Java Stream - 如何创建递归Lambda

import java.util.function.UnaryOperator;

public class Main {
  static UnaryOperator<Integer> factorial = null;

  public static void main(String[] args) {

    factorial = i -> i == 0 ? 1 : i * factorial.apply(i - 1);

    System.out.println(factorial.apply(6));

  }
}