Saw a blog post this morning titled Why Functional Programming in Java is Dangerous. Author Elliotte Rusty Harold sets up the world’s worst straw man I’ve seen. He talks about Uncle Bob’s post on the advantages of functional programming. Elliotte’s thesis is that Java and JVM just can’t handle this sort of thing, and then sets out to prove this with some terrible code that, unsurprisingly, is terrible. He takes this bit of Clojure

(take 25 (squares-of (integers)))

And “re-implements” it in Java. He does this by implementing integers to just allocate an array of all the integers.

Yeah.

That’s bad enough, but really, that line of Clojure isn’t functional per se, it’s just that aspects of a functional programming language (namely first-class functions and lazy evaluation) make it really easy to implement that code in an efficient way.

Elliotte is just flat wrong that you can’t do this in Java. I will now present the Java version of the Clojure version.

As always, laziness is the key to success

The key to making it work is to use a lazy list which, in Java, is called Iterator. If you have never used Java before, Iterator is basically this:

public interface Iterator<T> {
  public boolean hasNext(); // true if there are more elements
  public T next();          // get the next element
  public void remove();     // remove the element you just got (optional)
}

First, let’s make a list of all the integers. The top of the collection food chain in Java is Iterable, which has one method that just returns an Iterator. We’ll make an Iterator of all the integers and wrap it up in an anonymous Iterable for use by anyone:

private static class IntegersIterator implements Iterator<Integer> {
  private int nextInt = 0;
  public boolean hasNext() {
    return nextInt < Integer.MAX_VALUE;
  }
  public Integer next() {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }
    nextInt++;
    return nextInt - 1;
  }
  public void remove() {
    throw new UnsupportedOperationException();
  }
}

private static Iterable<Integer> integers() {
  return new Iterable<Integer>() {
    public Iterator<Integer> iterator() {
      return new IntegersIterator();
    }
  };
}

This is almost all there is to it. To make the squares-of function, we need to maintain the “laziness” of our implementation. To do this, we’ll create a squaring Iterator that proxies¹ through to another Iterator:

private static class SquaringIterator implements Iterator<Integer> {
  private Iterator<Integer> original;
  public SquaringIterator(Iterator<Integer> iter) {
    original = iter;
  }
  public boolean hasNext() {
    return original.hasNext();
  }
  public Integer next() {
    int value = original.next();
    return value * value;
  }
  public void remove() {
    original.remove();
  }
}

private static Iterable<Integer> squaresOf(final Iterable<Integer> seq) {
  return new Iterable<Integer>() {
    public Iterator<Integer> iterator() {
      return new SquaringIterator(seq.iterator());
    }
  };
}

We are starting to see some of Java’s warts here. Because we’re creating an anonymous inner class, that class cannot access parameters or local variables unless we declare them final - we’ve declared the parameter to squaresOf as such. This is because Java doesn’t support real closures. We could’ve made a class that implements Iterable<Integer> just for our squaresOf function, but that feels like overkill for this increasingly contrived example.

OK, now that we have a way to get all the integers and square them without actually instantiating all the integers, we can just take the first 25 off:

public static void main(String args[]) {
  System.out.println(take(25,squaresOf(integers())));
}

private static List<Integer> take(int numToTake, Iterable<Integer> seq) {
  List<Integer> results = new ArrayList<Integer>();
  Iterator<Integer> iterator = seq.iterator();
  for(int i=0;i<numToTake;i++) {
    if (iterator.hasNext()) {
      results.add(iterator.next());
    }
  }
  return results;
}

Running this code sure enough works quickly and without incident (full listing here):

> java -cp . Functional
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, \
361, 400, 441, 484, 529, 576]

So, it’s certainly possible to create lazy data structures in Java. There are a few things bad about this, however:

• It’s very special purpose - if we needed a doublesOf or squareRootsOf, we’d create a lot of duplicate code
• It’s not very OO - we’d expect our code to be something like squaresOf(integers).take(25).

Let’s fix it!

Functions to the rescue

Now we’re going to need a Function interface:

private static interface Function1<A,R> {
  public R apply(A a);
}

Think of A is “argument” and R as “return value”.

With this, we can genericize our SquaringIterator into a TransformingIterator that takes a function for transformation and the Iterator to transform:

private static class TransformingIterator<A,R> implements Iterator<R> {
  private Iterator<A> original;
  private Function1<A,R> function;

  public TransformingIterator(Function1<A,R> function, Iterator<A> iter) {
    this.original = iter;
    this.function = function;
  }
  public boolean hasNext() {
    return this.original.hasNext();
  }
  public R next() {
    A value = this.original.next();
    return this.function.apply(value);
  }
  public void remove() {
    original.remove();
  }
}

Now, we can make our squaresOf method like so:

private static Iterable<Integer> squaresOf(final Iterable<Integer> seq) {
  return new Iterable<Integer>() {
    public Iterator<Integer> iterator() {
      Function1<Integer,Integer> square = new Function1<Integer,Integer>() {
        public Integer apply(Integer i) {
          return i * i;
        }
      };
      return new TransformingIterator<Integer,Integer>(square, seq.iterator());
    }
  };
}

Using TransformingIterator, we could make a doublesOf method like so:

private static Iterable<Integer> doublesOf(final Iterable<Integer> seq) {
  return new Iterable<Integer>() {
    public Iterator<Integer> iterator() {
      Function1<Integer,Integer> square = new Function1<Integer,Integer>() {
        public Integer apply(Integer i) {
          return i + i;
        }
      };
      return new TransformingIterator<Integer,Integer>(square, seq.iterator());
    }
  };
}

Yes, creating a function in Java is messy, but it’s not impossible, and doesn’t affect the runtime performance of our application. This ability to extract structure and code vs. logic is the true power of functional programming.

Making it more OO

Now, what about that pesky take method? We can handle that pretty easily by creating an implementation of Iterable that adds the necessary methods:

private abstract static class RichIterable<T> implements Iterable<T> {

  public static <T> RichIterable<T> fromIterable(Iterable<T> seq) {
    return new ProxyRichIterable<T>(seq);
  }

  public RichIterable<T> take(int numToTake) {
    List<T> results = new ArrayList<T>();
    Iterator<T> iterator = this.iterator();
    for(int i=0;i<numToTake;i++) {
      if (iterator.hasNext()) {
        results.add(iterator.next());
      }
    }
    return fromIterable(results);
  }

  public List<T> toList() {
    List<T> list = new ArrayList<T>();
    for(T t: this) {
      list.add(t);
    }
    return list;
  }
}

private static class ProxyRichIterable<T> extends RichIterable<T> {
  private Iterable<T> seq;
  public ProxyRichIterable(Iterable<T> seq) {
    this.seq = seq;
  }
  public Iterator<T> iterator() {
    return seq.iterator();
  }
}

Notice that we create a means of converting any Iterable (i.e. any Java collection) into our RichIterable, and also provide a way to turn it back into a vanilla list. We replace all use of Iterable in our code with RichIterable and our main method looks more “OO”:

public static void main(String args[]) {
  System.out.println(squaresOf(integers()).take(25).toList());
}

And, it still runs great (updated source here):

> java -cp . Functional
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, \
361, 400, 441, 484, 529, 576]

So, what did we learn? We learned that you can’t just write shitty code in Java and then say Java and the JVM can’t handle functional programming. Java can absolutely handle functional programming, even though Java’s syntax makes it a bit painful.