This show how to test Event Handlers with JUnit & Mockito.

EventHandlerTest.java

import org.junit.Test;
import org.mockito.ArgumentCaptor;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;

class PizzaJava {

    private final String name;

    public PizzaJava(String name) {
        this.name = name;
    }

    public String getName() {

        return name;
    }
}

class PizzaServiceJavaAgain {

    public interface PizzaEventJava {
        void pizzaAvailable(PizzaJava pizza);
    }

    PizzaEventJava listener = null;

    public void orderPizza( String name) {
        if(listener != null) {
            listener.pizzaAvailable(new PizzaJava(name));
        }
    }
}

public class EventHandlerTest {

    @Test
    public void testEvent() {

        String pizzaName = "MargaritaJava";
        PizzaServiceJavaAgain service = new PizzaServiceJavaAgain();
        PizzaServiceJavaAgain.PizzaEventJava event = mock( PizzaServiceJavaAgain.PizzaEventJava.class );
        service.listener = event;

        service.orderPizza(pizzaName);

        ArgumentCaptor<PizzaJava> captor = ArgumentCaptor.forClass( PizzaJava.class );
        verify( event ).pizzaAvailable( captor.capture() );
        PizzaJava capturedPizza = captor.getValue();
        assertEquals( pizzaName, capturedPizza.getName() );

    }

}

//-------------------------------------------------------------------------------------------------------------

{in pom.xml}

    <dependencies>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-core</artifactId>
            <version>2.7.13</version>
            <scope>test</scope>
        </dependency>

    </dependencies>