public List<C3> getThemll(List<C1> list) {
     list.stream.flatmap(a->a.c2list.stream())
                .flatmap(b->b.c3list..stream())
                .collect(Collectors.toList());
}

这很管用，谢谢

正如注解中所建议的，这里有一个完整的示例，说明如何将一个流流展平为一个对象流（称为流连接）：标题应编辑为“合并流”或“连接列表”

public class Community {
    private List<Person> persons;
    // Constructors, getters and setters
}

--

public class Person {
    private List<Address> addresses;
    // Constructors, getters and setters
  }

--

public class Address {
    private String city;
    // Constructors, getters and setters
    @Override
    public String toString() {
        return "Address{" +
                "city='" + city + '\'' +
                '}';
    }
  }

演示：

public class AggregationDemo {
    public static void main(String[] args) {
        List<Community> communities = List.of(
                new Community(List.of(new Person(List.of(new Address("Paris"))))),
                new Community(List.of(new Person(List.of(new Address("Florence"))))));
        List<Address> addresses = communities.stream()
                .flatMap(community -> community.getPersons().stream())
                .flatMap(person -> person.getAddresses().stream())
                .collect(Collectors.toList());
        List<List<Person>> collectedListOfList = communities.stream()
                .map(community -> community.getPersons())
                .collect(Collectors.toList());
        addresses.forEach(System.out::println);
        collectedListOfList.forEach(System.out::println);
    }

}
输出：

Person{addresses=[Address{city='Paris'}]}
Person{addresses=[Address{city='Florence'}]}
[Person{addresses=[Address{city='Paris'}]}]
[Person{addresses=[Address{city='Florence'}]}]