A Functional Approach to Java

Chapter 1. An Introduction to Functional Programming

To better understand how to incorporate a more functional programming style in Java, you first need to understand what it means for a language to be functional and what its foundational concepts are.

This chapter will explore the roots of functional programming needed to incorporate a more functional programming style into your workflow.

What Makes a Language Functional?

Programming Paradigms, like object-oriented, functional, or procedural, are synthetic overall concepts that classify languages and provide ways to structure your programs in a specific style and use different approaches to solving problems. Like most paradigms, functional programming doesn’t have a single agreed-upon definition, and many turf wars are fought about what defines a language as actually functional. Instead of giving my own definition, I will go over different aspects of what makes a language functional.

A language is considered functional when there’s a way to express computations by creating and combining abstract functions. This concept is rooted in the formal mathematical system lambda calculus, invented by the logician Alonzo Church in the 1930s.¹ It’s a system to express computations with abstract functions and how to apply variables to them. The name “lambda calculus” came from the Greek letter “lambda” chosen for its symbol: $\lambda$.

Lambda Calculus

Three pillars build the foundation for the general concept of lambda calculus:

Abstraction ...