Groovy

This is something you bring into Java to compensate for Java's bad generics and to do stuff akin to the LINQ stuff of C#. (I think.)

 
 

Addendum 3/21/2018: Groovy is a sister language very much like Java, but it is not something you add onto Java. You can get a compiler that will compile both together and somewhat use objects from one in the other in a hodgepodge.