When IntelliJ IDEA 2017.3 creates a multi-Module project, there is a problem with multiple root modules in the right column. As shown.

 

I created a new project with three modules (Module) in it, and after it was built, three roots appeared. Then I found the pom file of the main module, which contains such a configuration

<modules>
    <module>design_pattern</module>
</modules>

I added two other modules with the attitude of trying it, and now there is only one root left.

Hey, my brother doesn't know the specific reason, but I solved a question anyway. Java engineers have a long way to go, they have to work hard.