Vaskaran Sarcar
Java Design PatternsA Hands-On Experience with Real-World Examples2nd ed.
Foreword by Sunil Sati
Vaskaran Sarcar
Bangalore, Karnataka, India
ISBN 978-1-4842-4077-9e-ISBN 978-1-4842-4078-6
https://doi.org/10.1007/978-1-4842-4078-6
Library of Congress Control Number: 2018964945
© Vaskaran Sarcar 2019
Standard Apress
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While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.
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This book is dedicated to Almighty God, my family, and the Gang of Four.

You are my inspiration.

Foreword

“A problem well stated is a problem half solved.”

—Charles Kettering, inventor and engineer

To build on this concept, I must say that thinking about all possible scenarios and coming out with a best possible option is the key to a robust and lasting solution. This new second edition of Java Design Patterns will serve as a mentor and guide to engineers and designers who are regularly challenged to come up with the best possible solution in resource-constrained environments. This book explains in very clear terms the design patterns, the alternatives, and the concept of antipatterns. The complete section on antipatterns is very thought-provoking and helps us appreciate the utility of design in the first place.

As in his previous books, Vaskaran has provided hands-on experience in implementing design patterns. His innate way of getting engineers to think of an alternative solution is very insightful. This book will serve as mentor and task master as one traverses the chapters.

When I reflect on my interactions with Vaskaran while facing some complex engineering issues in HPI, I find him to be a keen listener, deep thinker, and a person who doesn’t rush for a solution. After analyzing all possible alternatives, he picks the best one among the lot. In summary, this is what this book all about.

Sunil Sati

Senior Project Manager, BU Automotive Division, NXP Semiconductors

About Sunil Sati

Sunil Sati is an engineer with a major in electronics and communication from NIIT Surathkal and EGMP from IIM-Bangalore. He has 23 years of experience in various roles and capacities in process automation and semiconductor industries. Currently, he is working as senior manager with NXP Semiconductors in the automotive division. He was the brand ambassador in HPI for the Print Renaissance program. He loves to work and build teams across geographic locations.

Introduction

Welcome to your journey through Design Patterns in C# .

This is an introductory guide to the design patterns that you want to use in Java. You probably know that the concept of design patterns became extremely popular with the Gang of Four’s famous book Design Patterns: Elements of Reusable Object-Oriented Software (Addison-Wesley, 1994). Most important, these concepts still apply in today’s programming world. The book came out at the end of 1994, and it primarily focused on C++.

But Sun Microsystems released its first public implementation Java 1.0 in 1995. So, in 1995, Java was totally new to the programming world. But it grew rapidly, becoming rich with features. It has now secured its rank in world’s top programming languages. In today’s programming world, it is always in high demand. On the other hand, the concepts of design patterns are universal. So, when you exercise these fundamental concepts of design patterns with Java, you will be a better programmer and you’ll open new opportunities for yourself.

In 2015, I wrote Design Patterns in C#: Computer Science Interview Series , and in 2016, I wrote Java Design Pattern: A tour with 23 Gang of Four Design Patterns in Java . They are basically the companions to this book.

In those books, my core intention was to implement each of the 23 Gang of Four (GoF) design patterns with C# and Java implementations. I wanted to present each pattern with simple examples. One thing was always on my mind when writing Java Design Patterns (First Edition): I wanted to use the most basic constructs of Java, so that the code would be compatible with both the upcoming version and the legacy version of Java. I have found this method helpful in the world of programming.

In the last two years, I got a lot of constructive feedback from my readers. This fully revised and updated version is created keeping those feedback in mind. I also took the opportunity to update the formatting and correct some typos in the previous version of the book and add new content to this new edition.

This time, I wanted to focus on another important area; I call it the “doubt-clearing sessions.” I knew that if I could add some more information such as alternative ways to write these implementations, the pros and cons of these patterns, when to choose one approach over another, and so on, readers would find this book even more helpful. So, in this enhanced version of the original, I have added a “Q&A Session” section to each chapter that can help you learn about each pattern in more depth.

