Mocking Method with Wildcard Generic Return Type

[Ivan Zerin provides a primer on mocking methods that contain generic return types.]

There are several cases where construction when().thenReturn is not suitable during Unit tests. One of then generic return type. I have faced such issue, let's take a look. There is a method in interface declaration:

public interface FooInterface {    
    public Iterable<? extends SomeClass> getList();
    ...
}

Implementation looks like this:

public class Foo implements FooInterface {

    public List<SomeChildClass> getList() {
        ...
    }

    ...
}

At first mocking of such method should not be something unusual:

public class UnitTest {
    @Mock private FooInterface mockFoo;

    @Test
    public void someUnitTest() {
        ...
        List<SomeChildClass> testList = generateTestList();
        when(mockFoo.getList())
          .thenReturn(testList);
    }
}

But this code won't compile with error: Cannot resolve method 'thenReturn(List<SomeChildClass>). Seems to be some mistake, cause returned type are correct from the interface point of view.

This error happening cause compiler can not gurantee that returned type of method getList() will be List<SomeChildClass>. Actually return type of method getList() in this case is Iterable<? extends SomeClass> and this means "Return some Iterable object with any objects that extends SomeClass". Let's rename this type as 'X'. So when(mockFoo.getList()) will create object OngoingStubbing<Iterable<X>> and it has method thenReturn(Iterable<X>). Compiler can not tell what type X before runtime and cannot perform safe cast from List<SomeChildClass> to Iterable<X> (we perfrom actuall call of method thenReturn(List<SomeChildClass>)).

Sounds a little tricky but let's assume that SomeClass is standard Java class Number. Then classes Integer and Double are both fulfill the criteria of List<? extends Number>. Let's say that as return type of mock we will use List<Double>, in this case, compiler should be ready that actual work with code during runtime will be performed with List<Integer> too, but it is clearly that cast from Double to Integer will be incorrect (try to cast double value 12.6 to int).

You could argue that in the case of mockito compiler won't need to bother about the casting of returned type because call of original method would not produce anything, but it is known by mockito. From compiler point of view, it is only Java code, that should be checked for safety and correctness before compiling.

Ok, so how we should deal with such cases in mockito?

Use doReturn() method:

doReturn(testList).when(mockFoo).getList();

Such expression is not type safe, so it were designed for exceprional cases, so use it only when you cannot use standart when().thenReturn(), which is the type-safe, elegant and more readable syntax.