How to mock List<Interface> while using @Mock and @InjectMock Let's say you have a Service class which is holding, List of Handler. e.g. @Component @RequiredArgsConstructor public class Service { private final List<Handler> handlerList ; Now, it's turn to write the test cases, using Junit5, and Mockito, you may be trying to write like this. @ExtendWith (MockitoExtension. class ) class ServiceTest { @InjectMocks Service service ; @Mock List<Handler> handlers ; But mockito doesn't know anything about initialising the List. So putting @Mock is not going to work. There are 2 situations :- Scenario 1. You have setter method in the Service, for List<Handler> handlers;( assuming you haven't defined the List<Handler> as final) in that case you can approach like this :- Define all the Handler implemented(which you want use in test) class and annotate with @Mock. Define instance of Service class(which you are going to test)