table of Contents
Description
Version: Android Studio 3.5.3
common problem
A red J circular symbol appears in the lower left corner of the Java file.
Note: In IDEA, this small red mark means Java class located out of the source root (machine translation: java class located out of the source root). This means that the local resource file import failed, Android studio cannot compile this Module, and the java file cannot become a class file.
Solution: Re-import the module with the problem.
First check whether there is a problem Module in setting.gradle (located in gradle scripts),
and then
Download https://services.gradle.org/distributions/gradle-xxx-all.zip download stuck
Solution: Close Android Studio and go to the website services.gradle.org/distributions/ to download the required files using Thunder. Then open the local folder to be downloaded to, delete the files inside (there are only two files that fail to download), 
copy the downloaded compressed package into it, and restart AndroidStudio.