MessageProcessor implements Processor<Message<T>, Message<T>>, Subscription { @Override public void subscribe(Subscriber<? super Message<T>> subscriber) { this.subscriber = new FirstErrorOnlySubscriber<>(subscriber); this.subscriber.onSubscribe(this); }  @Override void subscribe(Subscriber<? super Message<T>> subscriber); @Override void request(long n); @Override void cancel(); @Override void onNext(Message<T> t); @Override void onError(Throwable t); @Override void onSubscribe(Subscription s); @Override void onComplete();  }
@Test public void subscribe() throws Exception { underTest.subscribe(subscriber); verify(subscriber).onSubscribe(underTest); }