static method using a default instance?

Hi there,

I've bumped into a pretty trivial problem, but probably I've got a dumb day today — can't find a solution :(

How does one write a method which can be called either over an instance, or over a class, in which case it would automatically use a default instance? Like this, in pseudo-code which does not work in Groovy/Java, but shows pretty well what I need:

===
class Foo {
  static defaultInstance=new Foo()
  def foo() { ... does its stuff on this ... }
  static foo() { defaultInstance.foo() }
}
===
// usage when I happen to have the instance:
Foo o= ...
o.foo() // does its stuff on o
// usage otherwise
Foo.foo() // does its stuff on the default instance
===

Thanks!
OC