Use javaslang.Option instead of java.util.Optional if null is permitted

README.TXT

The `Option` next is used to test if the iterator has more elements.
In particular the value of `new Some<>(iterator.next())` is always present, even if `iterator.next()` is null.
This would not be possible with `Optional.of(iterator.next())`.

Iterators.java

/**    / \____  _    ______   _____ / \____   ____  _____
 *    /  \__  \/ \  / \__  \ /  __//  \__  \ /    \/ __  \   Javaslang
 *  _/  // _\  \  \/  / _\  \\_  \/  // _\  \  /\  \__/  /   Copyright 2014 Daniel Dietrich
 * /___/ \_____/\____/\_____/____/\___\_____/_/  \_/____/    Licensed under the Apache License, Version 2.0
 */
package javaslang.collection;

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.function.Predicate;

import javaslang.monad.None;
import javaslang.monad.Option;
import javaslang.monad.Some;

public final class Iterators {

    private Iterators() {
        throw new AssertionError(Iterators.class.getName() + " is not intended to be instatiated.");
    }

    public static <T> Iterator<T> of(Iterator<T> iterator, Predicate<? super T> whileCondition) {
        return new Iterator<T>() {

            Option<T> next = testNext();

            @Override
            public boolean hasNext() {
                return next.isPresent();
            }

            @Override
            public T next() {
                final T result = next.orElseThrow(() -> new NoSuchElementException("no more elements"));
                next = testNext();
                return result;
            }

            Option<T> testNext() {
                return (iterator.hasNext() ? new Some<>(iterator.next()) : None.<T> instance())
                        .filter(whileCondition);
            }
        };
    }
}