Practical Design Patterns for Java Developers: Hone your software design skills by implementing popular design patterns in Java

BIRMINGHAM—MUMBAI

Practical Design Patterns for Java Developers

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I remember the days when I started exploring the Java platform at Sun Microsystems. It became not only a daily job but also a lifelong hobby and passion. This book is dedicated to my incredible wife, Tanja, my wonderful children, Maxi and Elli, and my entire family for giving me the energy, inspiration, and motivation to keep up the strength needed to complete this book.

– Miroslav Wengner

Foreword

It was November 2021. In a conversation about his developer career, a senior Java developer told me:

I’ve been a senior engineer for almost 20 years. I don’t know what to do to get beyond that level.

That is a typical statement. The length of time varies, but far too many developers and engineers feel stuck in a senior position.

Don’t get me wrong, it’s not bad to be a senior!

You may be working on great projects and using cool technologies. You are probably involved in solving sophisticated problems and facing deep technical challenges.

But there may come a day that you, as the aforementioned developer, feel like you could be doing more in your career, could have more influence in the direction of projects, and have independence and autonomy in your work, or maybe you would feel more fulfilled by inspiring and mentoring other developers.

Although this is something I have heard many times before, that particular instance got me thinking. You see, earlier that same day, I was looking at the results of the Java Community Process (JCP) election. Miroslav Wengner, my friend Miro, had just been elected to the Executive Committee of the premiere standard body of Java technology.

It was impossible not to think about the parallels...

I first met Miro when we worked together on the NetBeans team at Sun Microsystems, the company that created Java. Like the aforementioned developer, Miro had also worked with Java for many years. By the time I had a long conversation with him about his developer career, Miro too had been a senior engineer for a number of years. But he didn’t stop there. Now, just a few years later, Miro has gone way beyond senior: freelancer, speaker, open source developer, committer of the Java Mission Control project at OpenJDK, Java Champion, and JCP EC Member, and why not a book author!

Like Miro, you don’t have to be stuck at the senior level. You too can build your technical reputation, grow beyond senior positions, and have a larger impact on your project, company, and even the world.

All that leads us to this book that you hold in your hands.

What attracted me to Miro’s book is the importance of design patterns in achieving exactly this goal: growing beyond the senior level. Whether you want to become a manager, continue your technical career as a staff-plus engineer, or even if you prefer to be more independent and grow as a freelancer or entrepreneur, you will need to take on more responsibilities and go beyond the code.

Although design patterns are directly connected to code, they are also separate and independent. They encapsulate proven solutions to the common challenges of designing quality software. Design patterns hover above a specific piece of code or even a specific project, and create a common vocabulary for explaining problems and solutions. Anyone in the business of building software should master this vocabulary to understand, communicate, and participate in discussions. After all, growing beyond senior is taking responsibility for your career. You start as a senior in your project, and then you expand your influence by getting involved with more impactful things.

When you master the vocabulary of design patterns, you are more equipped to go beyond your immediate project and get involved in other projects across your company and the industry. You’ll maybe even get to the point of helping define the directions of technology through open source projects and foundations or through standards organizations, like Miro is doing inside the JCP.

This book will help you do exactly that. With a broad view of design patterns applied to the Java ecosystem, you will see not only the concepts and the vocabulary, but also the real implementation and impact of design patterns in software that you use daily.

And one of the things that always impresses me is Miro’s ability to connect software with real things. His open source project, Robo4J, which even won a Duke’s Choice Award, helps you turn your Java code into robots and drones capable of navigating the real world. I was delighted to see Miro taking the same approach to design patterns. The examples in this book, using multiple vehicles and their parts, are written using the latest Java 17+ features, bringing the designs to life and connecting them to real, concrete problems.

So, get started on your journey of building your reputation, growing your career, and reaching beyond the senior level. Knowing design patterns will help you fit right in, speak the language, and participate in the most important decisions. Let the amazing Miro be your guide, and feel free to reach out to me if you would like (as Miro once did) to talk about the next steps in your career.

