Question

I’ve written the following JUnit test, which uses ArgumentMatchers.

    MyClass classUnderTest = new MyClass();

    class AnyBooleanMatcher extends ArgumentMatcher<Boolean> {
        public boolean matches(Object argument) {
            return ((Boolean) argument).equals(Boolean.TRUE);
        }
    }

    class MyObjectMatcher extends ArgumentMatcher<MyObject> {
        public boolean matches(Object argument) {
            return ((MyObject) argument).getValue().equals("123");
        }
    }

    final Service mockService = mock(Service.class);

    when(mockService.search(Matchers.argThat(new MyObjectMatcher()),
            Matchers.argThat(new AnyBooleanMatcher())));

    classUnderTest.callMethod(mock(ActionEvent.class));

    verify(mockService).search(Matchers.argThat(new MyObjectMatcher()),
            Matchers.argThat(new AnyBooleanMatcher()));

Unfortunately I’m always getting a NullPointerException, at the when statement.

I even know why: argThat returns null, as specified in the JavaDoc. But I don’t know why my test won’t work, when the ones in the example – which are modelled in exactly the same way – do work.

Answer 1

You have a NullPointerException because the method signature is using a native type, and you are using Matchers.argThat, which returns an object.

When compiled Javac will add auto-unboxing code around Matchers.argThat(new AnyBooleanMatcher()), as you said it returns null, so you have the cause of the NPE.

I’m pretty sure the Javadoc of mockito also say to use intThat, booleanThat style method when dealing with native types. Using those will prevent the auto-unboxing code introduced by the compiler.

On another topic, I would recommand you to write code like that, in order to make it more readable.

when(mockService.search(argThat(hasValue("123")), booleanThat(isTrue()))).thenReturn("something");

// ...

private AnyBooleanMatcher isTrue() {
    return new AnyBooleanMatcher();
}

private MyObjectMatcher hasValue(String value) {
    return new MyObjectMatcher(value);
}