如何通过从另一个对象的列表中累积两个字段来创建对象？

Question

我有一个房间对象列表：

class Room {
    int guestCount;
    List<String> sunglassBrands;
}

我想让所有房间的客人总数和他们所有的太阳镜品牌像这样：

class Guest {
    int guestCount;
    List<Sunglass> sunglasses;
}

class Sunglass {
    String brandName;
}

现在，这可能是一种直接的方法：

AtomicInteger numberOfGuests = new AtomicInteger();
List<Sunglass> sunglassBrandsList = new ArrayList<>();
rooms.forEach(
            room -> {
              numberOfGuests.getAndAdd(room.getGuestCount());
              sunglassBrandsList.addAll(
                  room.getSunglassBrands.stream()
                      .map(
                          brand -> {
                            Sunglass sunglass = new Sunglass();
                            sunglass.setBrandName(brand);
                            return sunglass;
                          })
                      .collect(Collectors.toList()));
            });
Guest guest = new Guest();
guest.setGuestCount(numberOfGuests.get());
guest.setSunglassBrands(sunglassBrandsList);

这可以在一次迭代中以更优雅的方式完成吗？ 我想我可以在这里使用reduce但我不太确定如何使用。

Answer 1

你可以用下一个方法来做。

int numberOfGuests = rooms.stream().mapToInt(Room::getGuestCount).sum();
List<Sunglass> sunglassBrands = rooms.stream().map(Room::getSunglassBrands).flatMap(List::stream).map(Sunglass::ofBrandName).collect(Collectors.toList());
// creation of object

注意： Sunglass::ofBrandName是下一个实现的静态方法

public static Sunglass ofBrandName(String brandName) { 
  Sunglass sunglass = new Sunglass();
  sunglass.brandName = brandName;
  return sunglass;
}

你可以在你的类中创建一个私有方法来创建太阳镜或只是硬编码 lambda。

Answer 2

如果你想使用reduce ，你可以使用它的三参数版本。

Guest guest = rooms.stream()
       .reduce(new Guest(),
               (guest, room) -> {
                   guest.addGuests(room.getGuestCount());
                   guest.addSunglasses(room.getSunglassBrands()
                           .stream().map(Sunglass::new).collect(Collectors.toList()));
                   return guest;
               },
               (guest1, guest2) -> {
                   guest1.addGuests(guest2.getGuestCount());
                   guest1.addSunglasses(guest2.getSunglasses());
                   return guest1;
               });

为此，我在Guest类中添加了以下两个方法并初始化了实例变量。

class Guest {
    int guestCount = 0;
    List<Sunglass> sunglasses = new ArrayList<>();

    public void addGuests(int guestCount) {
       this.guestCount += guestCount;
    }

    public void addSunglasses(List<Sunglass> sunglasses) {
        this.sunglasses.addAll(sunglasses);
    }
}

并向Sunglass类添加了一个构造函数。 （如果您不想这样做，必须将.map(Sunglass::new)更改为 lambda 表达式以创建Sunglass并设置品牌名称（就像您所做的那样）。

public Sunglass(String brandName) {
    this.brandName = brandName;
}

Answer 3

您可以使用流将太阳镜列表转换为太阳镜对象，但除此之外，我会为房间使用常规循环。

• 只需分配一个访客实例。
• 并使用 getter 和 setter 更新实例。
• 根据您的要求，您可能希望使用Set<Sunglass>而不是List<Sungclass>以避免重复。

List<Room> rooms = new ArrayList<>(
        List.of(new Room(23, List.of("brandA, BrandB")),
                new Room(33, List.of("brandC", "brandD"))));

Guest guest = new Guest();

for (Room room : rooms) {
    // update the guest count here
    guest.setGuestCount(
            guest.getGuestCount() + room.getGuestCount());
    
    // create a new list of Sunglass instances here.
    // and add to the guest instance
    guest.getSunglasses().addAll(room.getBrands().stream()
            .map(Sunglass::new).collect(Collectors.toList()));
}
System.out.printl(guest);

印刷

[56, [brandA, BrandB, brandC, brandD]]

支持类定义。

class Room {
    int guestCount;
    List<String> sunglassBrands;
    
    public Room(int count, List<String> list) {
        this.guestCount = count;
        this.sunglassBrands = list;
    }
    
    public int getGuestCount() {
        return guestCount;
    }
    
    public List<String> getBrands() {
        return sunglassBrands;
    }
    
}

class Guest {
    int guestCount = 0;
    List<Sunglass> sunglasses = new ArrayList<>();
    
    public int getGuestCount() {
        return guestCount;
    }
    
    public void setGuestCount(int count) {
        this.guestCount = count;
    }
    
    public List<Sunglass> getSunglasses() {
        return sunglasses;
    }
    @Override
    public String toString() {
        return String.format("[%d, %s]", guestCount, sunglasses);
    }
}

class Sunglass {
    String brandName;
    
    public Sunglass(String name) {
        this.brandName = name;
    }
    @Override
    public String toString() {
        return brandName;
    }
}