How can I unit test void functions?

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Solution with Mockito Spy

import org.junit.Test;

import static org.mockito.Mockito.*;

public class ElephantTest {

    @Test
    public void shouldMakeNoise() throws Exception {

        //given
        Elephant elephant = spy(new Elephant("foo"));

        //when
        elephant.perform("friday");

        //then
        verify(elephant).makeNoise();

    }
}

Negative tests:

@Test
public void elephantShouldDontMakeNoisesOnMonday() {

    //given
    Elephant elephant = spy(new Elephant("foo"));

    //when
    elephant.perform("monday");

    //then
    verify(elephant, never()).makeNoise();

}

or

@Test
public void shouldDoNotMakeNoisesOnMonday() {

    //given
    Elephant elephant = spy(new Elephant("foo"));

    //when
    elephant.perform("monday");

    then(elephant).should(never()).makeNoise();

}

Dependency

org.mockito:mockito-core:2.21.0

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