How to mock generic method in Java with Mockito?

Question

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); }

Can you explain what about this doesn't work? What is the error/warning that you get? — Andy Turner
– Andy Turner, Commented Jul 24, 2015 at 23:48

M A · Accepted Answer · 2015-07-27 20:09:17Z

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);

thank you, this helps me with my understanding of java generics as well

Fariba · Accepted Answer · 2015-07-25 03:10:46Z

If you write like this it will be ok:

Mockito.doReturn(handler).when(registry).getHandlerFor(Mockito.any(route.class))

TheCodingFrog · Accepted Answer · 2015-07-26 13:43:42Z

1

Mockito.when argument should be a method not a mock.

The correct statement is: when(registry.getHandlerFor (route)).thenReturn(handler)

edited Jul 26, 2015 at 13:43

TheCodingFrog

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answered Jul 26, 2015 at 13:20

Gualtiero Testa

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1 Comment

Jon Erickson Over a year ago

You are correct, sorry I made an edit using my phone and didn't edit it correctly... this wasn't my issue though.

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