groovy main method is use static main(args) //ok

Groovy has done a lot to make Java more palatable, but at the same time has introduced some confusion as to what developers can get away with at the most fundamental level.

Living on The Main Line
In Java, main methods are typically, if not always

publicstaticvoid main(String[] args)

In Groovy, methods are public by default and args is typed dynamically, so you can get away with

static main(args)  //ok

but not

static main() //not ok
void main() //not ok
void main(args) //not ok

attempting to run a class with these bad main method declarations, the compiler will invariably return

Caught: groovy.lang.GroovyRuntimeException: This script or class could not be run.
It should either:
- have a main method,
- be a JUnit test, TestNG test or extend GroovyTestCase,
- or implement the Runnable interface.

IntelliJ is clever in that it can recognize the proper declaration on the fly. If you missing the required main method declaration you will get the normal Groovy class file type icon

If you have a valid main method you will see the runnable arrow

Groovy scripts
I have some sturdy bags I take with me to the grocery store. If I buy lot of food they just give me a paper bag in which to put the extra items. Every item is treated the same regardless of which bag they put it in.
A Groovy script is essentially any file with some code outside the confines of a class. Scripts can contain their own classes but all the stuff outside those is given its own class based on the filename and placed inside a working main(args) method. They just give you another bag - it's no big deal.

The following Groovy class and Groovy script are equivalent.

Groovy class with main method

class aGroovyClass {
  def myString = "I am an instance variable";
  static main(args) {
    def myObject = new aGroovyClass();//def required
    println myObject.myString;
}
}

Groovy script with class

class aGroovyClassWithinAScript {
  def myString = "I am an instance variable";
}
myObject = new aGroovyClass(); //no def needed
println myObject.myString;

Multiple main methods - you had to ask
Can classes inside a script have their own main method? Umm, yes, but that is pretty confusing (even to IntelliJ). I would have to explicitly call that main method from my class to use it - main is not a constructor.

Class with its own main method.

IntelliJ again
Both Groovy scripts and Groovy classes with main methods should be run as Groovy Script in the Run/Debug Configuration. Sometimes IntelliJ will default to attempting to run it as a Java application. If you attempt to run a Groovy script as a Java application you will get the following error:

Exception in thread "main" java.lang.NoSuchMethodError: main