|Java Generics and Collections|
|author||Maurice Naftalin, Philip Wadler|
|publisher||O'Reilly Media, Inc.|
|summary||Guide to Java generics; also includes interesting discussion of collection classes.|
Given the history of Generic Java, Naftalin and Wadler's Java Generics and Collections has a distinguished pedigree. In this review I'll argue that this is a new classic.
If you're a Java programmer you've probably heard of generics, an extension to the type system that was introduced in Java 5. They give you, as a programmer, a way to write code even when you don't know exactly what classes will be used.
The obvious example is collections — the author of a List class has no idea what type of objects will be stored when the code is used.
Before generics, if you wanted to write code that handled unknown classes you had to use make use of inheritance: write the code as if it would get Objects, and then let the caller cast the result as necessary. Since casts happen at runtime any mistakes may cause a runtime error (a ClassCastException).
Generics fix this. They let you write code in which the classes are named (parameters) and the compiler can then check that the use of these class parameters is consistent in your program. So if you have a List of Foo instances you write List<Foo> and the compiler knows that when you read that list you will receive a Foo, not an Object.
I'll get to the book in a moment, but first a little history. If you know any type theory — particularly as used in functional languages like ML and Haskell — then you'll recognize my quick description above as parametric polymorphism. You'll also know that it is incredibly useful, and wonder how Java programmers could ever have managed without it.
Which explains why Philip Wadler, one of the people responsible for Haskell, was part of a team that wrote GJ (Generic Java), one of the experimental Java mutations (others included PolyJ and Pizza) that, back in the day (late 90s) helped explore how parametric polymorphism could be added to Java, and which formed the basis for the generics introduced in Java 5.
So if you want to understand generics, Wadler is your man. Which, in turn, explains why I jumped at the chance to review O'Reilly's Java Generics and Collections, by Maurice Naftalin and Philip Wadler.
This is a moderately slim book (just under 300 pages). It looks like any other O'Reilly work — the animal is an Alligator this time. It's well organized, easy to read, and has a decent index.
There's an odd discrepancy, though: Wadler is the generics Guru; this is going to be `the generics reference'; generics are sexy (in relative terms — we're talking Java here) and collections are not; the title has "Java Generics" in great big letters with "and Collections" in little tiny ones down in a corner. Yet very nearly half this book is dedicated to collections.
Generics is a great, practical read. It starts simply, introducing a range of new features in Java 5, and then builds rapidly.
If you are completely new to generics, you'll want to read slowly. Everything is here, and it's very clear and friendly, but there are not the chapters of simple, repeated examples you might find in a fatter book. Within just 30 pages you meet pretty much all of generics, including wildcards and constraints.
If that makes your head spin, don't worry. Read on. The next hundred or so pages don't introduce any new syntax, but instead discuss a wide range of related issues. The chapters on Comparisons and Bounds and Declarations contain more examples that will help clarify what generics do. And the following chapters on Evolution, Reification, and Reflection will explain exactly why.
So the first seven chapters introduce generics and then justify the implementation — any programmer that takes the time to understand this will have a very solid base in generics.
There are even some interesting ideas on how Java could have evolved differently — section 6.9 Arrays as a Deprecated Type presents a strong case for removing arrays from the language. It's a tribute to the clarity and depth of this book that the reader is able to follow detailed arguments about language design. Fascinating stuff.
The next two chapters, however, were my favorites. Effective Generics and Design Patterns give sensible, practical advice on using generics in your work, including the best explanation of <X extends Foo<X>> I've seen yet (so if you don't know what I am talking about here, read the book).
(A practical word of advice — if at all possible, use Java 6 with generics. Java 5 has a sneaky bug).
The Collections part of the book was more along O'Reilly's `Nutshell' lines: the different chapters explore different collection types in detail. I must admit that at first I skipped this — it looked like API docs re-hashed to extend the size of the book.
Then I felt bad, because I was supposed to be reviewing this book (full disclosure: if you review a book for Slashdot you get to keep it). And you know what? It turned out to be pretty interesting. I've programmed in Java for (too many) years, and I guess I've not been quite as dedicated to tracking how the library has changed as I should have been — I learned a lot.
Again, a wide range of readers are welcome. This is more than a summary of the Javadocs, ranging from thumbnail sketches of trees and hashtables to a discussion of containers intended for multi-threaded programming.
The way I see it now, this part is a bonus: the first half, on generics, makes this book one of the standards; the second half is an extra treat I'm glad I stumbled across (I guess if you're some kind of weird collection-fetishist maybe it's even worth buying the book for).
I've used generics since the first beta release of Java 5 and had experience with parametric polymorphism in functional languages before that (in other words, I can tell my co- from my contra-variance). So I guess I'm heading towards the more expert end of the spectrum and I was worried I'd find the book boring. It wasn't. After claiming to be expert I don't want to spoil things with evidence that I'm actually stupid, but reading this book cleared up a few `misunderstandings' I'd had. I wish I had read it earlier.
If you're new to generics, and you don't mind thinking, I recommend this book. If you're a Java programmer who's a bit confused by <? super Foo> then this is the book for you.
The only people who shouldn't read this are people new to Java. You need to go elsewhere first. This is not a book for complete beginners. This is a great book in the classic — practical, concise and intelligent — O'Reilly mould.
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