Calling an overloaded java generic method from scala

Question

I'm using Mockito to mock an object with a method which returns an un-parametrized ArrayList, and I cannot figure out how to get this to work

Method signature to mock

public java.util.ArrayList getX()

Test code

var mockee = mock(classOf[Mockee])
when(mockee.getX).thenReturn(Lists.newArrayList(x): ArrayList[_])

This actually compiles fine in IntelliJ, but at runtime throws:

[error] ....scala:89: overloaded method value thenReturn with alternatives:
[error]   (java.util.ArrayList[?0],<repeated...>[java.util.ArrayList[?0]])org.mockito.stubbing.OngoingStubbing[java.util.ArrayList[?0]] <and>
[error]   (java.util.ArrayList[?0])org.mockito.stubbing.OngoingStubbing[java.util.ArrayList[?0]]
[error]  cannot be applied to (java.util.ArrayList[_$1])
[error]       when(mockee.getX).thenReturn(Lists.newArrayList(x): ArrayList[_])

I may be wrong, but I thought ArrayList[_] in Scala is not the same as an unparameterized ArrayList, it is an existential type -- ie it has a type parameter, but it's bound at a funny place. I think the unparameterized type would be ArrayList[AnyRef] ie parameterized by java.lang.Object. — Owen
– Owen, Commented Sep 9, 2011 at 17:33
This looks like a compile-time error, not a run-time error. Scala's existential type ArrayList[_] is its closest approximation to Java's wildcard type ArrayList<?>. You actually have a raw type, ArrayList. Two SO questions to look into: Scala's existential types and Raw types and wildcard... — Kipton Barros
– Kipton Barros, Commented Sep 9, 2011 at 19:48
You're right it is a compile time error, but not highlighted by the scala plugin. I can work around this by using thenAnswer(new Answer[ArrayList[_]]{ def answer(arg: InvocationOnMock) = Lists.newArrayList(x)}) but that's not very nice — Jon Freedman
– Jon Freedman, Commented Sep 26, 2011 at 17:26

user500592 · Accepted Answer · 2011-10-11 15:24:04Z

8

The following works for me:

val mockee = mock(classOf[Mockee])
when[ArrayList[_]](mockee.getX).thenReturn(Lists.newArrayList)

assuming the "Lists" class is from the Google collections (now Guava).

answered Oct 11, 2011 at 15:24

user500592

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1 Comment

Jon Freedman Over a year ago

Perfect - I never thought to add the method parameter

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