Y combinator in Java 8

YComb.java

package ch.claude_martin;

import java.util.function.Function;
import java.util.function.UnaryOperator;

public class YComb {

	interface Fn2Op<T> extends Function<Fn2Op<T>, UnaryOperator<T>> {
		@Override
		UnaryOperator<T> apply(Fn2Op<T> x);
	}

	public static void main(String args[]) {
		UnaryOperator<UnaryOperator<Long>> factorial = f -> n -> {
			if (n == 0)
				return 1L;
			else
				return n * f.apply(n - 1);
		};
		System.out.println(Y(Long.parseLong(args[0]), factorial));
	}

	private static <T> T Y(T arg, UnaryOperator<UnaryOperator<T>> factorial) {
		return ((Function<UnaryOperator<UnaryOperator<T>>, UnaryOperator<T>>) r -> ((Fn2Op<T>) f -> f.apply(f))
				.apply(f -> r.apply(x -> f.apply(f).apply(x)))).apply(factorial).apply(arg);
	}
}