dart Error: Field ‘xxx’ should be initialized because its type ‘String’ doesn’t allow null

最近在学dart class的构造函数时候出现 这个问题

原因是firstName没有初始化，不能为null

解决方案：

1.在String后面加个?

class Person {
  String? firstName ;
  Person.fromJson(Map data) {
    print('in Person');
  }
}

2.使用late关键字

class Person {
  late String firstName ;
  Person.fromJson(Map data) {
    print('in Person');
  }
}

完整代码：

class Person {
  late String firstName ;
  Person.fromJson(Map data) {
    print('in Person');
  }
}

class Employee extends Person {
  // Person does not have a default constructor;
  // you must call super.fromJson(data).
  Employee.fromJson(Map data) : super.fromJson(data) {
    print('in Employee');
  }
}

main() {
  var emp = new Employee.fromJson({});

  // Prints:
  // in Person
  // in Employee
  if (emp is Person) {
    // Type check
    emp.firstName = 'Bob';
  }
  (emp as Person).firstName = 'Bob';
}

参考：https://stackoverflow.com/questions/67034475/non-nullable-instance-field-must-be-initialized