PECS in Action

One funny bit at JavaOne was Josh Bloch introducing the mnemonic PECS. Well actually what was most funny about it was the picture of Arnold Schwarzenegger that accompanied it... Anyways, PECS stands for producer-extends, consumer-super. Instead of repeating what Josh said or plagiarizing Effective Java, I will give an example of how PECS helped me out yesterday.

One there was an API that existed back in olden times, before Java 1.5 It looked like this:

void runInboundCycles(final Module[] modules)

There was also a runOubound, but you get the picture. The class Module is an interface that has many implementations. This API got tweaked courtesy of Java 1.5:

void runInboundCycles(final List<Module> modules);

Before the change you could do this:

runInboundCycles(new Module[] { new MyModule() } );

A logical uprev would be:

runInboundCycles(Arrays.asList(new MyModule()));

Turns out that won't compile! The Arrays.asList call will return a List and the Java compiler says that is not a List. So instead you have to do something annoying like:

List<Module> modules = new ArrayList<Module>(1);
modules.add(new MyModule());
runInboundCycles(modules);

This is particularly annoying if modules is actually a member variable, as now you cannot declare it to be final. Enter PECS.

My API is using the Modules, thus my parameter is a producer to the API. Remember producer-extends, so refactor the API like this:

void runInboundCycles(List<? extends Module> modules);

Now you can pass a List<MyModule> and the compiler won't complain.

There are a couple of things about this that bother me. When my crazy brain looks at the API, it thinks "the API consumes Modules." Maybe that's just me. The other thing that bothers me is writing ? extends Module because Module is an inteface. Now granted it would suck to have to write ? implements Module just because Module is an interface and write ? extends Module just because Module was a class, so I am not advocating the altnernative. It just feels weird to write extends in front of an interface type. Maybe I have been programming in Java too long.

JavaOne 2008

I've been at JavaOne all week, but not blogging. That is partially because of the horrible Wi-Fi at JavaOne this year. Luckily it is good today, so I am blogging before Josh Bloch's Effective Java talk. I also just picked up the new edition of Effective Java, as if I could somehow read the whole thing before the talk ... Probably should have waited to buy it at the end of the day since I won't be reading it until the train ride home. Anyways...

So what's been good at JavaOne this year? Well ... no big news, really. We see more meat on JavaFX and I guess that was the lead story at the opening keynote. It seems like JFX is about where Silverlight was a year ago. The JVM 6, update 10 does rock. You can do so much more with Java than with Flash or Silverlight, regardless of JFX, so it will be interesting to see if people leverage that.

Along those lines, the most interesting session I saw this week was by the guys from Ajaxian, Dion Almaer and Ben Galbraith. Their talk was titled "What's new in Ajax." The subject matter was interesting, but that's not what made their talk so awesome. They are outstanding speakers, and they seem to have a great rapport between them. Their talk seemed very conversational. It also definitely stood out for NOT having to follow the Sun presentation template. It was a Keynote presentation full of cool graphics, animations, and even some short video interviews with some of the top dogs in the world of Ajax frameworks. 

They also did two cool demos, one with Fluid. I fee like such a nov for not having seen this before. It is awesome. They also did an awesome demo where they "threw" a dart at a dart board. The dart board was an Ajax app running the in the browser. The dart was a Wiimote! They used the Bluetooth connection of the Wiimote to send to the PC, and then bridged the Bluetooth stack on the PC to the browser using (drum roll please) a Java applet. The applet was just for communicating with the OS, all of the graphics were in HTML and the interactivity was JavaScript. Very freakin' cool. 

The other good session I went to was Alex Miller's talk on design patterns. Most of this was stuff I was familiar with, like problems with the Singleton and Visitor patterns. What was interesting to me was that Alex showed how closures could dramatically change the implementation of some of these patterns. He showed this for a template pattern. Both template and strategy patterns are kind of obvious targets for refactoring with closures. What surprised me was how a visitor pattern could be refactored using closures. I was inspired to start playing with design patterns in Scala, since that is the future of Java (in my opinion.) I tweeted this to Alex, and he pointed me to some writings on that exact topic. I still plan on doing my own exposition, as I think it will be fun.

JavaOne Day Three

I'll keep this pretty short for now, since the Wi-Fi is definitely not holding up as well today as it did earlier in the week!

I went to two sessions this morning. The first was on Web Beans. Gavin King did a great job explaining the ideas and the possibilities created by Web Beans. Bob Lee added some bits on some of the things in Guice that will make it into Web Beans. You can tell things are at an early stage, and it's hard to know if Web Beans will just be a superset of Seam and Guice. From my perspective, Seam is more powerful, but Guice is more elegant. Two interesting questions in the Q&A. First, somebody asked about involvement by the Spring guys on Web Beans. Bob Lee says they have been invited to the expert group, but so far no interest. There's a lot you can imply about that, enough for another post some other time. Next, somebody asked about the compatibility between Web Beans and the component model for the Service Component Architecture JSR. Gavin King balked at this, as seemed to think the whole notion of Service Component was a contradiction of terms. He said he couldn't say much more without being extremely negative.

Next session was Effective Java Reloaded by the venerable Joshua Bloch. First surprising thing was that Bloch is the "Chief Java Architect" at Google. And I thought they didn't have titles at Google and everybody was a software engineer! His talk was great. He had some really cool things with generics in particular. He talked about the SuperType Tokens that I had fist read about in Bob Lee's blog last year. It was a very useful talk. The only disappointing part is that he told us that book by the same title (the sequel to his successful Effective Java) won't be ready until the end of the year. I don't buy too many Java books anymore, but that's definitely one I will buy.