A simple attempt at Higher Order functions in Java.

HigherOrder.java

import java.util.Arrays;
import java.util.Iterator;
import java.util.NoSuchElementException;

class HigherOrder {

    public static void main(String[] args) {
        Iterable<Integer> it = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        Iterable<Integer> evens = keep(it, new Function1<Boolean, Integer>() {
                public Boolean map(Integer x) {
                    return x % 2 == 0;
                }
            });
        Iterator<Integer> iter = evens.iterator();
        for (Integer i : evens) { System.out.println(i); }
    }

    interface Function1<R,T> {
        R map(T t);
    }

    public static <E> Iterable<E> keep(final Iterable<? extends E> itr, final Function1<Boolean,E> f) {
        return new Iterable<E>() {
            public Iterator<E> iterator() {
                return new Iterator<E>() {
                    Iterator<? extends E> oldIter = itr.iterator();
                    E cache = null;
                    public boolean hasNext() {
                        while (oldIter.hasNext() && cache == null) {
                            E elem = oldIter.next();
                            if (f.map(elem)) {
                                cache = elem;
                                return true;
                            }
                        }
                        return cache != null;
                    }

                    public E next() {
                        if (cache != null || hasNext()) {
                            E tmp = cache;
                            cache = null;
                            return tmp;
                        }
                        throw new NoSuchElementException();
                    }

                    public void remove() { oldIter.remove(); }
                };
            }
        };
    }
}