Question

why does this throw an exception?

messageSource.getMessage('UserService.msg.forgot.unknown', ["[e-mail]"], null) 

unless I do this…

def Object[] args = ["[e-mail]"]
messageSource.getMessage('UserService.msg.forgot.unknown', args, null) 

Answer 1

Because [“[e-mail]”] evaluates to an ArrayList, not an array:

groovy:000> o = ["asdf"]
===> [asdf]
groovy:000> o.getClass()
===> class java.util.ArrayList

OTOH your declaration creates an array of Objects:

groovy:000>  Object[] args = ["asdf"]
===> [Ljava.lang.Object;@14e113b

and the method you’re calling needs an array. You can create an array using as:

Object[] args = ["asdf"] as Object[]

The Groovy creators made a point of making higher-level data structures like lists idiomatic, while arrays are present for interoperability with Java.

In his interview in Coders at Work Guy Steele talks about choices made in designing a language:

“There’s this Huffman encoding problem. If you make something concise,
something is going to have to be more verbose as a consequence. So in
designing a language, one of the things you think about is, ‘What are
the things I want to make very easy to say and very easy to get
right?’ But with the understanding that, having used up characters or
symbols for that purpose, you’re going to have to make something else
a little bit harder to say.”

It certainly looks like Groovy made lists more concise, with the side effect that arrays became more verbose.