Start Concurrent: A Gentle Introduction to Concurrent Programming

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Recent update!

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About

Welcome to Start Concurrent! This book is intended as an entry point into the challenging realm of multicore computing. It’s designed to introduce students to concurrent programming at the same time they are learning the basics of sequential programming, early in their college days. After mastering the concepts covered here, students should be prepared when they encounter more complex forms of concurrency in advanced courses and in the workplace. A generation of students who learn concurrency from their first course will be ready to exploit the full power of multicore chips by the time they join the workforce.

Multicore processors are omnipresent. Whether you use a desktop, a laptop, or even just a smart phone, chances are that your computer has a multicore chip at its heart. Inexpensive parallel computers have been prophesied for years. That time has come. Parallel computers sit on our laps and in our pockets. This progress in microprocessor technology has thrown a challenge to educators: How can we teach concurrent programming?

Computer programming has been taught in academia for decades. However, the unwritten goal in nearly every beginning programming class has been teaching students to write, compile, test, and debug sequential programs. Material related to concurrent programming is often left to courses about operating systems and programming languages or courses in high performance computing. Now that parallel computers are everywhere, should we consider introducing the fundamentals of concurrent programming in beginner classes in programming?

For our part, we believe that concurrent programming can be, and should be, taught to first year students. This book aims at introducing concurrent programming from almost the first day. The rationale for our belief stems from another belief that procedural thinking, sequential as well as concurrent, is natural. People knew how to solve problems in a sequential manner, long before the study of algorithms became a formal subject and computer science a formal discipline. And this rationale applies to problem solving using a collection of sequential solutions applied concurrently. Watch a cook in the kitchen and you’ll see concurrency in action. Watch a movie and you’ll see concurrency in action as various subplots, scenes, and flashbacks weave the story together. Parents use concurrent solutions to solve day-to-day problems as they juggle childcare, a career, and a social life.

If people naturally solve problems sequentially and concurrently, why do we need to teach them programming? Programming is a way to map an algorithmic solution of a problem to an artificial language such as Java. It’s an activity that requires formal analysis, specialized vocabulary, and razor sharp logic. The real intellectual substance of programming lies in this mapping process. What’s the best way to transform a sequential solution to an artificial language? How can a sequential solution be broken into concurrent parts that run faster than the original? How can a large problem be divided into small, manageable chunks that can be programmed separately and then integrated into a whole? In addition, there are issues of testing, debugging, documentation, and management of the software development process, which combine to make programming a limitless field for intellectual effort.

Target audience

This book is intended to teach college level students with no programming experience over a period of two semesters. Although we start with concurrency concepts from the very beginning, it’s difficult for students with no prior programming experience to write useful multithreaded programs by the end of their first semester. By the end of the second semester, however, this book can lead a student from a blank slate to a capable programmer of complex parallel programs that exploit the power of multicore processors.

The content in this book could also be used for single semester courses. Chapters 1 through 12 are intended for the absolute beginner. If you don’t want to introduce concurrent programming in a first course, these chapters should prove adequate. The concurrency material and exercises in these chapters can be ignored without negatively impacting the other material. For a second course in programming, Chapters 1 through 12 should be used as review material as well as an introduction to concurrent programming. Most material from Chapters 13 onward could then be covered in a single semester.

Material covered

Java is a complex language. Its long list of features makes it difficult for an instructor to decide what to cover and what to leave out. Often there’s a tendency to cover more material than less. We’ve noticed that today’s student uses not only a textbook but also the large volume of material available on the web to learn any subject, including programming. Our focus is consequently more on fundamental elements of programming and less on giving a complete description of Java, which is itself still a living and evolving language. Where appropriate we direct the student to websites where relevant reference material can be found.

Classes and objects are an essential part of Java. Some educators have adopted an “objects early” approach that focuses heavily on object oriented principles from the very beginning. Although we see many merits in this approach, we feel compelled to start with logic, arithmetic, and control flow so that students have a firm foundation of what to put inside their objects. A full treatment of classes and objects unfolds throughout the book, moving naturally from monolithic programs to decomposition into methods to full object orientation.

Organization

The material covered can be divided up in different ways depending on the needs of the instructor or the student. Chapters 1 through 12, with the exception of Chapter 7, are designed to introduce the student to Java and programming in general. Chapters 7 and 15 cover material related to graphical user interfaces and can be skipped if these topics are not of interest. Chapters 13 and 14 give an in-depth treatment of the concurrency features of Java. Although we make an effort to mark concurrency material and keep it independent from the rest of the content, those chapters numbered 15 and higher will assume some knowledge and interest in concurrency. Chapter 15 itself covers debugging and testing, which is even more crucial in a concurrent environment. The rest of the book covers advanced material relating to OO design, data structures, and I/O.

Chapter layout

One feature of this book that separates it from many Java textbooks is its problem-driven approach. Most chapters are divided into the following parts.

