Lucene: for full text search/information retrieval/text mining
https://www.youtube.com/watch?v=yst6VQ7Lwpo https://www.youtube.com/watch?v=SCsS5ePGmCs
OkHttp for HTTP clients.
RxJava: Reactive programming.
Guava: Multi utility Collections/data structures
Gson/moshi: json handling
Mockito: mocking in testing
Dagger/guice: Dependency Injection.