如何修復類名的錯誤實體？

添加覆寫 toSting() 后，輸出如下所示..

I/flutter (25471): [London11.0811.741012815.3]
I/flutter (25471): [London11.0811.741012815.3, London11.1111.861011776.45]
I/flutter (25471): [London11.0811.741012815.3, London11.1111.861011776.45, London10.4311.291011756.1]
I/flutter (25471): [London11.0811.741012815.3, London11.1111.861011776.45, London10.4311.291011756.1, London6.379.541012766.93]

我只需要遍歷每個專案的地名、溫度、濕度、風速、壓力和感覺。我在列印輸出時做錯了什么？

在預測資料中

import 'package:forecast/models/weather_data.dart';

class ForecastData {
  final List list;

  ForecastData({required this.list});

  factory ForecastData.fromJson(Map<String, dynamic> json) {
    var list = json['list']?.map((e) => e)?.toList(growable: true) ?? [];
    List weatherData = [];
    list.forEach((e) {
      WeatherData w = WeatherData(
          placeName: json['city']['name'],
          temperature: e['main']['temp'],
          feels_like: e['main']["feels_like"],
          pressure: e['main']['pressure'],
          humidity: e['main']['humidity'],
          wind_speed: e['wind']['speed']);

      weatherData.add(w);
      print(weatherData.toList());
    });

    return ForecastData(list: weatherData);
  }
}

在weatherdata.dart中

class WeatherData {
  String? placeName;
  num? temperature;
  num? feels_like;
  num? pressure;
  num? humidity;
  num? wind_speed;

  WeatherData({
    this.placeName,
    this.temperature,
    this.feels_like,
    this.pressure,
    this.humidity,
    this.wind_speed,
  });
  @override
  String toString() =>
      '${placeName ?? 'unknown'}${feels_like ?? 'unknown'}${temperature ?? 'unknown'}${pressure ?? 'unknown'}${humidity ?? 'unknown'}${wind_speed ?? 'unknown'}';

  WeatherData.fromJson(Map<String, dynamic> json) {
    placeName = json['city']['name'];
    temperature = json['list'][0]['main']['temp'];
    feels_like = json['list'][0]['main']['temp'];
    pressure = json['list'][0]['main']['temp'];
    humidity = json['list'][0]['main']['temp'];
    wind_speed = json['list'][0]['main']['temp'];
  }
}

json

{
  "cod": "200",
  "message": 0,
  "cnt": 40,
  "list": [
    {
      "dt": 1641124800,
      "main": {
        "temp": 11.74,
        "feels_like": 11.08,
        "temp_min": 11.74,
        "temp_max": 12,
        "pressure": 1012,
        "sea_level": 1012,
        "grnd_level": 1008,
        "humidity": 81,
        "temp_kf": -0.26
      },
      "weather": [
        {
          "id": 803,
          "main": "Clouds",
          "description": "broken clouds",
          "icon": "04d"
        }
      ],
      "clouds": {
        "all": 75
      },
      "wind": {
        "speed": 5.3,
        "deg": 229,
        "gust": 11.75
      },
      "visibility": 10000,
      "pop": 0,
      "sys": {
        "pod": "d"
      },
      "dt_txt": "2022-01-02 12:00:00"
    },
    {
      "dt": 1641135600,
      "main": {
        "temp": 11.86,
        "feels_like": 11.11,
        "temp_min": 11.86,
        "temp_max": 12.09,
        "pressure": 1011,
        "sea_level": 1011,
        "grnd_level": 1006,
        "humidity": 77,
        "temp_kf": -0.23
      },
      "weather": [
        {
          "id": 500,
          "main": "Rain",
          "description": "light rain",
          "icon": "10d"
        }
      ],
      "clouds": {
        "all": 83
      },
      "wind": {
        "speed": 6.45,
        "deg": 223,
        "gust": 15.9
      },
      "visibility": 10000,
      "pop": 0.7,
      "rain": {
        "3h": 0.6
      },
      "sys": {
        "pod": "d"
      },
      "dt_txt": "2022-01-02 15:00:00"
    },
{
      "dt": 1641546000,
      "main": {
        "temp": 3.96,
        "feels_like": 0.41,
        "temp_min": 3.96,
        "temp_max": 3.96,
        "pressure": 1010,
        "sea_level": 1010,
        "grnd_level": 1007,
        "humidity": 82,
        "temp_kf": 0
      },
      "weather": [
        {
          "id": 804,
          "main": "Clouds",
          "description": "overcast clouds",
          "icon": "04d"
        }
      ],
      "clouds": {
        "all": 100
      },
      "wind": {
        "speed": 4.29,
        "deg": 239,
        "gust": 11.29
      },
      "visibility": 10000,
      "pop": 0.02,
      "sys": {
        "pod": "d"
      },
      "dt_txt": "2022-01-07 09:00:00"
    }
  ],
  "city": {
    "id": 2643743,
    "name": "London",
    "coord": {
      "lat": 51.5085,
      "lon": -0.1257
    },
    "country": "GB",
    "population": 1000000,
    "timezone": 0,
    "sunrise": 1641110761,
    "sunset": 1641139355
  }
}

有人能告訴我我是否正確地迭代了forecast.data 中的每個專案嗎？以及如何解決它？

uj5u.com熱心網友回復：

return placeName ?? '';

你只需要處理它是否為空

uj5u.com熱心網友回復：

輸入字串？意味著它必須在這種型別的變數中具有字串或空值所以解決方案是將型別更改為字串并將所需的關鍵字添加到建構式中的命名引數中。試試這可能有用...

uj5u.com熱心網友回復：

//'A value of type 'String?' can't be returned from the method 'toString' because it has a return type of 'String'

顯示這一點是因為您必須在那里處理空例外。做placeNmae? 或

if (placeName != null) {
    return placeName;
  //2
  } else {
    codeExceptionsomething();
  }
}

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