Bruno Souza

Principal Consultant, Java Champion, JCP Executive Committee Member

@brjavaman

https://java.mn

Contributors

About the author

Miroslav Wengner is an engineer with a passion for resilient distributed systems and product quality. He is a co-author and contributor to the Robo4J project (a reactive soft real-time framework for robotics/IoT). Miro contributes to OpenJDK and participates in other open source technologies. He uses his passion for helping build resilient and scalable solutions.

Miro was selected for the Java Champions Program, recognized as a JavaOne Rockstar, and elected to the Java Community Process (JCP) as an executive committee member.

In addition to his day-to-day duties as a principal engineer at OpenValue, he shares his knowledge at conferences (JavaOne, Devoxx, and so on) and in blogs. Miro believes in the Java ecosystem and helps move it forward!

About the reviewer

Werner Keil works in areas such as Agile, BDD, Cloud-native DevOps, Java, Java EE/Jakarta EE, IoT, security, and microservices, helping Global 500 clients across various industries and IT vendors. Having worked for over 30 years as PM, coach, software architect, and consultant for different sectors, Werner is an Eclipse and Apache Committer and a JCP member in JSRs. Werner has won multiple JCP awards, including Member of the Year and Outstanding Spec Lead, and was recognized as Speaker of All Times by Java2Days, a large Java conference held in eastern Europe. He is an Eclipse Babel Language Champion, a project lead of Eclipse UOMo, and a committer member in the Jakarta EE Specification Committee.

Table of Contents

Preface

Part 1: Design Patterns and Java Platform Functionalities

1

Getting into Software Design Patterns

Technical requirements

Code – from symbols to program

Examining OOP and APIE

Only exposing what’s required – encapsulation

Inevitable evolution – inheritance

Behavior on demand – polymorphism

Standard features – abstraction

Gluing parts to APIE

Understanding the SOLID design principles

The single-responsibility principle (SRP) – the engine is just an engine

The open-closed principle (OCP)

The Liskov Substitution Principle (LSP) – substitutability of classes

The interface segregation principle (ISP)

The dependency inversion principle (DIP)

Significance of design patterns

Reviewing what challenges design patterns solve

Summary

Questions

Further reading

2

Discovering the Java Platform for Design Patterns

Technical requirements

Knocking on Java’s door

Exploring the model and functionality of the Java platform

The JDK

The JRE

The JVM

Reviewing GC and the Java memory model

The JMM

GC and automatic memory management

Examining the core Java APIs

Primitive data types and wrappers

Working with the String API

Introducing arrays

Discovering a collection framework

Math APIs

Functional programming and Java

Introducing lambdas and functional interfaces

Using functional interfaces in lambda expressions

Getting to grips with the Java Module System

A quick review of Java features from 11 to 17+

The local variable syntax for lambda parameters (Java SE 11, JEP-323)

Switch expressions (Java SE 14, JEP-361)

Text blocks (Java SE 15, JEP-378)

Pattern matching for instanceof (Java SE 16, JEP-394)

Records (Java SE 16, JEP-395)

Sealed classes (Java SE 17, JEP-409)

UTF-8 by default (Java SE 18, JEP-400)

Pattern matching for switch (Java SE 18, Second Preview, JEP-420)

Understanding Java concurrency

From a basic thread to executors

Executing tasks

Summary

Questions

Further reading

Part 2: Implementing Standard Design Patterns Using Java Programming

3

Working with Creational Design Patterns

Technical requirements

It all starts with a class that becomes an object

Creating objects based on input with the factory method pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Creating objects from different families using the abstract factory pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Instantiating complex objects with the builder pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Cloning objects with the prototype pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Ensuring only one instance with the singleton pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Improving performance with the object pool pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Initiating objects on demand with the lazy initialization pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Reducing class dependencies with the dependency injection pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Summary

Questions

Further reading

4

Applying Structural Design Patterns

Technical requirements

Incompatible object collaboration with the adapter pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Decoupling and developing objects independently with the bridge pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Treating objects the same way using the composite pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Extending object functionality by using the decorator pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Simplifying communication with the facade pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Using conditions to select desired objects with the filter pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Sharing objects across an application with the flyweight pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Handling requests with the front-controller pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Identifying instances using the marker pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Exploring the concept of modules with the module pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Providing a placeholder for an object using the proxy pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Discovering multiple inheritance in Java with the twin pattern

