@jackfrosch compares Java to Groovy in three different angles: interoperability, language differences and Groovy add-ons.
how to keep Java 8 compatibility:
Wherever you find a Java 8 method reference or Lambda you can replace it with a closure
What Groovy brings to the table: ASTs, Traits, Multiline Strings, operator override.
Several topics (equality, method dispatching and truth evaluation ... ) to highlight the difference between both languages.