tests mockito using verification methods

MockitoTestVerifyCalling.java

package org.example.observer;

import org.junit.jupiter.api.Test;
import org.mockito.Mockito;

class ConcreteSubjectTest {

  @Test
  void observersHandleEventsFromSubject() {
    // given
    ConcreteSubject subject = new ConcreteSubject();
    ConcreteObserverA observerA = Mockito.spy(new ConcreteObserverA());
    ConcreteObserverB observerB = Mockito.spy(new ConcreteObserverB());

    // when
    subject.notifyObservers("No listeners");

    subject.registerObserver(observerA);
    subject.notifyObservers("Message for A");

    subject.registerObserver(observerB);
    subject.notifyObservers("Message for A & B");

    subject.unregisterObserver(observerA);
    subject.notifyObservers("Message for B");

    subject.unregisterObserver(observerB);
    subject.notifyObservers("No Listeners");

    // then
    Mockito.verify(observerA, Mockito.times(1)).observe("Message for A");
    Mockito.verify(observerA, Mockito.times(1)).observe("Message for A & B");
    Mockito.verifyNoMoreInteractions(observerA);

    Mockito.verify(observerB, Mockito.times(1)).observe("Message for A & B");
    Mockito.verify(observerB, Mockito.times(1)).observe("Message for B");
    Mockito.verifyNoMoreInteractions(observerB);
  }

  @Test
  void subjectLeveragesLambdas() {
    Subject<String> subject = new ConcreteSubject();

    subject.registerObserver(e -> System.out.println("A: " + e));
    subject.registerObserver(e -> System.out.println("B: " + e));
    subject.notifyObservers("This message will receive A & B");
  }
}