How can we implement Flow API using Publisher-Subscriber in Java 9?

Flow API (java.util.concurrent.Flow) has introduced in Java 9. It helps to understand different ways in which the Publisher and Subscriber interfaces interact to perform desired operations.

Flow API consists of Publisher, Subscriber, Subscription, and Processor interfaces, which can be based on reactive stream specification.

In the below example, we can implement Flow API by using Publisher-Subscriber interfaces.

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

Output

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