Problem: A motivating problem is given at the beginning of almost all chapters. This problem is intended to show the value of the material covered in the chapter as well as sketching a practical application.
Concepts: One or more short sections devoted to concepts is given in each chapter. The concepts described in these sections are the fundamental topics covered in the chapter, as well as main ideas needed to solve the chapter’s motivating problem. These concepts are intended to be broad and language neutral. Java syntax is kept to an absolute minimum in these sections.
Syntax: Each chapter has one or more sections describing the Java syntax needed to implement the concepts already described in the Concepts sections. These sections are typically longer and have numbered examples in Java code sprinkled throughout.
Solution: After the appropriate concepts and Java syntax needed to solve the motivating problem have been given, a solution to the motivating problem is provided near the end of the chapter. In this way, students are given plenty of time to think about the approach needed to solve the problem before the answer is given.
Concurrency: For all of the chapters except for Chapters 13 and 14, the dedicated concurrency chapters, additional relevant concurrency concepts and syntax are introduced in these specially marked sections, spreading concurrency throughout the book.
Exercises: Each chapter ends with exercises, which are divided into three sections: Conceptual Problems, Programming Practice, and Experiments. Most Conceptual Problems are simple and are intended as a quick test of the student’s understanding. Problems in Programming Practice require students to implement a short program in Java and can be used as homework assignments. Experiments are a special feature of this textbook and are especially appropriate in the context of concurrency. Experiments focus on the performance of a program, usually in terms of speed or memory usage. Students will need to run short programs and measure their running time or other features, gaining practical insight into speedup and other advantages and challenges of concurrency. References to exercises are given throughout the chapter text.

We hope that structuring chapters in this way can be useful for many different kinds of readers. Novice programmers may wish to read each chapter from start to end. Experienced programmers who have never programmed Java may focus primarily on the Syntax sections to learn the appropriate Java syntax and semantics. Rusty Java programmers may prefer to focus on the clearly numbered examples and exercises. Of course, instructors are encouraged to use the motivating problems to motivate their lectures as well.

In addition, specially marked Pitfall sections throughout the book highlight common programming errors and mistakes.

What topics does this book not cover?

This book is not intended to be a comprehensive guide to Java. Instead, it’s intended to teach how to use computers to solve problems, especially concurrently. Java has a marvelous wealth of packages and libraries that we don’t have the space to cover. For example, the Swing package for building user interfaces is discussed but not in its entirety. For material not found in this book, we expect students to refer to the material available on the Oracle Java tutorial website and other reference books and websites.

Suggestions

Java IDE: It’s important that the students be introduced to a Java IDE very early in the course. We recommend that students use a simpler rather than a more complex IDE. We have successfully used DrJava though other simple IDEs might work just as well. For instructors hoping to give their students experience with an industry-level IDE, we give examples using Eclipse as well as DrJava in the chapter on testing and debugging and a few other times when relevant.
Concurrency at the start: Many courses begin with a lecture or two on the relationship between problem-solving and computers. Chapter 2 covers this topic. During these very early lectures, instructors can introduce the notions of both sequential and concurrent solutions. One could use simple problems from areas such as mathematics or physics or even day-to-day life that lead to sequential and concurrent solutions. Early exposure to solutions these problems, programmed in Java, can be beneficial students even if they don’t understand all the syntax.
Input and output: The issue of what input and output material to cover can be dealt with in several ways. While programming attractive GUIs may be exciting, some instructors prefer to postpone detailed treatment of GUI-related material until late in a course. In this book we’ve decided to follow a flexible approach. We begin by discussing the use of System.out.print() and Scanner and the JOptionPane class as alternatives for basic input and output. Our assumption is that most instructors will prefer the simplicity of command line I/O; however, those who favor a more GUI-heavy approach can start early in Chapter 7 for GUI basics and eventually move onto Chapter 15 for more depth in GUIs and Swing. Instructors who wish to concentrate only on command line I/O are free to ignore these chapters.

Acknowledgements

A number of people have played a significant role in motivating the authors to undertake the task of writing this book and in the choice of topics. First, during the spring of 2008, several faculty from the Department of Computer Science and a scientist from Purdue’s ITaP, participated in early discussions related to the teaching of concurrent programming in freshman classes. Despite the multitude of issues raised, all participants seemed to agree on one point: that we ought to introduce concurrency early in the Computer Science undergraduate curriculum. Thanks to all the participants, namely, Buster Dunsmore, Ananth Grama, Suresh Jagannathan, Sunil Prabhakar, Faisal Saied, and Jan Vitek. We benefited from advice, encouragement, and support from a number of alumni and corporate partners; special thanks to Kevin Kahn, Andrew Chien, and Carl Murray.

Thanks to the many anonymous reviewers who carefully read through Draft 3.0 of this manuscript and made valuable suggestions. This current draft would not exist without the many suggestions and critiques of these reviewers.

In the fall of 2008, we offered an experimental freshman class entitled “Introduction to Programming with Concurrency.” This class was certainly one of the best we have taught to freshmen. Thanks to Alexander Bartol, Alexander Coe, Eric Fisher, Benjamin Gilliland-Sauer, John Graff, Tyler Holzer, Kelly, Jordan Kelly, Azfar Khandoker, Zackary Naas, Ravi Pareek, Carl Rhodes, Robert Schwalm, Andrew Wirtz, and Christopher Womble.

Special thanks to Azfar Khandoker who not only attended this initial class, but also worked on an independent study project to create exercises using Lego robots to help students learn programming. Azfar’s work has led to the use of robots in two freshman programming classes taught at Purdue.

We appreciate the support and cooperation of the faculty, and their students, who are our first test users: Professor David John of Wake Forest University and Professor Sunil Prabhakar of Purdue University.

Finally, we thank everyone in the open source community who has submitted issues, questions, or pull requests through Github.