Functional Programming in Java: How functional techniques improve your Java programs

Chapter 1. What is functional programming?

This chapter covers

• The benefits of functional programming
• Problems with side effects
• How referential transparency makes programs safer
• Reasoning about programs with the substitution model
• Making the most of abstraction

Not everybody agrees on a definition for functional programming (FP). In general terms, functional programming is a programming paradigm, and it’s about programming with functions. But this doesn’t explain the most important aspect: how FP is different from other paradigms, and what makes it a (potentially) better way to write programs. In his article “Why Functional Programming Matters,” published in 1990, John Hughes writes the following:

Functional programs contain no assignment ...