Ide-java errors in a default Eclipse "Hello Horld" project



I am new to Java and only learning the basics of the language. I wanted to use Atom with atom-ide-ui and ide-java packages for writing my code, while compiling and running it in console. At first I stumbled upon the “Classpath is incomplete” warning which suggests

You need to open a folder containing a pom.xml, build.gradle or at least default eclipse setting files, so that a complete classpath and project hierarchy can be set.

After a quick google search I decided that POM files would be too big of a topic to get into, so instead I installed Eclipse with java-oxygen and created a “Hello World” program with all defaults. After opening it in Atom, this is what I got:

It does not complain about classpath any more, but now there are 5 errors on 7 lines of code. The application itself compiles and runs fine both in Eclipse and with javac/java (obviously).

So why is Atom unhappy with the code and how should I go about fixing that?
Thank you kindly C:

[xubuntu 17.10; openjdk-8 and openjdk-9 installed; Java home path in ide-java settings is set to /usr/lib/jvm/java-1.8.0-openjdk-amd64]