In the world of programming, there is no shortage of patterns, and each has its own significance. So, in addition to the 23 GoF design patterns covered in Part I, I discuss three design patterns that are equally important in today’s world of programming in Part II. Finally, in Part III, I discuss the criticism of design patterns and give you an overview of antipatterns, which are also important when you implement the concepts of design patterns in your applications.

Before jumping into these topics, I want to highlight few more points.

  • You are an intelligent person. You have chosen a subject that can assist you throughout your career. If you are a developer/programmer, you need these concepts. If you are an architect at a software organization, you need these concepts. If you are a college student, you need these concepts, not only to score high on exams but to enter the corporate world. Even if you are a tester who needs to take care of white-box testing or needs to know about the code paths of a product, these concepts will help you a lot.

  • I already mentioned that this book was written using the most basic features of Java so that you do not need to be familiar with advanced Java topics. These examples are simple and straightforward. I believe that these examples are written in such a way that even if you are familiar with another popular language, such as C#, C++, and so on, you can still easily grasp the concepts in this book.

  • There are many books about design patterns and related topics. You may be wondering why I would want to write a new one about the same topics. The simple answer is that I have found other reference material to be scattered. Second, in most cases, many of those examples are unnecessarily large and complex. I like simple examples. I believe that anyone can grasp a new idea with simple examples, and if the core concept is clear, you can easily move into more advanced areas. I believe that this book scores high in this context. The illustrated examples are simple. I wanted to keep this book concise so that it motivates you to continue your journey of learning.

  • Each chapter is divided into six parts: a definition (which is basically called intent in Design Patterns: Elements of Reusable Object-Oriented Software ), a core concept, a real-world example, a computer/coding–world example, a sample program with various output, and the Q&A Session section. These Q&A Session sections help you learn about each pattern in more depth.

  • Please remember that you have just started on this journey. As you learn about these concepts, try to write your own code; only then will you master the area.

  • You will be able to download all the source code in the book from the publisher’s website. I plan to maintain the “Errata,” and if required, I can also make update/announcements there. So, I suggest that you visit those pages to receive any corrections or updates.

Guidelines for Using This Book

Here are some suggestions for you to use the book more effectively.

  • You should have a basic understanding of Java and you should know how to create classes, interfaces, and so forth. It is helpful to be familiar with common terms in object-oriented programming; for example, encapsulation, abstraction, polymorphism, and inheritance.

  • I assume that you have some idea about the GoF design patterns. If you are absolutely new to design patterns, I suggest you quickly go through Appendix A. This appendix will help you become familiar with the basic concepts of design patterns.

  • If you are confident about the content in Appendix A, you can start with any part of the book. But I suggest you go through the chapters sequentially. The reason is that some fundamental design techniques may be discussed in the “Q&A Session” section of a previous chapter, and I did not repeat those techniques in the later chapters.

  • There is only one exception to the previous suggestion. There are three factory patterns: simple factory, factory method, and abstract factory. These three patterns are closely related, but the simple factory pattern does not directly fall into the GoF design catalog, so it appears in Part II of the book. So, I suggest that when you start learning about these three factory patterns, you begin with the simple factory pattern.

  • These programs are tested with Java 8 (update 172). I used the Eclipse editor in a Windows 10 environment. So, in the Eclipse Package Explorer view, you may notice the string jdk1.8.0_172. At the time of this writing, Photon is the latest edition of Eclipse (released in June 27, 2018), Java 8 is the long-term support (LTS) version, and Java 10 is the rapid release version. Java 11 is the next LTS version after Java 8 and planned for September 2018. But all of this version information should not matter much because I used the most basic constructs of Java. So, I believe that the code should execute smoothly in the upcoming versions of Java/Eclipse as well.

  • One of my reviewers tested the code in a Linux environment. So, I believe that the results should not vary in other environments, but you know the nature of software—it is naughty. So, I recommend that if you want to see the same output, it is best if you can mimic the same environment.

