intellij icon over project files

Question

intellij has placed what look like no entry signs over my java files (red circles with a line through) searched around but cannot find why? Would anyone know please.

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Answer 1

For maven projects:

In my case I had to import the project through Maven. Some modules were missing (they weren't specified in the root pom) so adding those as maven projects fixed the issue.

click on the 'plus' icon on the right hand side maven menu and then add the project. This imported all sources / structure automatically.