Invalid duplicate class definition…One of the classes is a explicit generated class using the class statement, the other is a class generated from the script body based on the file name. Solutions are to change the file name or to change the class name.

When I was first learning Groovy I would get this error from time to time. It was puzzling to me because sometimes I’d get this error, and sometimes it would seem like the same situation and I wouldn’t. It seemed quite random to me when the error would crop up. When it did, I’d usually just rename the class and go on, resolving to figure it out later. To save you the trouble, here’s what is happening.

Groovy has two ways to treat a .groovy file: either as a script, or as a class definition file. If it is a script you can not have a class by the same name as the file. If it is a class definition file you can. It is very easy to tell whether a .groovy file is going to be treated as a script or as a class definition file. If there is any code outside a class statement in the file (other than imports), it is a script. What is happening is that if there is any code to be executed in the file then Groovy needs a containing class for that code. Groovy will implicitly create a containing class with the name of the file. So if you have a file called Grapher.groovy that has some code in it that isn’t inside a class definition, Groovy will create an implicit containing class called Grapher. This means that the script file Grapher.groovy can not itself contain a class called Grapher because that would be a duplicate class definition, thus the error. If, on the other hand, all you do in the file Grapher.groovy is define the class Grapher (and any number of other classes), then Groovy will treat that file as simply a collection of class definitions, there will be no implicit containing class, and there is no problem having a class called Grapher inside the class definition file Grapher.groovy.

It’s worth mentioning that the script version of Grapher.groovy will be compiled into a class called Grapher that extends groovy.lang.Script. In the other case, when Grapher.groovy merely defines classes, one of which is Grapher, that Grapher class will be compiled into a class that implements groovy.lang.GroovyObject.

I’m sure this is all explained somewhere in the Groovy documentation, but it didn’t soak in to me until I read this Nabble post from which I extracted this explanation.

UPDATE: The text of this error message has changed (at least in some cases) to be a bit more informative. Now it reads:

One of the classes is an explicit generated class using the class statement, the other is a class generated from the script body based on the file name. Solutions are to change the file name or to change the class name. The underlying mechanics behind this error are the same.