  • To draw class diagrams, ObjectAid Uml Explorer is used in the Eclipse editor. It is a lightweight tool for Eclipse. At the time of this writing, it is free if you want to draw the class diagrams, but to draw the sequence diagrams, you need to purchase a license. The website at www.objectaid.com/home gives more information about licenses, terms, and conditions.

Lastly, I hope that this enhanced edition provides more help to you.

Conventions Used in This Book

  • All the output and code in the book follow the same font and structure. To draw your attention, in some places, I made it bold, like the following.

    ***Mediator Pattern Demo***
    At present, registered employees are:
    Amit
    Sohel
    Raghu
    Communication starts among participants...
    Amit posts: Hi Sohel, can we discuss the mediator pattern? Last message posted at 2018-09-09T17:41:21.868
    Sohel posts: Hi Amit, yup, we can discuss now. Last message posted at 2018-09-09T17:41:23.369
    Raghu posts: Please get back to work quickly. Last message posted at 2018-09-09T17:41:24.870

    An outsider named Jack trying to send some messages.

  • In some cases, to present a cleaner class diagram and focus on the important parts, the less important dependencies are not shown. For example, consider the diagram presented in Chapter 18 .

../images/395506_2_En_BookFrontmatter_Figf_HTML.jpg
But if I need to show all the dependencies, it may look like the following.
../images/395506_2_En_BookFrontmatter_Figg_HTML.jpg
You can see that the later one is much more complex and difficult to understand. For the ObjectAid class diagrams in Eclipse, you can always show these dependencies by selecting an element in the diagram, right-clicking and selecting Add ➤ Dependencies.
  • I like to put curly braces on a new line. and I love to see a method body like the following.
    public void myFunction()
    {
    //Some code
    }
    Instead of the following:
    public void myFunction(){
    //Some code
    }

I used the same format for most of the methods, except main() methods.

Acknowledgments

At first, I thank the Almighty. I sincerely believe that with His blessings only, I could complete this book. I extend my deepest gratitude and thanks to

Ratanlal Sarkar and Manikuntala Sarkar. My dear parents, with your blessings only, I could complete the work.

Indrani, my wife, and Ambika, my daughter. Sweethearts, once again, without your love, I could not proceed at all. I know that we needed to limit many social gatherings and invitations to complete this work on time and each time I promise you that I’ll take a long break and spend more time with you.

Sambaran, my brother. Thank you for your constant encouragement toward me.

Shekhar, Ritesh, and Ankit. You are my friends and technical advisors. I know that whenever I was in need, your supports were there. Thank you one more time.

Anupam. My friend and another technical advisor. Though this time, you were not involve but still I acknowledge your support and help toward me in the development of Java Design Patterns first edition.

Sunil Sati. My ex-colleague cum senior. A special thanks to you for investing your time to write a foreword for my book. From the moment when experts like you agreed to write for me, I got some additional motivation to enhance the quality of my work.

Celestin, thanks for giving me another opportunity to work with you and Apress. Laura, Amrita, Nagarajan, Sivachandran, Pradapsankar and Vinoth thank you for your exceptional support to beautify my work.

Lastly, I extend my deepest gratitude to my publisher, the editorial board members, and everyone who directly or indirectly supported this book.

