traitLikeDefineTest.groovy

interface Cat {
  String mew()
}
class Zash implements Cat {
  String mew(){ "雑種" }
}
class Nora implements Cat {
  String mew(){ "野良" }
}
def scottish = [mew:{"すこてぃっしゅ"}] as Cat

Cat.metaClass {
  def oldMew = Cat.metaClass.getMetaMethod('mew', [] as Class[])
  mew = {->
    "[Cat] ${oldMew.invoke(delegate, null)}"
  }
}

Cat[] cats = [new Zash(), new Nora(), scottish] as Cat[]
for (c in cats) println c.mew()

// output ↓
// [Cat] 雑種
// [Cat] 野良
// [Cat] すこてぃっしゅ