如何将子类中的超类静态变量实例化为final

Question

I am simulating a parking lot application. And have a class Vehicle which is abstract and extended by concrete classes like Car , Truck etc. I have a variable parkingFare which for each sub class of Vehicle will be instantiated as class variable and final so that all instances of that particular class can use the same value. eg If I set parkingFare for Car as $2 it should be the same for all instances of Car . The problem is I am unable to access this variable in my concrete class as class variable.

public abstract class Vehicle {

 String plateNo;
 LocalTime startTime;
 static int parkingFare;
 public abstract int getFare();
 public abstract String getPlateNo();
 public abstract LocalTime getParkedTime();

}


 public class Car extends Vehicle {

  final parkingFare = 2; // how I want to instantiate it.
   public Car(String plateNo){
    this.plateNo = plateNo;
    this.startTime = LocalTime.now();     
   }

   @Override
   public LocalTime getParkedTime(){
    return startTime;
   }

   @Override
   public String getPlateNo(){
    return plateNo;
   } 

   @Overrid
   public int getFare(){
    return parkingFare;
   }   
}

Answer 1

Use a static-block:

public class Car extends Vehicle {

    static {
        parkingFare = 2;
    }

    // ...
}

However, this makes parkingFare equal in all Vehicle-classes, regardless of subclass. For example, having another Vehicle subclass Bus setting parkingFare = 10 and running:

Car car = new Car("Car");
Bus bus = new Bus("Bus");
System.out.println(car.getFare());
System.out.println(bus.getFare());

Results in 10 being printed twice.

Adding parkingFare to each subclass will solve this.

(Double edit: confused with how java stores static variables, but then I tested, and the current description is accurate)

Answer 2

Use this in Your SubClass

static {
 parkingFare=10;
}

Since A static variable is related to a class not in the instance, so all instances of Class will share the static variable.

So after initializing parkingFare=10 all the instances of Class Car will have the same parking fare .

Now Coming to your Comment you need to make them final in child class you can't ,if you want to make them final you need to declare parkingFare as final itself in Vehicle Class

Answer 3

I think if you want to make cars cost 2$, then you should move the static final variable to that class.

public abstract class Vehicle {

 String plateNo;
 LocalTime startTime;
 public abstract int getFare();
 public abstract String getPlateNo();
 public abstract LocalTime getParkedTime();

}


 public class Car extends Vehicle {

   static final int parkingFare = 2;

   ...
   @Override
   public final int getFare() {
     return parkingFare;
   }   
}