How to capture a list of specific type with mockito

java

mockito

argument-captor

test-verification

byAlex Kataev·Oct 23, 2024

In Mockito, to capture a list of a specific type, use the ArgumentCaptor class with a wildcard ?. Here's an example with List<String>:

ArgumentCaptor<List<?>> argumentCaptor = ArgumentCaptor.forClass(List.class);
verify(mockedList).someMethod(argumentCaptor.capture());

List<?> captured = argumentCaptor.getValue();
assertTrue(captured.stream().allMatch(item -> item instanceof String));

Assertions employ the instanceof to guarantee the list comprises solely String types due to type erasure.

If you're confident about the list's uniform type (bold assumption), suppress warnings to avoid the pesky linting monster:

@SuppressWarnings("unchecked")
ArgumentCaptor<List<String>> captor = (ArgumentCaptor<List<String>>) (Object)ArgumentCaptor.forClass(List.class);

The "Catching" Setup

Kickstart the captor with either the @Captor annotation along with MockitoAnnotations.initMocks(this) in your setup method, or by sparking an instance of ArgumentCaptor<ArrayList<SomeType>>.

@Captor
ArgumentCaptor<List<SomeType>> captor;

@Before
public void init() {
    MockitoAnnotations.initMocks(this);
}

Just like catching Pokémon, capture the list in your test method verification phase:

verify(service).performAction(captor.capture());
List<SomeType> capturedList = captor.getValue();
assertThat(capturedList, hasItem(someExpectedType));

For our nerdy satisfaction, let's verify behaviors and analyze each element of the list.

Dodging Generics' Grenades

Type erasure in Java generics can throw off your Mockito game:

Type token: A parameterized type like ArrayList<SomeType> doesn't have a Class object, hence casting is your savior.
Introspection: Brace yourself to inspect individual list items to spot their types if ArgumentCaptor falls short.
Nested generics: Are the bane of readable tests. Use the @Captor annotation to simplify and demystify.