Guava

Guava is the set of core Java libraries Google pulled out of its own internal code and published under Apache 2.0. It fills gaps the JDK left: multimaps and multisets, immutable collections, a graph library, and utilities for concurrency, I/O, hashing, primitives, and strings. The same Maven coordinates ship in two flavours, a JRE build requiring JDK 8 or later and an Android build for projects that must stay compatible with Android. What is unusual is that the compatibility contract is written down rather than assumed: anything marked @Beta can change or vanish without notice, while everything else stays binary compatible indefinitely. A separate Beta Checker exists so library authors do not pass unstable APIs on to the people depending on them. According to the project, most Java work inside Google rests on it, along with a long list of companies outside. Javadoc for any class is reachable by appending its name to guava.dev.