Functional programming is not a new idea; actually, it's pretty old. For example, Lisp, which is a functional language, is the second oldest of today's commonly-used programming languages.

Functional programs are built using small pieces of reusable pure functions (lambdas). The program logic is composed of small declarative steps and not complex algorithms. That's because functional programs minimize the use of state, which makes imperative programs complex and hard to refactor/support.

With Java 8, the Java world got the lambda expressions and the ability to pass functions to functions. With them, we can code in a more functional style and get rid of a lot of the boilerplate code. The other new thing we got with Java 8 is the streams—something very similar to RxJava's observables but not asynchronous. Combining these streams and the lambdas, we are able to create more functional-like programs.

We are going to familiarize ourselves with...