Null pointer exception when using Mockito to mock interface

I’m using Mockito to test the following method:

  protected void produceProducedFieldValue(Target target) throws ValueProducerCreationException {
    String mappedValue = (String) valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get();
    target.setProducedField(mappedValue);
  }

Here is my test:

     @Test
    public void TestProducingProducedFieldValue() throws ValueProducerCreationException {

        Target mockedTarget = Mockito.mock(Target.class);
        ValueProducerFactory valueProducerFactory = Mockito.mock(ValueProducerFactory.class);
        List<Object> creationParameters = new ArrayList<>();

        String mappedValue= "SomeString";

        Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get()).thenReturn(mappedValue);
        SourceToTargetMapper sttm = new SourceToTargetMapper(targetModelObjectFactory,mappingOperatorFactory,mapperFactory,valueProducerFactory);
        sttm.produceProducedFieldValue(mockedTarget);
        Mockito.verify(mockedTarget, Mockito.times(1)).setProducedField(Mockito.any());
    }

But I keep getting NullPointerExeption for this line:

    Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get()).thenReturn(mappedValue);

ValueProducerFactory is an Interface

and the createValueProducer method signature is as follows:

    public interface ValueProducerFactory {

        <T> Supplier<T> createValueProducer(String producerName, List<Object> creationParameters) throws ValueProducerCreationException;

    }

I have a class named CachingValueProducerFactory that implements the interface

    @Override
    public <T> Supplier<T> createValueProducer(String producerName, List<Object> creationParameters) throws ValueProducerCreationException {
        Map<List<Object>, Supplier<?>> parametersToOperatorsMap = producersCache.computeIfAbsent(producerName, s -> new HashMap<>());

        List<Object> key = Optional.ofNullable(creationParameters).orElse(Collections.emptyList());

        if (!parametersToOperatorsMap.containsKey(key)) {
            parametersToOperatorsMap.put(key, nestedProducersFactory.createValueProducer(producerName, creationParameters));
        }

        return (Supplier<T>) parametersToOperatorsMap.get(key);
    }

It seems like using Collections.emptyList() in the test is the problem, but I don’t see any other solution for this.

Answer

You are trying to mock two methods at once. In this line:

Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get()).thenReturn(mappedValue);

you are calling get() on the result of the createValueProducer() call, which is null because you haven’t mocked it yet.

There are two solutions:

Use deep stubbing:

ValueProducerFactory valueProducerFactory = Mockito.mock(ValueProducerFactory.class, Mockito.RETURNS_DEEP_STUBS);
Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList()).get()).thenReturn(mappedValue);

Mock the Supplier first (pay attention to the removed get()):

Supplier<Object> supplier = () -> mappedValue;
ValueProducerFactory valueProducerFactory = Mockito.mock(ValueProducerFactory.class);
Mockito.when(valueProducerFactory.createValueProducer("IdProducer", Collections.emptyList())).thenReturn(supplier);