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Previously, I posted Void methods and Mockito in Java that verifies if any of the methods of a object passed as a argument were invoked.

This time we want to alter the behavior of a method that returns void.

Mock to alter a Method Behavior

Consider this code snippet:

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public class Animal {
    private boolean isFriend;
    
    public void beFriend(Animal anotherAnimal) {
        anotherAnimal.isFriend = true;
    }
}

public class Animal {

private boolean isFriend;

public void beFriend(Animal anotherAnimal) {

anotherAnimal.isFriend = true;

}

How are we to mock this using Mockito? We want to test for a scenario where

anotherAnimal.isFriend = false;

1	anotherAnimal.isFriend = false;

Introducing Mockito.doAnswer

doAnswer allows us to change something within a method particularly objects passed as arguments.

@Test
public void testUnfriendly() {
    Animal dog = Mockito.mock(Animal.class);
    Mockito.doAnswer(new Answer<Animal>() {

        @Override
        public Animal answer(InvocationOnMock invocation) throws Throwable {
            Object[] args = invocation.getArguments();
            Animal animal = (Animal) args[0];

            // We changed the behavior
            animal.isFriend = false;
            return null;
        }
    }).when(dog).beFriend(Matchers.<Animal> any());
}