Using any<String>() with a spy throws IllegalArgumentException

any<String>() returns null which works with mocks but fails with spies. A simple test to reproduce.

    @Test
    fun passAnyStringToSpy() {
        val my = spy(MyClass())
        whenever(my.foo(any())).thenReturn("mocked") // this throws java.lang.IllegalArgumentException: Parameter specified as non-null is null
        expect(my.foo("hello")).toBe("mocked") 
    }

    private interface MyInterface {
        fun foo(value: String): String
    }

    private open class MyClass : MyInterface {
        override fun foo(value: String) = value
    }

It seems that making any<String>() return "" solves the problem, but one of the current tests fails: stubbingTwiceWithCheckArgumentMatchers_throwsException

[nhaarman]

Due to any() returning null and the method foo being executed with it, the IllegalArgumentException is thrown.

To prevent null from being passed to the actual code, you can use the alternative syntax as such:

    @Test
    fun passAnyStringToSpy() {
        /* Given */
        val my = spy(MyClass())

        /* When */
        doReturn("mocked").whenever(my).foo(any())

        /* Then */
        expect(my.foo("hello")).toBe("mocked")
    }

    private interface MyInterface {

        fun foo(value: String): String
    }

    private open class MyClass : MyInterface {

        override fun foo(value: String): String = value
    }

Since actually fixing this requires any() to not return null which brings a lot of other problems, I propose to close this issue as resolved.

[jarrodmoldrich]

It would be nice to have this in the wiki. I spent a few hours trying to figure out why the mock syntax didn’t work for spies 😞 I could amend the wiki, and submit a PR if that helps, @nhaarman ?