Can't choose class as main class in IntelliJ

Question

I have a Java project in IntelliJ to which I just added a bunch of files in a nested folder hierarchy. Many of these files are tests and include the main method, so I should be

Answer 1

Here is the complete procedure for IDEA IntelliJ 2019.3:

1. File > Project Structure

2. Under Project Settings > Modules

3. Under 'Sources' tab, right-click on 'src' folder and select 'Sources'.

4. Apply changes.