Define factorial as lambda

@FunctionalInterface
public interface Function2<A1, A2, R> {
    R apply(A1 a1, A2 a2);
}

// first solution
class Run {

    public static class Self<T> {
        public final Function2<Self<T>, T, T> func;
        public Self(Function2<Self<T>, T, T> func) {
            this.func = func;
        }
    }

    public static final Function<Long, Long> factorial = n -> {
        final Self<Long> trampoline = new Self<>((cont, m) -> {
            if (m == 0L)
                return 1L;
            else
                return m * cont.func.apply(cont, m - 1);
        });
        return trampoline.func.apply(trampoline, n);
    };

    public static void main(String... args) {
         println("factorial(10) = " + factorial.apply(10L));
    }
}