Java Y Combinator

y.java

public class Y<T> implements Function<Function<Function<T, T>, Function<T, T>>, Function<T, T>> {

    @FunctionalInterface
    private interface FuncToFunc<F> extends Function<FuncToFunc<F>, F> {}

    @Override
    public Function<T, T> apply(Function<Function<T, T>, Function<T, T>> r) {
        final FuncToFunc<Function<T, T>> funcToFunc = f -> f.apply(f);
        return funcToFunc.apply(f -> r.apply(x -> f.apply(f).apply(x)));
    }
  
    public static void main(String[] args) {
        final Function<Integer, Integer> factorial = new Y<Integer>().apply(fact -> n -> {
            if (n == 0) return 1;
            return n * fact.apply(n - 1);
        });
        System.out.println(factorial.apply(10));
        System.out.println(factorial.apply(5));
    }
}