Java Closures: first prototype

I've finally had some time to make progress on a prototype of closures. If you want to see what an API looks like, you can compare Doug Lea's jsr166y fork-join framework to the same API ported to use the language features of the prototype.

If you want to try it, you can download an executable version of the prototype here. Make sure a JDK6 version of java and javac are on your path. This is binary-licensed under the JRL, but if a JSR is created I expect to license it under GPLv2. There are a few small test cases included.

This prototype supports

• the BGGA function type syntax
• closure literals
• the closure conversion
• the null type, disjunctive types, and exception transparency
• definite assignment
• Unreachable and completion transparency.
• Catching multiple exceptions at once like catch(X1|X2 ex) { ...
  [not closely related to closures but the implementation was simple once closures are there]

This prototype does not yet support

• a closure using a mutated variable from the enclosing scope
• nonlocal control-flow (break, return, and continue)
• the control invocation statement and loop abstractions

I'm intentionally distributing it before these features are available. The idea is that people can try this version, and compare it to the next version with these features working.

Separately, I'm working on a set of smaller language extensions for JDK7, some of which interact very nicely with Closures. For example, "extension methods" enable you to have the effect of adding methods to existing interfaces (e.g. adding "each", "filter", etc to Collection) without breaking backward compatibility. I'll write more about these over the next few days.

This is still rough around the edges, but any feedback you have is most welcome.