Groovy Editor's content assist proposals for fields/properties and methods/constructors are missing or have unresolved generics. The examples below show the proposals in the Java Editor and Groovy Editor respectively for a class that implements Comparator<String>.

Notice in compare Groovy has type T for the params and Java has resolved to String. Also, reversed returns Comparator<String> for Java and just Comparator for Groovy. And so on for method params with wildcards, extends, super, etc.