Want to improve your Java code? Use Guava. Guava is a set of core Java libraries from Google that includes new collection types (such as multimap and multiset), (importantly and usefully) immutable collections, a graph library, and utilities for concurrency, I/O, hashing, primitives, strings, and more! It is widely used on most Java projects within Google, and widely used by many other companies as well.

Guava comes in two flavours:

• The JRE flavour requires JDK 1.8 or higher.
• If you need support for Android, use the Android flavour.