YFact.java

//based on code from http://www.arcfn.com/2009/03/y-combinator-in-arc-and-java.html and the generic version https://gist.github.com/2571928

class YFact {
  // T function returning a T
  // T -> T
  public static interface Func<T> {
    T apply(T n);
  }

  // Higher-order function returning a T function
  // F: F -> (T -> T)
  private static interface FuncToTFunc<T> {
    Func<T> apply(FuncToTFunc<T> x);
  }

  //Next comes the meat. We define the Y combinator, apply it to the factorial input function, and apply the result to the input argument. The result is the factorial.
  // Formulation : λr.(λf.(f f)) λf.(r λx.((f f) x))
  public static <T> Func<T> Y(final Func<Func<T>> r) {
    return ((FuncToTFunc<T>) f -> f.apply(f))
        .apply(
            f -> r.apply(
                x -> f.apply(f).apply(x)));
  }

  public static void main(String args[]) {
    System.out.println(
        // Y combinator
        Y(
            // Recursive function generator
            new Func<Func<Integer>>() {
              public Func<Integer> apply(final Func<Integer> f) {
                return n -> n == 0 ? 1 : n * f.apply(n - 1);
              }
            }

        ).apply(
            // Argument
            Integer.parseInt(args[0])));
  }
}