Chapter 1. Getting Started with Functional Programming

Functional programming languages have been used successfully for decades and present a different and often more elegant way of expressing the program logic. Functional languages, such as Lisp, Clojure, or Haskell, incorporate techniques that may seem odd and hard to follow by programmers who are used to imperative programming techniques.

A language such as Java, while not initially developed with a functional orientation, can incorporate functional techniques. This is the major change to the language made with the release of Java 8. Java now incorporates imperative, procedural, object-oriented, and functional techniques.

It is possible to write a non-object-oriented program in Java. Likewise, it is possible to write a nonfunctional program in Java 8. The goal of this book is to enlighten the reader to the nature of functional programming techniques and how to incorporate these techniques in Java 8 applications.

We will start with a discussion of the attributes commonly associated with functional programming. From there, we will examine the support, Java provides for developing applications using a functional-style programming approach.

A predominant feature of functional programming languages is the use of functions. The term, function, is generally understood to be:

• A way of expressing an algorithm
• A mathematical function
• Where the goal is to avoid state changes and mutable data

In functional programming, applications are constructed using only pure functions. A pure function is a function which does not have side effects. A side effect occurs when a function does something else besides simply returning a value, such as mutating a global variable or performing IO. In this chapter, we will examine the major aspects of functional programming including:

• Functions and function composition
• Fluent interfaces
• Strict and non-strict evaluation
• Persistent data structures, monads, and the Optional class
• Recursion and parallelism

This is followed by the support Java 8 provides for functional programming, including:

• Lambda expressions
• Default methods
• Functional interface
• Method and constructor references
• Collections

In addition, to our discussion of functional programming support as provided by Java 8, refactoring, debugging, and testing Java 8 code are also important topics, which need to be addressed. These topics are covered in Chapter 8, Refactoring, Debugging, and Testing.

So, let's begin with an overview of what constitutes the functional programming approach.