Java Puzzlers: Traps, Pitfalls, and Corner Cases

My wife popped this book open after dinner. Big mistake -- we had planned to spend the night watching Firefly on DVD. She read the first puzzle. We went to the blackboard (yes, we're so geeky and our NY apartment's so small that there's a blackboard in the dining nook). Between us, we had half a dozen possible answers about what a three-line program was going to do. We found at least four boundary conditions and were pretty sure about two of them. For the record, the first puzzle she opened to involved the compound XOR assignment statement x^=y^=x^=y. They're not all that bit-fiddly; some of the other puzzles include class and method mazes, integer or double arithmetic oddities, unexpected exception/initialization interactions, string/charset twistiness, etc.

I thought I'd be good at this kind of puzzle. As an academic, I wrote about programming languages. I read Bloch's "Effective Java" book. Twice. I follow its advice religously and make my coworkers read it. I've read most of the source code for String, StringBuffer and the collections framework and I/O streams. I just came off a week-long coding project where I did exclusively bit-level I/O with all the shifts and masks you could ask for. I was wrong. I got about 1/5 of the puzzles right if I give myself partial credit for diagnosing the boundary condition in the question and having the right answer be in my top two or three guesses.

Unless you've written the bit fiddling parts of a JVM implementation, or are the kind of person who can remember minute details of the specification, you'll most likely suffer. And love it. Then you can relate the puzzles at gatherings of geeks and look on with a smug grin as they twist in the wind. These would be perfect interview questions for a sadistic HR person.

Overall, this book's a jaw-dropping, eye-opening, brain-melting overview of the kind of boundary conditions you can run into with very simple constructions. Most of the puzzles seem to involve implicit conversions done by the JVM, some involve 1.5 features, some involve class inheritance, others exceptions. Almost all of the puzzles contain links to the boundary-condition definition in the Java language spec.

I'll do better next time. Really.

This book is filled with brainteasers about the Java programming language and its core libraries. Anyone with a working knowledge of Java can understand these puzzles, but many of them are tough enough to challenge even the most experienced programmer. Puzzlers are grouped according to the features they use, but you cannot assume that the trick to a puzzle is related to its chapter heading.

Most of the puzzles exploit counterintuitive or obscure behaviors that can lead to bugs. Every platform has them, but Java has far fewer than other platforms of comparable power. The goal of the book is to entertain the reader with puzzles while teaching you to avoid the underlying traps and pitfalls. By working through the puzzles, you become less likely to fall prey to these dangers in your own code and more likely to spot them the code of others over which you have maintenance priveleges.

This book is meant to be read while you have access to a computer that has a Java development environment installed, ideally JDK 5.0, which is the latest release at the time I am writing this. That is because some of the puzzles rely on pitfalls in this particular release of Java.

Most of the puzzles take the form of a short program that appears to do one thing but actually does something else. It's the reader's job to figure out what each program does. It would be best if you first study the program/puzzle and determine what you think it will do. Next, run the program and see if its expected behavior matches its actual behavior. Try to fix the program if you believe it is "broken". Finally, read the solution and see if it matches your answer. What is really great about this book is that it sticks to pitfalls in the core language and doesn't delve into any of the add-on API's or J2EE. You'll be surprised that so many pitfalls can be conjured up in the core language. Amazon does not show the table of contents, so I do that here along with a brief description of the type of puzzles in each chapter.

1. Introduction
2. Expressive Puzzles - The puzzles in this chapter are simple but not necessarily easy and involve only expression evaluation. My personal favorite : the statement "System.out.println(2.00 - 1.10);" displays 0.8999999999999999 instead of .9. There is a solution, but it is not pretty and showcases a pretty bad weakness in the Java language.
3. Puzzlers with Character - This chapter contains puzzles that concern strings, characters, and other textual data. This section contains several puzzles involving unicode characters, and one is a cautionary tale for language designers in character overloading. Example: System.out.print('H' + 'a'); prints the number 169 not the word "Ha" as you might imagine.
4. Loopy Puzzlers - All the puzzles in this chapter concern loops, such as coming up with declarations that turn simple loops into infinite ones.
5. Exceptional Puzzlers - The puzzles in this chapter concern exceptions and the closely related Try-finally statement. Most exhibit odd behavior such that simple changes in the program cause completely different types of exception handling to occur.
6. Classy Puzzlers - This chapter contains puzzlers that concern the use of classes and their instances, methods, and fields.
7. Library Puzzlers - The puzzles in this chapter concern basic library-related topics, such as Object methods, collections, Date, and Calendar. One particularly interesting puzzler illustrates that, in Java, integer literals beginning with a "0" are interpreted as octal values. This obscure construct is a holdover from the C programming language and the 1970s, when octal was much more commonly used than today. Thus "012" is seen by Java as 10 base 10.
8. Classier Puzzlers - The puzzles in this chapter concern inheritance, overriding, and other forms of name reuse.
9. More Library Puzzlers - The puzzles in this chapter feature more advanced library topics, such as threading, reflection, and I/O. Here you will learn, for example, that "write(int)" is the only Java output method that does not flush a PrintStream on which automatic flushing is enabled. Thus you must explicitly invoke "flush" on its stream to print any message, making the "write" method seem unfriendly and outright pointless.
10. Advanced Puzzlers - The puzzles in this chapter concern advanced topics, such as nested classes, generics, serialization, and binary compatibility.
A. Catalog of Traps and Pitfalls - This chapter contains a concise taxonomy of traps and pitfalls in the Java platform. Each entry in the catalog is divided into three parts - A short description of the pitfall, how to avoid the trap, and pointers to additional information on the trap.
B. Notes on the Illusions - This appendix contains brief descriptions of the graphical illusions that appear throughout the book. The descriptions are grouped loosely by category. Within each category, the order is roughly chronological.

This book is very good practice for anybody who enjoys programming in the Java language, but it will probably appeal the most to the geekiest among us of which I proudly count myself.

Many C and C++ books exist that discuss traps and pit falls with each language. Now Java has such a book. This book is fun to read and will challenge even the best Java programmers. Be sure to get the source code from [...] Study each puzzle and try figure out what it does or does not do. Then run the example code and see if you were right. If you weren't right, then try to figure out why you guessed wrong and figure out how to fix the program. Then turn the page and read the solution.

Working through the puzzlers is not only fun, but it will definitely make you a much better Java programmer and a better troubleshooter.

I've been passing this book around my office, describing it alternately as "C Traps and Pitfalls on steroids" or "The most fun that can be had with the Java Language Specification with your clothes on." Thus far, it has not failed to frustrate, educate, enlighten, and amuse each reader, usually in that order. I can't think of any better way to become more familiar with the ins, outs, and warts of the Java language: the puzzles, memorable and clever as they are, stick with you.

The writing is fluid, friendly, and full of wit, from the names of the puzzles (like "The Lock Mess Monster" or "Dyslexic Monotheism") to the commentary on the finer points. Even the layout shows thought, with the answers out of plain sight behind the next page.

If your brain burns out on the puzzles, check out the optical illusions in between. (Note: do not, during a car ride, show them to anyone who is prone to motion sickness. I learned this the hard way, so you don't have to.)

I strongly recommend this book if you spend any time in or around Java code. I've had to deal with many of the pitfalls it highlights, and boy, would it have been more pleasant if I'd already read this book.