Flow API – Subscriber

As its name says, the Subscriber is a component that works as a consumer of data. This means it consumes the data from a producer. It acts as a destination of data. So, it is also known as a consumer or destination of data:

In the Java 9 Flow API, this Subscriber is an interface with a set of methods and is defined as follows:

public static interface Subscriber<T> {   
  public void    onSubscribe(Subscription subscription);   
  public void    onNext(T item);   
  public void    onError(Throwable throwable);   
  public void    onComplete();   
}       

It has a set of methods:

• onSubscribe(): This creates a new subscription. It is invoked prior to invoking any other Subscriber methods for the given Subscription.
• onNext(): Once a Subscription is created, this is invoked to receive the next data, item, or element from the Publisher.
• onError(): This is invoked upon an unrecoverable error encountered by a Publisher or Subscription, after which no other Subscriber methods are invoked by the Subscription.
• onComplete(): This is invoked when there is no requirement to invoke any further Subscriber methods on that Subscription that is not already terminated in error, after which no other Subscriber methods are invoked by that Subscription.

It is also defined within another class as a static component. We will see it in the next section.