import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import static java.util.stream.Collectors.toList;
public class Main {
  public static void main(String[] args) {
    List<String> list1 = Arrays.asList("A", "B", "C");
    List<String> list2 = Arrays.asList("1", "2");
    List<String> result = product(list1, list2);
    result.forEach(System.out::println);
}

/**
 * @param lists 可变长度参数列表
 * */
public static List<String> product(List<String>... lists) {
    List<String> temp = new ArrayList<>();
    for (List<String> list : lists) {
        if (temp.isEmpty()) {
            temp = list;
        } else {
            // stream流 flatMap 和 map 方法
            temp = temp.stream().flatMap(o1 -> list.stream().map(o2 -> o1 + " " + o2)).collect(toList());
        }
    }
    return temp;
  }
}