Flow.Subscription;

public class FlowAPITest {
   public static void main(String args[]) {
      <strong>Publisher<Integer></strong> publisherSync = new <strong>Publisher<Integer></strong>() {   <strong>// Create publisher</strong>
         <strong>@Override</strong>
         public void <strong>subscribe</strong>(Subscriber<? super Integer><!--? super Integer--> subscriber) {
            for(int i = 0; i < 10; i++) {
               System.out.println(Thread.currentThread().getName() + " | Publishing = " + i);
               subscriber.<strong>onNext</strong>(i);
            }
            subscriber.<strong>onComplete</strong>();
         }
      };
      <strong>Subscriber<Integer></strong> subscriberSync = new <strong>Subscriber<Integer></strong>() {   <strong>// Create subscriber</strong>
         <strong>@Override</strong>
         public void <strong>onSubscribe</strong>(Subscription subscription) {
         }
         <strong>@Override</strong>
         public void <strong>onNext</strong>(Integer item) {
            System.out.println(Thread.currentThread().getName() + " | Received = " + item);
            try {
               Thread.sleep(100);
            } catch(InterruptedException e) {
               e.printStackTrace();
            }
         }
         <strong>@Override</strong>
         public void <strong>onError</strong>(Throwable throwable) {
         }
         <strong>@Override</strong>
         public void <strong>onComplete()</strong> {
         }
      };
      publisherSync.<strong>subscribe</strong>(subscriberSync);
   }
}

Output

<strong>main | Publishing = 0
main | Received = 0
main | Publishing = 1
main | Received = 1
main | Publishing = 2
main | Received = 2
main | Publishing = 3
main | Received = 3
main | Publishing = 4
main | Received = 4
main | Publishing = 5
main | Received = 5
main | Publishing = 6
main | Received = 6
main | Publishing = 7
main | Received = 7
main | Publishing = 8
main | Received = 8
main | Publishing = 9
main | Received = 9</strong>

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