Functional Programming in Java: From Lambdas to Monads

The book "Functional Programming in Java" is a practical guide that will help Java developers master the functional paradigm and apply it in their daily work. The author, Alexey Zakharov, consistently guides the reader from basic concepts (lambda expressions, method references) to complex topics (monoids, functors, monads).

What is this book about?

• Fundamentals of functional programming: immutability, pure functions, higher-order functions.
• Lambda expressions and functional interfaces in Java 8+.
• Stream API: working with collections, parallel streams, optimization.
• Optional as an alternative to null — safe call chains.
• Function composition, currying, partial application.
• Monads in Java: from simple (Maybe, Either) to industrial (Try, Validation).
• Integrating the functional approach with existing imperative code.
• Testing functional code: property-based testing, mocking.

Who is this book for?

• Java developers who want to write more concise and reliable code.
• Programmers transitioning from imperative languages (C++, Python) to Java.
• Students and beginners studying modern paradigms.
• Those preparing for interviews at large IT companies (knowledge of FP is often a plus).

What will the reader learn?

After reading, you will be able to:

• Write code without side effects and mutations — this reduces the number of bugs.
• Effectively use the Stream API for data processing instead of cumbersome loops.
• Apply Optional to forget about NullPointerException.
• Build data transformation chains using function composition.
• Understand monads and use them for error handling, asynchrony, and validation.
• Refactor old code by introducing functional patterns without losing performance.

Why should you read this book?

Functional programming is not just a passing trend. It is a way to write code that is easier to test, parallelize, and maintain. Starting with version 8, Java has been actively embracing functional ideas, and this book will help you keep up with the modern industry.

All examples are based on real-world tasks: from parsing JSON to building ETL pipelines. The code is available for download and ready to run.

Functional Programming in Java is your guide to the world of clean, expressive, and reliable code.