Rewrite object-oriented code using functional techniques. Get started with lambdas, method references, functional interfaces, and the Streams API in Java Welcome back to this two-part tutorial ...

first off you question sounds like "how do I do first class functions in java" historically that answer was "make a class with one method, it's shit suck it" things are better now though. But really ...

Kotlin is a modern alternative to Java that supports functional programming in the JVM. Here's a first look at programming with Kotlin using some of the concepts and syntax you already know from Java.