Usages of doThrow() doAnswer() doNothing() and doReturn() in mockito

doThrow : Basically used when you want to throw an exception when a method is being called within a mock object.

public void validateEntity(final Object object){}
Mockito.doThrow(IllegalArgumentException.class)
.when(validationService).validateEntity(Matchers.any(AnyObjectClass.class));

doReturn : Used when you want to send back a return value when a method is executed.

public Socket getCosmosSocket() throws IOException {}
Mockito.doReturn(cosmosSocket).when(cosmosServiceImpl).getCosmosSocket();

doAnswer: Sometimes you need to do some actions with the arguments that are passed to the method, for example, add some values, make some calculations or even modify them doAnswer gives you the Answer<?> interface that being executed in the moment that method is called, this interface allows you to interact with the parameters via the InvocationOnMock argument. Also, the return value of answer method will be the return value of the mocked method.

public ReturnValueObject quickChange(Object1 object);
Mockito.doAnswer(new Answer<ReturnValueObject>() {

        @Override
        public ReturnValueObject answer(final InvocationOnMock invocation) throws Throwable {

            final Object1 originalArgument = (invocation.getArguments())[0];
            final ReturnValueObject returnedValue = new ReturnValueObject();
            returnedValue.setCost(new Cost());

            return returnedValue ;
        }
}).when(priceChangeRequestService).quickCharge(Matchers.any(Object1.class));

doNothing: (From documentation) Use doNothing() for setting void methods to do nothing. Beware that void methods on mocks do nothing by default! However, there are rare situations when doNothing() comes handy:

• Stubbing consecutive calls on a void method:

  doNothing().
  doThrow(new RuntimeException())
  .when(mock).someVoidMethod();
  
  //does nothing the first time:
  mock.someVoidMethod();
  
  //throws RuntimeException the next time:
  mock.someVoidMethod();
  

• When you spy real objects and you want the void method to do nothing:

  List list = new LinkedList();
  List spy = spy(list);
  
  //let's make clear() do nothing
  doNothing().when(spy).clear();
  
  spy.add("one");
  
  //clear() does nothing, so the list still contains "one"
  spy.clear();
  

It depends on the kind of test double you want to interact with:

• If you don't use doNothing and you mock an object, the real method is not called
• If you don't use doNothing and you spy an object, the real method is called

In other words, with mocking the only useful interactions with a collaborator are the ones that you provide. By default functions will return null, void methods do nothing.

To add a bit to accepted answer ...

If you get an UnfinishedStubbingException, be sure to set the method to be stubbed after the when closure, which is different than when you write Mockito.when

Mockito.doNothing().when(mock).method()    //method is declared after 'when' closes

Mockito.when(mock.method()).thenReturn(something)   //method is declared inside 'when'