Code and Class obligations:
As you note the error type, it should have been declared as simple_comment, and then compiled as  "simple_comment.java."

public class simple_comment
{
public static void main(String args[])
{
//this is a comment
//compile as
//javac simple_comment.java
//call as java simple_comment
System.out.println("Hell World");
}
}

When you compile, a java class is created

now you can call the class