I am trying to understanding how the generics work in case of Mockito.

I am trying to mock the RestTemplate, however the following code does not compile and I don't know why:

var expectedResponse = createServiceResponse();

when(restTemplate.exchange(anyString(), any(HttpMethod.class),
        Mockito.<HttpEntity<Object>>any(),
        Mockito.<ServiceResponse>any().getClass(), anyMap()))
        .thenReturn(ResponseEntity.ok(expectedResponse));

The compiler keeps giving me the error message:

Cannot resolve method 'thenReturn(org.springframework.http.ResponseEntity<T>)

But when I switch Mockito.<ServiceResponse>any().getClass() with Mockito.<Class<ServiceResponse>>any() the compilation succeeds.

Why is this happening ?

What is the difference between Mockito.<ServiceResponse>any().getClass() and Mockito.<Class<ServiceResponse>>any() ?

They should be the same thing right ?

I mean the class of ServiceResponse should be Class<ServiceResponse>. Right ?

I don't have a good grasp of advanced generics so I can't understand what the compiler is doing.

Please do add some description in the answer so that I can understand why this is working the way it is suppose to.

CodePudding user response：

Only in these cases, where the compiler cannot garantuee the returned type of the method (here restTemplate.exchange) we can use the non-type-safe equivalent syntax:

Mockito.doReturn(ResponseEntity.ok(expectedResponse))
    .when(restTemplate)
    .exchange(anyString(), any(HttpMethod.class),
        Mockito.<HttpEntity<Object>>any(),
        Mockito.<ServiceResponse>any().getClass(), anyMap()));

CodePudding user response：

What is the difference between Mockito.<ServiceResponse>any().getClass() and Mockito.<Class<ServiceResponse>>any()?

Mockito.<ServiceResponse>any() is an instance of ServiceResponse. And calling getClass() from any instance return Class<?>. getClass() is created to get the actual class of the instance, usually for runtime comparison.

On the other hand, Mockito.<Class<ServiceResponse>>any() is an instance of Class<ServiceResponse>

I mean the class of ServiceResponse should be Class. Right ?

Not really, consider following code

    private void dummy(ServiceResponse response) {
        System.out.println(response.getClass().equals(ServiceResponse.class));
    }

You can't guarantee the output is always true, as response can be of type CustomServiceResponse(extending ServiceResponse)

For your problem, just use ServiceResponse.class, simple and clear.

Further reading:
What is the difference between a.getClass() and A.class in Java? Why does a wildcard on a generic parameter require an explicit cast?
Java: getClass() of bounded type