Functional Programming in Java: From Lambdas to Monads

The book "Functional Programming in Java" by Alexey Petrov is a comprehensive practical guide that will help Java developers master the modern functional programming style. In the era of Java 8 and beyond, knowledge of lambda expressions, Stream API, and immutable data structures has become essential for professional growth. The author guides the reader step by step from the basics to advanced concepts such as monads and functional compositions.

The book is intended for developers already familiar with Java but wishing to transition from imperative to declarative thinking. It will be useful both for beginners in functional programming and for experienced engineers who want to write more concise, testable, and thread-safe code. No abstract theoretical digressions — only working examples and real-world cases from industrial development.

What you will learn from this book:

• Lambda expressions and functional interfaces — how to replace anonymous classes and reduce code volume.
• Stream API — efficient collection processing, filtering, mapping, reductions, and parallel streams.
• Optional values (Optional) — eliminating NullPointerException and safely working with missing data.
• Functional combinators — function composition, currying, and partial application.
• Monads in Java — a practical introduction to monadic design (Maybe, Try, Either) and their implementation in pure Java.
• Testing functional code — how to write tests for pure functions and immutable structures.
• Refactoring and antipatterns — how to rewrite old imperative code in a functional style and avoid common mistakes.

Each chapter contains not only explanations but also exercises for independent work. Code examples are available for download and have been tested on Java 11 and 17. The author pays special attention to practical aspects: performance, debugging, and integrating the functional approach with existing projects.

After reading the book, you will be able to:

• Write concise and readable chains of data operations.
• Confidently use Optional instead of null checks.
• Apply monadic constructs for error handling and asynchrony.
• Refactor legacy code, making it more modular and testable.
• Prepare for interviews that require an understanding of functional programming in Java.

The book is written in a lively style, with humor and many analogies. Suitable both for self-study and for reading in a team. If you want to reach a new level of Java proficiency and stop being afraid of lambdas — this book is for you.