Table of Contents

Part I: Gang of Four Patterns
Chapter 1:​ Singleton Pattern 3
GoF Definition 3
Concept 3
Real-World Example 3
Computer-World Example 4
Illustration 4
Class Diagram 4
Package Explorer View 5
Discussion 5
Implementation 6
Output 7
Q&​A Session 7
Output 11
Eager Initialization 12
Bill Pugh’s Solution 14
Double-Checked Locking 15
Chapter 2:​ Prototype Pattern 19
GoF Definition 19
Concept 19
Real-World Example 19
Computer-World Example 20
Illustration 20
Class Diagram 20
Package Explorer View 22
Implementation 23
Output 25
Q&​A Session 26
Demonstration 29
Output 31
Chapter 3:​ Builder Pattern 33
GoF Definition 33
Concept 33
Real-World Example 34
Computer-World Example 34
Illustration 35
Class Diagram 36
Package Explorer View 36
Implementation 38
Output 42
Q&​A Session 42
Modified Illustration 46
Modified Package Explorer View 46
Modified Implementation 48
Modified Output 52
Analysis 53
Chapter 4:​ Factory Method Pattern 55
GoF Definition 55
Concept 55
Real-World Example 56
Computer-World Example 56
Illustration 57
Class Diagram 57
Package Explorer View 58
Implementation 58
Output 61
Modified Implementation 61
Modified Output 63
Analysis 63
Q&​A Session 63
Chapter 5:​ Abstract Factory Pattern 67
GoF Definition 67
Concept 67
Real-World Example 68
Computer-World Example 68
Illustration 68
Class Diagram 70
Package Explorer View 71
Implementation 72
Output 76
Q&​A Session 76
Simple Factory Pattern Code Snippet 77
Factory Method Pattern Code Snippet 78
Abstract Factory Pattern Code Snippet 78
Conclusion 79
Modified Illustration 80
Modified Implementation 80
Modified Output 85
Chapter 6:​ Proxy Pattern 87
GoF Definition 87
Concept 87
Real-World Example 87
Computer-World Example 88
Illustration 88
Class Diagram 88
Package Explorer View 89
Implementation 90
Output 92
Q&​A Session 92
Alternate Implementation 93
Output Without Lazy Instantiation 95
Analysis 96
Output with Lazy Instantiation 96
Analysis 96
Modified Package Explorer View 98
Modified Implementation 99
Modified Output 101
Chapter 7:​ Decorator Pattern 103
GoF Definition 103
Concept 103
Real-World Example 103
Computer-World Example 105
Illustration 106
Class Diagram 106
Package Explorer View 107
Implementation 107
Output 110
Q&​A Session 111
Chapter 8:​ Adapter Pattern 117
GoF Definition 117
Concept 117
Real-World Example 117
Computer-World Example 118
Illustration 119
Class Diagram 120
Package Explorer View 120
Implementation 121
Output 123
Modified Illustration 123
Modified Class Diagram 123
Key Characteristics of the Modified Implementation 124
Modified Package Explorer View 126
Modified Implementation 127
Modified Output 130
Types of Adapters 130
Q&​A Session 132
Chapter 9:​ Facade Pattern 135
GoF Definition 135
Concept 135
Real-World Example 135
Computer-World Example 136
Illustration 136
Class Diagram 137
Package Explorer View 138
Implementation 139
Output 143
Q&​A Session 144
Chapter 10:​ Flyweight Pattern 147
GoF Definition 147
Concept 147
Real-World Example 148
Computer-World Example 148
Illustration 149
Class Diagram 150
Package Explorer View 150
Implementation 151
Output 157
Analysis 159
Q&​A Session 159
Chapter 11:​ Composite Pattern 165
GoF Definition 165
Concept 165
Real-World Example 166
Computer-World Example 166
Illustration 166
Class Diagram 167
Package Explorer View 168
Implementation 169
Output 174
Q&​A Session 176
Chapter 12:​ Bridge Pattern 179
GoF Definition 179
Concept 179
Real-World Example 179
Computer-World Example 180
Illustration 180
Class Diagram 183
Package Explorer View 184
Key Characteristics 185
Implementation 185
Output 189
Q&​A Session 190
Chapter 13:​ Visitor Pattern 193
GoF Definition 193
Concept 193
Real-World Example 194
Computer-World Example 194
Illustration 194
Class Diagram 195
Package Explorer View 196
Implementation 196
Output 198
Modified Illustration 198
Modified Class Diagram 204
Modified Package Explorer View 204
Modified Implementation 206
Modified Output 212
Q&​A Session 213
Chapter 14:​ Observer Pattern 217
GoF Definition 217
Concept 217
Real-World Example 220
Computer-World Example 220
Illustration 221
Class Diagram 222
Package Explorer View 222
Implementation 224
Output 227
Analysis 227
Q&​A Session 227
Chapter 15:​ Strategy (Policy) Pattern 233
GoF Definition 233
Concept 233
