这个用不到 lambda 表达式，但我猜你是想要这样：

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

public class User {

private int id;

private int parentId;

private List<User> userList = new ArrayList<>();

public User(int id, User... users) {
this.id = id;
this.userList.addAll(Arrays.asList(users));
this.userList.forEach(u -> u.parentId = id);
}

public static void main(String[] args) {
User root = new User(1,
new User(2,
new User(3),
new User(4)),
new User(5,
new User(6)
)
);
}
}
```