Functional Programming in Java: From Lambdas to Monads

The book "Functional Programming in Java" is a practical guide for developers who want to go beyond the imperative style and master modern functional approaches in the Java ecosystem. The author, Alexey Sokolov, a senior developer with 10 years of experience, consistently guides the reader from basic concepts to advanced patterns.

What is this book about?

The book covers all key aspects of functional programming (FP) as applied to Java:

• Lambda expressions and functional interfaces — how to write compact anonymous functions and use built-in interfaces (Predicate, Function, Consumer, Supplier).
• Stream API — pipeline processing of collections: map, filter, reduce, flatMap, parallel streams, and performance optimization.
• Option, Either, and Try — dealing with null-safe code and error handling without exceptions (functional approach).
• Monads and functors — from theory to practice: how to apply monadic constructs in Java (Optional, CompletableFuture, third-party libraries vavr, functionaljava).
• Pure functions and immutability — designing immutable data structures and refactoring legacy code.
• Parallelism and concurrency — functional approaches to multithreading: CompletableFuture, reactive streams, actors.

Who is this book for?

• Java developers at Junior and Middle levels who want to improve code quality.
• Architects looking for ways to reduce coupling and increase system testability.
• Programmers transitioning from other languages (Scala, Kotlin, JavaScript) — to understand how to apply FP in Java.
• Students and instructors of modern Java courses.

What will you learn?

• How to replace loops and conditional statements with functional constructs.
• How to write code that is easier to test, debug, and maintain.
• How to effectively use multithreading without locks and race conditions.
• How to integrate functional libraries (Vavr, Javaslang, RxJava) into an existing project.
• How to avoid side effects and write predictable functions.

All examples in the book are accompanied by full code and comments. Each chapter ends with practical exercises and self-test questions. The book is suitable both for self-study and for use as a textbook in advanced training courses.

"Functional Programming in Java" is not just a translation of academic concepts, but a real tool for improving daily work. After reading it, you will be able to write more concise, safe, and parallel code using the full power of modern Java (8+).