Functional programming in Java shouldn't be so hard. Streams is an attempt to make it a little easier on us all by providing Haskell-style functionality in an object-oriented format. A Stream is a simple object that wraps an iterator (or an iterable) and allows you to (mis)treat it as you please. Everything is lazily evaluated, so nothing happens until you ask for the results.

Personally, I find this sort of thing is much better explained with an example:

Iterator<Integer> numbers = theIteratorOfAMassivelyLongList();
Stream<Integer> numberStream = Stream.wrap(numbers);
Stream<String> htmlStream = numberStream.map(add(3))
                                        .filter(isEven())
                                        .concat(defaults())
                                        .zipWith(definitions(), formatAsHtml());

for (String item : htmlStream.take(5)) {
    // This will loop through only the first five values of that horrendously complicated endeavour.
    // In fact, it will only evaluate the first five values.
    // No wasted effort.
}

All you need to do is define those functions. Streams backs onto Google's Guava when it can to allow you to reuse code wherever possible.

private Function<Integer, Integer> add(final int n) {
    return new Function<Integer, Integer>() {
        public Integer apply(Integer input) {
            return input + n;
        }
    };
}

All you have to do is download the JAR, grab Guava 11.0.1 and add them to your classpath.

Streams is built through [Maven][]. Just fire up a command line and type:

mvn package

This will run all the tests and create a JAR in the target directory. Please run this command and ensure it succeeds before pushing up to your public repository as it will make sure anyone downloading it can build it.