The @Rule'mActivityRule' must be public. If you convert the Espresso test to Kotlin, an error will occur.

Thing you want to do

Kotlinization of Espresso test

Executed Convert Java File To Kotlin File

trouble

When I run the test I get the following exception

org.junit.internal.runners.rules.ValidationError: The @Rule 'mActivityRule' must be public.

Cause

This is because the properties declared in Kotlin have only getters and setters when viewed from the Java side, and cannot be accessed as public fields.

Solution

If you want to access the field of the class written in Kotlin from Java, add @JvmField annotation as follows

@Rule @JvmField
var mActivityRule = ActivityTestRule(HogeActivity::class.java)

At the end

Since I don't know Kotlin's language specifications, this happens, so I need to learn the basics. I hope it will be helpful for you.

Helpful site

https://kotlinlang.org/docs/reference/java-to-kotlin-interop.html https://proandroiddev.com/fix-kotlin-and-new-activitytestrule-the-rule-must-be-public-f0c5c583a865