Guava

What it is

Guava is a Java library that adds collection types like multimaps and multisets, immutable collections, a graph package, and helpers for caching, concurrency, hashing, I/O, strings, and primitive types. It ships in a JRE flavor for Java 8 and later and an Android flavor. Java developers pull it into Maven or Gradle builds to fill gaps in the standard library, relying on its long-term binary compatibility for non-beta APIs.