Real-World Example 233
Computer world Example 234
Illustration 234
Class Diagram 235
Package Explorer View 235
Implementation 237
Output 240
Q&​A Session 240
Chapter 16:​ Template Method Pattern 251
GoF Definition 251
Concept 251
Real-World Example 251
Computer-World Example 252
Illustration 252
Class Diagram 252
Package Explorer View 253
Implementation 254
Output 256
Q&​A Session 256
Modified Implementation 257
Modified Output 260
Chapter 17:​ Command Pattern 263
GoF Definition 263
Concept 263
Real-World Example 263
Computer-World Example 264
Illustration 264
Class Diagram 265
Package Explorer View 266
Implementation 267
Output 270
Q&​A Session 270
Modified Class Diagram 271
Modified Package Explorer View 272
Modified Implementation 274
Modified Output 280
Chapter 18:​ Iterator Pattern 285
GoF Definition 285
Concept 285
Real-World Example 286
Computer-World Example 287
Illustration 287
Class Diagram 288
Package Explorer View 290
First Implementation 291
Output 293
Key Characteristics of the Second Implementation 294
Second Implementation 294
Output 296
Q&​A Session 297
Third Implementation 299
Output 302
Chapter 19:​ Memento Pattern 303
GoF Definition 303
Concept 303
Real-World Example 303
Computer-World Example 304
Illustration 304
Class Diagram 305
Package Explorer View 306
Implementation 306
Output 309
Q&​A Session 310
Modified Caretaker Class 311
Modified Output 312
Analysis 313
Shallow Copy vs.​ Deep Copy in Java 321
Chapter 20:​ State Pattern 329
GoF Definition 329
Concept 329
Real-World Example 330
Computer-World Example 330
Illustration 330
Key Characteristics 332
Class Diagram 332
Package Explorer View 334
Implementation 335
Output 339
Q&​A Session 340
Modified Package Explorer View 343
Modified Implementation 345
Modified Output 350
Chapter 21:​ Mediator Pattern 353
GoF Definition 353
Concept 353
Real-World Example 353
Computer-World Example 354
Illustration 355
Class Diagram 356
Package Explorer View 357
Implementation 359
Output 363
Analysis 363
Modified Illustration 363
Modified Class Diagram 365
Modified Package Explorer View 366
Modified Implementation 367
Modified Output 372
Analysis 373
Q&​A Session 373
Chapter 22:​ Chain-of-Responsibility Pattern 377
GoF Definition 377
Concept 377
Real-World Example 378
Computer-World Example 378
Illustration 379
Class Diagram 380
Package Explorer View 381
Implementation 382
Output 385
Q&​A Session 386
Chapter 23:​ Interpreter Pattern 389
GoF Definition 389
Concept 389
Real-World Example 391
Computer-World Example 391
Illustration 391
Class Diagram 393
Package Explorer View 394
Implementation 395
Output 399
Analysis 400
Modified Illustration 400
Modified Class Diagram 400
Modified Package Explorer View 400
Modified Implementation 401
Modified Output 406
Analysis 406
Q&​A Session 407
Part II: Additional Design Patterns
Chapter 24:​ Simple Factory Pattern 411
Intent 411
Concept 411
Real-World Example 411
Computer-World example 412
Illustration 413
Class Diagram 413
Package Explorer View 414
Implementation 415
Output 417
Q&​A Session 419
Chapter 25:​ Null Object Pattern 421
Concept 421
A Faulty Program 422
Output with Valid Inputs 424
Analysis with an Unwanted Input 424
Encountered Exception 425
Immediate Remedy 425
Analysis 425
Real-World Example 426
Computer-World Example 426
Illustration 426
Class Diagram 427
Package Explorer View 428
Implementation 429
Output 432
Analysis 433
Q&​A Session 433
Chapter 26:​ MVC Pattern 437
Concept 437
Key Points to Remember 438
Variation 1 439
Variation 2 439
Variation 3 440
Real-World Example 440
Computer-World Example 441
Illustration 442
Class Diagram 442
Package Explorer View 444
Implementation 444
Output 452
Q&​A Session 453
Modified Output 455
Part III: Final Discussions on Design Patterns
Chapter 27:​ Criticisms of Design Patterns 461
Q&​A Session 463
Chapter 28:​ AntiPatterns:​ Avoid the Common Mistakes 467
What Is an Antipattern?​ 467
Brief History of Antipatterns 468
Examples of Antipatterns 469
Types of Antipatterns 471
Q&​A Session 471
Chapter 29:​ FAQs 475
Appendix A:​ A Brief Overview of GoF Design Patterns 481
Key Points 482
A.​ Creational Patterns 483
B.​ Structural Patterns 483
C.​ Behavioral Patterns 484
Q&​A Session 486
Appendix B:​ Winning Notes and the Road Ahead 489
Appendix C:​ Bibliography 491
Index 493

