What does this symbol mean in IntelliJ? (red circle on bottom-left corner of file name, with 'J' in it)

You need to specify the source dir

File> Project Structure > Modules

click the directory and click the Sources button

It's weird because usually it's done automatically. Maybe it's better if you recreate the project again.

If it is a maven project

right click on the pom.xml Add As Maven Project

Thanks

The first answer from irreputable above that starts out with "you need to specify the source dir" is correct, but I don't see him telling you the easy way to do so.

Simply right click on the java sources folder ("java" under src/main/java if it is a Maven project for example) and select Mark Directory As > Sources Root (see screenshot below).

https://i.stack.imgur.com/clMWS.png

This situation happens when the IDE looks for src folder, and it cannot find it in the path. Select the project root (F4 in windows) > Go to Modules on Side Tab > Select Sources > Select appropriate folder with source files in it> Click on the blue sources folder icon (for adding sources) > Click on Green Test Sources folder ( to add Unit test folders).

Find all IntelliJ (v15) symbols over here: https://www.jetbrains.com/idea/help/symbols.html

This site states that this icon stands for "Java class located out of the source root. Refer to the section Configuring Content Roots for details."

First mark the directory as the source root. Then go to the pom excel and right click on it. select add as maven project .

Another option if you're using Flavors in Android Studio:

Click Build -> Select Build Variant.

In the list click the variant you're working in and it will turn green and the others will have the red J.

Press F4 at the project root and select the the root source pressing the source button. It resolved my problem!

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As mentioned earlier,

right click on the pom.xml

https://i.stack.imgur.com/PJIhM.png

Add As Maven Project

https://i.stack.imgur.com/i1PmQ.png

I downloaded a project from github with .idea folder. After deleting that folder, everything was ok.

Actually this can happens because of two reason.

Your project not getting/ Updating your dependencies. Go to your terminal and enter mvn clean install. Or right click on pom.xml and click Add as Mevan Project. Check your jdk has set properly to the project.

This was killing me as I had the same issue. I just going over old projects. Really redoing old lessons from class to get more practice. The repos just have compatibility issues. So I import the project. Then add the source like others state. Then close the project. Delete the .idea directory in the root and re-import project.

It means those files aren't part of the project settings.

mark the java folder as source root.It will solve.

Maybe your project has not been as android project by android studio, please make sure that plugin 'android support' has been enabled（Android Studio Preferences -> Plugins -> Select Android Support）

Close Android Studio and delete .idea and .gradle folder from project structure and start Studio again.

It means your Java source files aren't part of the project.

If the suggestions mentioned here don't resolve the issue, you may have hit a rare bug like I did. Researching the exceptions found in the log helped me. In my case, disabling the "Plugin DevKit", deleting the .idea directory, and reimporting the project worked.

This is what worked for me

Press File from the toolbar Press Sync Project with Gradle Files

For a gradle project "./gradlew --stop" will help to resolve the problem when the source directory is correctly specified and project is properly synced.