Failed resolution of: Lcom/throrinstudio/android/example/validator/R$string;

[XinwenCheng]

When I create an instance of any type of Validator, for example, NotEmptyValidator, there always be an error with detailedMessage "Failed resolution of: Lcom/throrinstudio/android/example/validator/R$string;".

My source code is following:
NotEmptyValidator notEmptyValidator = new NotEmptyValidator(fragment.getContext());

And the cause info of this error is:
java.lang.ClassNotFoundException: Didn't find class "com.throrinstudio.android.example.validator.R$string" on path: DexPathList[[zip file "/data/app/com.fourthdimension.gaodingv1android-2/base.apk"],nativeLibraryDirectories=[/vendor/lib64, /system/lib64]]