How to set up BungeeCord source code using Intellij IDEA?

I'm very interesting in BungeeCord and want to contribute code for it . Before this, I use Intellij IDEA to do development of my Android project and haven't used maven as the build tool.
So after I import the source code into Intellij IDEA as maven project, compile errors reported everywhere. But
all of "import code" do not show errors.

Anyone can help me to set up BungeeCord correctly?