Jon Erickson Jon Erickson - 1 month ago 12
Java Question

How to mock generic method in Java with Mockito?

How can we mock the

IRouteHandlerRegistry
? The error is
Cannot resolve method thenReturn(IHandleRoute<TestRoute>)


public interface RouteDefinition { }

public class TestRoute implements RouteDefinition { }

public interface IHandleRoute<TRoute extends RouteDefinition> {
Route getHandlerFor(TRoute route);
}

public interface IRouteHandlerRegistry {
<TRoute extends RouteDefinition> IHandleRoute<TRoute> getHandlerFor(TRoute route);
}

@Test
@SuppressWarnings("unchecked")
public void test() {
// in my test
RouteDefinition route = new TestRoute(); // TestRoute implements RouteDefinition
IRouteHandlerRegistry registry = mock(IRouteHandlerRegistry.class);
IHandleRoute<TestRoute> handler = mock(IHandleRoute.class);

// Error: Cannot resolve method 'thenReturn(IHandleRoute<TestRoute>)'
when(registry.getHandlerFor(route)).thenReturn(handler);
}

Answer

Even though TestRoute is a subtype of RouteDefinition, a IHandleRoute<TestRoute> is not a subtype of IHandleRoute<RouteDefinition>.

The when method from Mockito returns an object of type OngoingStubbing<IHandleRoute<RouteDefinition>>. This is due to the compiler inferring the type parameter TRoute from the method

<TRoute extends RouteDefinition> IHandleRoute<TRoute> getHandlerFor(TRoute route);

to be RouteDefinition since the argument passed to getHandlerFor is declared of type RouteDefinition.

On the other hand, the thenReturn method is given an argument of type IHandleRoute<TestRoute> whereas it expects an IHandleRoute<RouteDefinition>, that is the type argument of the OngoingStubbing mentioned earlier. Hence the compiler error.

To solve this, the simplest way is probably to change the declaration type of route to be TestRoute:

TestRoute route = new TestRoute();

IRouteHandlerRegistry registry = mock(IRouteHandlerRegistry.class);
IHandleRoute<TestRoute> handler = mock(IHandleRoute.class);

when(registry.getHandlerFor(route)).thenReturn(handler);