Design Patterns in Java: From Basics to Advanced Techniques

This book is a comprehensive guide to design patterns, adapted for Java developers. The author, Alexey Smirnov, a senior developer with 12 years of experience, has systematized knowledge from the classic "Gang of Four" (GoF) book and supplemented it with modern implementations characteristic of the Java ecosystem (Stream API, Optional, lambda expressions, module system).

The book covers 23 patterns divided into three groups: creational (Singleton, Factory Method, Abstract Factory, Builder, Prototype), structural (Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy), and behavioral (Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, Visitor). Each pattern is examined according to a unified scheme:

• Motivation — a real-world problem from practice that the pattern solves.
• Structure — UML class and sequence diagrams.
• Implementation in Java — clean code with comments, from simple to complex.
• Common Mistakes — anti-patterns and pitfalls that beginners encounter.
• Application in Real Projects — how the pattern is used in Spring, Hibernate, Java SE/EE.

The book is intended for junior and middle developers who already know the basics of Java (classes, inheritance, interfaces, polymorphism) and want to move to the next level: writing code that is easy to maintain, test, and extend. It will also be useful for students of technical specialties and team leads who want to standardize architecture within a team.

Special attention is paid to practical exercises: at the end of each chapter, there are 3-5 exercises with step-by-step solution breakdowns. For example, you will learn to rewrite spaghetti code using Strategy, build complex objects via Builder, manage state through State, and organize subscriptions via Observer. All code examples are available in the GitHub repository.

After reading, you will be able to:

• Confidently choose a pattern for a specific task without overcomplicating the architecture.
• Refactor legacy code by introducing patterns without risking breaking functionality.
• Understand the internal workings of popular Java frameworks (Spring IoC is a combination of Factory + Proxy + Singleton).
• Pass technical interviews by answering questions about patterns with real-world examples.

The book is written in a lively, accessible language, without excessive academicism. Each chapter is ready material for studying in 30-40 minutes. At the end of the book, there is a cheat sheet for choosing patterns and a list of references for in-depth study.