Cannot resolve symbol InstantTaskExecutorRule

Replace testImplementation by androidTestImplementation. So the tests on folder androidTest can have access to the library.

dependencies {    androidTestImplementation "androidx.arch.core:core-testing:2.1.0"}

If you are not using androidx then use android.arch.core:core-testing:1.1.1

I know it's late but I would like to add one thing to this accepted answer.

If you want to use,

@Rulepublic InstantTaskExecutorRule instantTaskExecutorRule = new InstantTaskExecutorRule();

in your JUnit test case, i.e., in test folder then use following dependency, i.e, with testImplementation

dependencies {testImplementation "android.arch.core:core-testing:1.0.0"}

If you want to use InstantTaskExecutorRule for your UI or integration test cases(androidTest folder), use androidTestImplementation. that is:

androidTestImplementation "android.arch.core:core-testing:1.0.0"

And if you want to add for both, use androidTestImplementation & testImplementation that is:

androidTestImplementation "android.arch.core:core-testing:1.0.0"

testImplementation "android.arch.core:core-testing:1.0.0"

For Android-X use below dependency:

androidTestImplementation 'androidx.arch.core:core-testing:2.0.0'

OR

testImplementation 'androidx.arch.core:core-testing:2.0.0'

for androidX migration, add

androidTestImplementation "androidx.arch.core:core-testing:2.0.0"