Stubbing a method that takes Class<T> as parameter with Mockito

The problem is, you cannot mix argument matchers and real arguments in a mocked call. So, rather do this:

when(serviceValidatorStub.validate(
    any(),
    isA(UserCommentRequestValidator.class),
    eq(UserCommentResponse.class),
    eq(UserCommentError.class))
).thenReturn(new UserCommentResponse());

Notice the use of the eq() argument matcher for matching equality.

see: https://static.javadoc.io/org.mockito/mockito-core/1.10.19/org/mockito/Matchers.html#eq(T)

Also, you could use the same() argument matcher for Class<?> types - this matches same identity, like the == Java operator.

Just in order to complete on the same thread, if someone want to stubb a method that takes a Class as argument, but don't care of the type, or need many type to be stubbed the same way, here is another solution:

private class AnyClassMatcher extends ArgumentMatcher<Class<?>> {

    @Override
    public boolean matches(final Object argument) {
        // We always return true, because we want to acknowledge all class types
        return true;
    }

}

private Class<?> anyClass() {
    return Mockito.argThat(new AnyClassMatcher());
}

and then call

Mockito.when(mock.doIt(this.anyClass())).thenCallRealMethod();

Nice one @Ash. I used your generic class matcher to prepare below. This can be used if we want to prepare mock of a specific Type.(not instance)

private Class<StreamSource> streamSourceClass() {
    return Mockito.argThat(new ArgumentMatcher<Class<StreamSource>>() {

        @Override
        public boolean matches(Object argument) {
            // TODO Auto-generated method stub
            return false;
        }
    });
}

Usage:

    Mockito.when(restTemplate.getForObject(Mockito.anyString(), 
            **streamSourceClass(),**
            Mockito.anyObject));