java - Run a Maven Project using IntelliJ IDEA - Stack Overflow
IntelliJ - Convert a Java project/module into a Maven project/module - Stack Overflow
Document says to build the project using maven command: mvn clean install (how do I do this?)
I want to create a new project, and simply import the work from a previous project into it, but I cannot figure how to do it in IntelliJ IDEA...
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Refresh the Maven project in the Maven Projects tool window (Reimport All Maven Projects), if it doesn't help, refer to this answer for diagnostics.

The issue is that your source roots were not configured correctly from the Maven model for some reason and the .java file appears in a plain directory instead of the Source root.
You can tell that by the color of the folders and by the icon of the file.
You need to create a Java Project.
File->New->Project->Java->Next->Next->Finish
Then go to the top and do:
Right click parent folder of project->Add Framework Support->Click Maven->Ok
Now Maven should be added to your project and also you should be able to run it
If this doesn't allow you to run the project, click the little drop down box to the left of the grayed out run button and click Edit Configurations.
Then click the + and click Application and set your Main class to the class that has the main method in it, and then it should allow you to run the application.
Hope that helps!
Right-click on the module, select "Add framework support...", and check the "Maven" technology.


(This also creates a pom.xml for you to modify.)
If you mean adding source repository elements, I think you need to do that manuallyβnot sure.
Pre-IntelliJ 13 this won't convert the project to the Maven Standard Directory Layout, 13+ it will.
A visual for those that benefit from it.

After right-clicking the project name ("test" in this example), select "Add framework support" and check the "Maven" option.
Where do I build the project using the maven command? I'm assuming this is the terminal bottom tab? But it's not in a build section.. hmm. Any help is appreciated, thank you!