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The Closures Controversy in Java


I just finished viewing Joshua Bloch's talk at JavaPolis 2007 entitled "The Closures Controversy", where he critiques one of the leading proposals for adding closures to the Java language (BGGA, named as an acronym after its authors). The presentation is eye-opening, and it is thought-provoking to see two of Java's biggest lumunaries on opposite sides of the debate. Of course, Bloch himself is one of the authors of an alternative proposal for adding closures, CICE (Concise Instance Creation Expressions).

It's not Clarence Darrow in the "Scopes Monkey Trial", but it's the closest we programmers can get. I won't go into huge detail about the two proposals except to say that BGGA adds full support for functional types whereas CICE is a more concise way of expressing anonymous inner classes in Java (there are many places on the web to go into the details). The heart of the debate really boils down to Defensive Programming VS. (for lack of a better term) "Let the programmer do whatever he/she wants (and let the language support it)". The former position being more conservative and more pragmatic, the later a more libertarian view that experts (programmers) are smart and should be allowed to do whatever they want. Ironically, these two opposites are also embodied in many ways in differing philosophies in design between Java and its antecedent language, C++. Java was, after all, "C++ --", the language that made smart decisions to remove features from C++ that were considered dangerously error-prone: pointers, explicit memory deallocation, etc.

So clearly Bloch is in the defensive programming camp whereas Neal Gafter, the main proponent of BGGA, is on the other side. And, I have to admit, I am on Bloch's side as well, for many reasons.

  • BGGA is complex. Non-local returns, continues, and breaks have a great potential to confuse the average programmer. The syntax, when viewed in even moderately complex code, looks cryptic. Even if you get past the syntax and non-local oddities you have to acknowledge, as Bloch so eloquently pointed out, that new features tend to add somewhere between quadratic and exponential complexity to a language because of their interaction with existing features. This was the *gotcha* with generics, and I have no desire to see that repeated with closures.
  • Defensive programming is at the very heart of what Java is as a language. And, by and large, it is this philosophy which has made it so successful. Trying to make it into an "experts language", as C++ is, cuts against the grain of the spirit of the language. (As an aside, it is telling to note that Gafter himself was a member of the C++ Standards Committee.)
  • The Java camp is already losing programmers to newer languages like Ruby because it is "too complex". Do we really want to add more complexity to drive away more programmers from the language? Simplicity is beauty, and (as someone once said) the most important decisions in a language have to do with what features you should leave out.
  • There are already other languages that target the JVM (and hence at least theoretically can interact with existing Java code) which handle closures in a much more natural way: JRuby, Groovy, Scala. Static typing without type inference tends to make control structures like closures much more challenging, if not downright awkward. Both Ruby and Groovy are dynamically typed and Scala uses static type inference. Perhaps one of these languages is a more suitable vehicle for those who feel they really need closures.

Whatever the result of the debate, it is likely that closures in some form will land in the Java language in the not-too-distant future. Right now, BGGA seems to have the momentum, which is unfortunate. Neal Gafter may, in a very well-intentioned way, be leading Java down a path of bloated complexity from which it will never recover; and which will ultimately result in Java yielding its leading position to a new language that is far more agile.