About the Author and About the Technical Reviewers

About the Author

Vaskaran Sarcar
../images/395506_2_En_BookFrontmatter_Figb_HTML.jpg

obtained his Master of Engineering degree from Jadavpur University, Kolkata. Currently, he is senior software engineer and team lead in the R&D Hub at HP Inc. India. He was a national Gate Scholar and has more than 12 years of experience in education and the IT industry. He is an alumnus of prestigious institutions in India, such as Jadavpur University, Vidyasagar University, and Presidency University (formerly Presidency College). Reading and learning new things are his passions. You can connect with him at [email protected] or find him on LinkedIn at www.linkedin.com/in/vaskaransarcar .

Other books by Vaskaran include the following:

  • Design Patterns in C# (Apress, 2018)

  • Interactive C# (Apress, 2017)

  • Interactive Object-Oriented Programming in Java (Apress, 2016)

  • Java Design Patterns (First Edition) (Apress, 2016)

  • C# Basics: Test Your Skill (CreateSpace, 2015)

  • Operating System: Computer Science Interview Series (CreateSpace, 2014)

 

About the Technical Reviewers

Shekhar Kumar Maravi
../images/395506_2_En_BookFrontmatter_Figc_HTML.jpg

is a system software engineer whose main interests are programming languages, algorithms, and data structures. He obtained his master’s degree in computer science and engineering from the Indian Institute of Technology, Bombay. After graduation, he joined Hewlett-Packard’s R&D Hub in India to work on printer firmware. Currently, he is a technical lead for automated lab diagnostic device firmware and software at Siemens Healthcare India. He can be reached by email at [email protected] or via LinkedIn at www.linkedin.com/in/shekharmaravi .

 
Ritesh Jha
../images/395506_2_En_BookFrontmatter_Figd_HTML.jpg

is passionate about large-scale distributed systems. Currently, he is working as a senior development engineer for the Supply Chain Technology Group at Walmart Labs. Before Walmart, he worked at eBay and Hewlett-Packard. He has a BE in computer science from Jadavpur University, Kolkata. When he is not exploring new technologies, he can be found exploring new places on his bike. He can be reached by email at [email protected] or via LinkedIn at www.linkedin.com/in/riteshjha9/ .

 
Ankit Khare
../images/395506_2_En_BookFrontmatter_Fige_HTML.jpg

is a senior software engineer with expertise in software architecture and designing, programming languages, algorithms, and data structure. After obtaining a BE in computer science, he joined Hewlett-Packard’s R&D Center in India in 2010, where he worked with various laser-jet firmware teams. He is currently involved in future machine vision development for print image diagnostic tools involving ink-jet, large-format, and laser-jet printers. He can be reached by email at [email protected] or via LinkedIn https://www.linkedin.com/in/khareankit/ .

 
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