Error: Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'

Question

void main() {
  Car c1 = new Car('E1001');
}

class Car {
  String engine;
  Car(String engine) {
    this.engine = engine;
    print("The engine is : ${engine}");
  }
}

Answer 1

In the dart null-safety feature,

1. either make the engine variable nullable by ?,

   class Car {
     String? engine;
     Car(String engine){
        this.engine = engine;
        print("The engine is : ${engine}");
     }
   }
   

2. or add the late keyword to initialise it lazily,

   class Car {
     late String engine;
     Car(String engine){
        this.engine = engine;
        print("The engine is : ${engine}");
     }
   }
   

3. or initialize the variable in the constructor's initialize block.

   class Car {
     String engine;
     Car(String engine) : engine = engine {
        print("The engine is : ${engine}");
     }
   }
   

Answer 2

Update, for initialize the variable in the constructor's:

class Car{
    String name;
    int wights; 
    Car(this.name,this.wigths);
    Car.origin(): this.name='', this.wigths=0;
  }

Answer 3

Using late keyword you can make a null variable.

Ex: late String engine;

"?" used for making null value.

Ex: String? engine;