Apache Groovy's dynamic typing, closures, and shorthand collection syntax (from v. 1.0) are great for testing and playing with Java classes, and I'm told Groovy's Ruby-like MOP is useful for Grails developers, but the stuff added later (like generics in v. 1.5) made the language klunky and overdone. Best switch to Java (or Scala/Kotlin/etc) once you need anything to be typed, let alone with generics.