Javaslang core is a functional library for Java 8+. It helps to reduce the amount of code and to increase the robustness. A first step towards functional programming is to start thinking in immutable values. Javaslang provides immutable collections and the necessary functions and control structures to operate on these values. The results are beautiful and just work.
There are several additional modules that group around Javaslang's core. All of these are supported by the Javaslang open source organization.