I’ve just been playing with the first fruits of the Lambda project. It takes a bit of messing around to get going; you need to downgrade the version of HotSpot in the lambda repository to b93 as so:

$ hg fclone http://hg.openjdk.java.net/lambda/lamdba
$ cd lambda/hotspot
$ hg update -r jdk7-b93

Once done, you can build code using closures like so:

$/mnt/builder/lambda/j2sdk-image/bin/javac -XDallowFunctionTypes Closures.java

I found that it’s pretty easy to implement a simple version of map which takes a collection of objects and a function to produce a collection of the results:

public static <S,T> List<S> map(List<T> s, #S(T) func)
    List<S> list = new ArrayList<S>();
    for (T t : s)
    return list;

public static void main(String[] args)
  #Integer(Integer) square = #(Integer x)(x * x);
  List<Integer> list = Arrays.asList(new Integer[]{4,5,6});
  List<Integer> squares = map(list, square);
  System.out.println(list + " squared is " + squares);

This can be run as follows:

$ /mnt/builder/lambda/j2sdk-image/bin/java Closures
[4, 5, 6] squared is [16, 25, 36]

Sadly, the current implementation doesn’t like autoboxing too much. javac crashes if you pass an Integer into a function requiring an int, and you need an #Integer(Integer) function for the above; javac throws an error if you try to use a #int(int).