John Mitchell points out in his blog, penned after attending the Java Puzzlers session at JavaOne, that:

The moral of these puzzles isn't merely "be careful of using wrapper types with the ternary operator", but that auto-boxing is evil and should have never been added to the language in the first place.

Of course, it goes without saying that this is just one example of how new "features" added to a mature language are very dangerous. One of the big, currently brewing brouhahas is over the proposed addition of closures to Java. Perhaps folks might want to pause their rampant fervor for a bit and actually look at the history of these attempts and realize just how costly these changes really are. Especially when there are plenty of alternatives like using other languages which have their pet features which target the underlying JVM.

As Josh so eloquently put, "APIs, like diamonds, are forever."

What newly proposed Java feature do you think could confuse developers the most?

abstract class Maybe<T> {}
 
class Nothing<T> extends Maybe<T> {}
 
class Just<T> extends Maybe<T> {
   final T value;
   Just(T value) { this.value = value; }
}
 
String s = "hi";
Maybe<String> ms = new Just<String>("hi");
 
String greet(Maybe<String> name) {
   if (name instanceof Just) {
      return "Hello, " + ((Just)name).value;
   } else {
      return "Hello, Anonymous";
   }
}