How to fix problem with List<Null> on dart?-CodePudding

I wrote simple code to parse json data on flutter. An error occurs -

A value of type 'List<Null>' can't be assigned to a variable of type 'List<Office>'.
Try changing the type of the variable, or casting the right-hand type to 'List<Office>'

My code:

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

class OfficesList {
  List<Office> offices;
  OfficesList({required this.offices});

  factory OfficesList.fromJson(Map<String, dynamic> json) {
    var officesJson = json['offices'] as List;

    List<Office> officesList = officesJson.map((element) {
      Office.fromJson(element);
    }).toList();

    return OfficesList(
      offices: officesList,
    );
  }
}

class Office {
  final String name;
  final String address;
  final String image;

  Office({required this.name, required this.address, required this.image});

  factory Office.fromJson(Map<String, dynamic> json) {
    return Office(
      name: json['name'] as String,
      address: json['address'] as String,
      image: json['image'] as String,
    );
  }
}

Future<OfficesList> getOfficesList() async {
  var url = "https://about.google/static/data/locations.json";

  final response = await http.get(Uri.parse(url));

  if (response.statusCode == 200) {
    return OfficesList.fromJson(json.decode(response.body));
  } else {
    throw Exception("Error: ${response.reasonPhrase}");
  }
}

I tried adding "cast" to the problem line, but that didn't help. I am writing my code in Visual Studio code (if it matters). I'd be glad if you can help.

CodePudding user response：

The following code creates a new list with no type and all items being null, since you create a new office item, but do not return it from the map call:

List<Office> officesList = officesJson.map((element) {
  Office.fromJson(element);
}).toList();

This is what you want, to create a List<Office>

List<Office> officesList = officesJson.map((element) {
  return Office.fromJson(element);
}).toList();

or shorter:

var officesList = officesJson.map((json) => Office.fromJson(json)).toList();

CodePudding user response：

You should change the following since you are never returning anything:

List<Office> officesList = officesJson.map((element) {
  Office.fromJson(element);
}).toList();

To:

List<Office> officesList = officesJson.map((element) {
  return Office.fromJson(element);
}).toList();

Or do it shorter like this:

List<Office> officesList =
  officesJson.map((element) => Office.fromJson(element)).toList();

Or even shorter if you are using Dart 2.15 or later:

List<Office> officesList = officesJson.map(Office.fromJson).toList();

And shortest (I think?):

List<Office> officesList = [...officesJson.map(Office.fromJson)];