Skip to main content

Java Puzzler

I bought the book Java Puzzlers by Josh Bloch and Neal Gafter a few years ago and enjoyed it. If you aren't familiar with it, it covers rare, odd and usually counter-intuitive edge cases of Java in a brain-teaser style of presentation. For both a Java user and an analytical mind its a fascinating book.

Well today I stumbled onto a puzzler of my own and thought I'd share. Can you tell me what the main method of this class will output?
1 : public class Parser {
2 :
3 : private static <T> T parse(String s, Class<T> type) {
4 : // Simple implementation only supports long primitive
5 : if(type == long.class) {
6 : return (T) Long.parseLong(s);
7 : }
8 : throw new UnsupportedOperationException(
9 : "parse() only supports long right now!");
10: }
11:
12: public static void main(String[] args) {
13: System.out.println(parse("1234", long.class).equals(1234));
14: }
15: }

So what gets written to standard output? If you said 'true', then you'd be wrong. If you said 'false', then you'd also be wrong! The code above won't even compile! There are a few features of Java that are colliding here to give odd results, including generics, type erasure, primitive types and autoboxing.

So what happens exactly? You'll get a compile error like this:
Error:line (6) inconvertible types
found : long
required: T
The error is on the return statement line of the parse() method. Long.parseLong() returns a primitive long, but since a parameter type can only represent an Object type, you can't cast a primitive to an parameter type. Also, it appear auto-boxing doesn't work in this case, so it fails with a compiler error.

But wait, doesn't the code explicitly use the Class object that represents the long primitive, long.class? But see 'long.class' is syntactic sugar for the constant field Long.TYPE which is of the parameterized type Class<Long> since type variables only support using Objects.

So realizing this, we can change the return statement to use a Long instead of a long:
    return (T) Long.valueOf(s);
Now the code will compile, but something is still wrong because it prints out 'false'.

Knowing that the return type of parse("1234", long.class) is Long, take another look at the puzzle. Can you see the problem? This one is subtle. The numeric literal '1234' that is inside the call to equals() needs to be auto-boxed and since it is a literal int, it is boxed into a Integer. But an Integer object isn't considered equal to a Long object because they are different types, so equals() returns false.

So it a few interesting features combine for some strange behavior. Hope you enjoyed this puzzle!

Comments

Neal Gafter said…
Most of the puzzlers in our book aren't rare; in fact, they're taken out of actual problems people have encountered while writing production code. However, the issues are distilled to their essence in the book.
My apologies if I mischaracterized it. It is a great book and one I recommend to other programmers as a must-read because its so informative and accessible.

Popular posts from this blog

TeamCity build triggering by GitHub

So I started using GitHub for a side project and discovered their very cool feature of service hooks. A service hook allows a repository administrator to setup a callback to another service when a commit is made to the repository. For example it can send an email, or chat a message via Jabber.

Now continuous integration servers, like TeamCity, can poll source control systems every few minutes to see if any changes have been committed. But wouldn't it be more efficient to use a service hook to trigger a build?
Looking at GitHub's service hooks, there wasn't one already available to callback a TeamCity server, but right on that same page was a link to the open source repository for GitHub Service Hooks. They "eat their own dogfood" so to speak and make it very easy to contribute new service hooks back to them. So I took an evening, did my first Ruby coding in a while which included more time getting Ruby setup and working on my Macbook than actually coding. In a …

My Journey to Fitness, a 5K, and my first Triathlon

My name is Brian and Sunday I became a triathlete. My journey started ten months ago when I decided to get back into shape after 15 years of being obese and out-of-shape with some yo-yo dieting in the middle. What changed? I'll get to that.
This weekend I competed in the first ever Rocketman Florida Triathlon which took place on the grounds of Kennedy Space Center at Cape Canaveral. In preparation I lost 50 lbs and 12 inches from my waist. But I'm getting ahead of myself.
I'm a huge space buff. As a kid I wanted to become an astronaut. I went to Space Camp in Titusville when I was 10. Before that, I saw my first shuttle launch at 7 while on vacation. It was the final launch of the Challenger. I've written about that experience. I've seen three other launches since then including John Glenn's famous return to space as well as the final launch that ended the U.S. Shuttle Program.
The idea of biking on the restricted grounds and getting closer than any civilian h…

Paperless

I've been slowly going paperless over the past decade. The first step on my journey started in 2000 when I signed up to use a payment service, PayTrust, to receive my incoming bills, scan them, and put them online for me to pay. The next major step was probably when I got a digital camera to replace my traditional film cameras. It might not be considered a "paperless" use case, but it has lead to very little hardcopies over the years as monitors and HDTV with screensavers and AppleTVs have become so beautiful.  Back to the paperless office, my next big step was eFileing my taxes but that didn't come until about 5 years later. Then suddenly about two years ago, I hit a real shift in my desire to go completely paperless when I got my iPad and installed Evernote.

digital notes...
If you aren't familiar with Evernote its an excellent app, available on all the major desktop and mobile OSes, that makes note-taking and organizing really simple. The killer feature is …