Motivation

Sample code

Conclusion

Summary

Questions

Further reading

5

Behavioral Design Patterns

Technical requirements

Limiting expensive initialization using the caching pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Handling events using the chain of responsibility pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Turning information into action with the command pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Giving meaning to the context using the interpreter pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Checking all the elements with the iterator pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Utilizing the mediator pattern for information exchange

Motivation

Finding it in the JDK

Sample code

Conclusion

Restoring the desired state with the memento pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Avoiding a null pointer exception state with the null object pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Keeping all interested parties informed using the observer pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Dealing with instance stages by using the pipeline pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Changing object behavior with the state pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Using the strategy pattern to change object behavior

Motivation

Finding it in the JDK

Sample code

Conclusion

Standardizing processes with the template pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Executing code based on the object type using the visitor pattern

Motivation

Finding it in the JDK

Sample code

Conclusion

Summary

Questions

Further reading

Part 3: Other Essential Patterns and Anti-Patterns

6

Concurrency Design Patterns

Technical requirements

Decoupling a method execution with an active object pattern

Motivation

Sample code

Conclusion

Non-blocking tasks using async method invocation pattern

Motivation

Sample code

Conclusion

Delay execution until the previous task is completed with the balking pattern

Motivation

Sample code

Conclusion

Providing a unique object instance with a double-checked locking pattern

Motivation

Sample code

Conclusion

Using purposeful thread blocking via a read-write lock pattern

Motivation

Sample code

Conclusion

Decoupling the execution logic with a producer-consumer pattern

Motivation

Sample code

Conclusion

Executing isolated tasks with the scheduler pattern

Motivation

Sample code

Conclusion

Effective thread utilization using a thread-pool pattern

Motivation

Sample code

Conclusion

Summary

Questions

Further reading

Answers

7

Understanding Common Anti-Patterns

Technical requirements

What anti-patterns are and how to identify them

Theoretical principles challenges

Collecting technical debt as a bottleneck

Inappropriately squeezing the capabilities of the Java platform

Selecting the right tool

Conclusion of the code smell anti-pattern

Examining typical software anti-patterns

Spaghetti code

Cut and paste programming

Blob

Lava flow

Functional decomposition

Boat anchor

Conclusion

Understanding software architecture anti-patterns

Golden hammer

Continuous obsolescence

Input kludge

Working in a minefield

Ambiguous viewpoint

Poltergeists

Dead end

Conclusion

Summary

Further reading

Assessments

Index

Other Books You May Enjoy

Preface

The Java language is a tool for communicating with a very rich platform that provides many features ready to serve for application development. This book explores the latest developments in improving language syntax with examples of the most useful design patterns. The book reveals the relationship between features, patterns, and platform efficiency through example implementations. The book explores how theoretical foundations help improve the maintainability, efficiency, and testability of source code. The content helps the reader solve different tasks and provides guidance on how to approach programming challenges using a variety of sustainable and transparent approaches.

Who this book is for

This book is dedicated to all hungry engineers who want to improve their software design skills with new language enhancements and a closer look at the Java platform.

What this book covers

Chapter 1, Getting into Software Design Patterns, introduces us to the initial foundations of source code design structure and outlines the principles that should be followed to achieve maintainability and readability.

Chapter 2, Discovering the Java Platform for Design Patterns, discusses the Java platform, which is a very broad and powerful tool. This chapter exposes the features, functions, and design of the Java platform in more detail to continue building the foundation needed to understand the purpose and value of using a design patterns.

Chapter 3, Working with Creational Design Patterns, explores object instantiation, which is a key part of any application. This chapter describes how to approach this challenge while keeping the requirements in mind.

Chapter 4, Applying Structural Design Patterns, shows how to create source code that allows for clarity of relationships between required objects.

Chapter 5, Behavioral Design Patterns, explores how to create source code that allows objects to communicate and exchange information while maintaining a transparent form.

Chapter 6, Concurrency Design Patterns, discusses the Java platform and how it is a concurrent environment by nature. It shows how to harness its power for the designed application’s purposes.

Chapter 7, Understanding Common Anti-Patterns, deals with anti-patterns that can be found in any application development