IDEA run a sampe code and get the issue: java: illegal character：'#'

Question

IDEA version： 2021.3.2

JAVA version: 1.8.0

The encoding format is UTF-8

I am a beginner in java, I try to install IEDA and run a sample code to output "HelloWorld", then get the error java: illegal character：'#' and other errors

What I tried:

1. Check the env path, right.

2. Create a txt file-> enter test code-> change the file type to .java, then run the cmd javac test.java and java xxx, it output HelloWorld

3. Change the encoding format to GBK and then back to UTF-8

4. Add -encoding UTF8 in IDEA settings

update1

public class test{
    public static void main(String[] args) {
        System.out.println("Just a test");
    }
}