Accessing "final" fields of a superclass in a subclass (and updating them) in Java

Question

I have a field for Buildings and subclasses of different types of Buildings that inherit from the Buildings class. The Buildings class has a field called "totalNumber" which I want to update every time I add a building in one of my subclasses.

I want totalNumber in Buildings class to always show the total number of buildings that have been made.

Shall I use the "final" keyword for the "totalNumber" field in this case?

What happens if inside an object of the subclass "Cottage" i.e. cott I call

cott.totalNumber = cott.totalNumber += 1?

Will it update the field it inherited?

Please help. Thank you.

Final fields cannot be mutated once initialized, so you would not want to use them to count the total number of buildings made. I would also consider some kind of state manager that keeps track of buildings. — jdphenix, Apr 13 '15 at 05:01
However, if the final field is a reference to a mutable object, then that object can be updated even if the instance field that refers to it can not. You could use `AtomicInteger`, for example to contain your count. — scottb, Apr 13 '15 at 05:05

score 3 · Answer 1 · answered Apr 13 '15 at 05:06

final means you can once assign a value when initializing the variable, and afterwards it's readonly. what you're looking for would be in the simplest case some sort of static variable that counts created instances of Buildings. But this could be easily done this way:

public class Buildings{
     //a simple counter for created instances
     private static int createdInstances = 0;

     public Buildings(){
          //new instance created -> increment counter
          createdInstances++;
     }
}

If you create an instance of a subclass, the constructor of the superclass is always called aswell, so you don't need to do anything in the subclasses.

jdphenix · Answer 2 · 2015-04-13T05:41:38.987

In your question you expressed a desire to count instances of subclasses that you create. This really isn't in the responsibilities that any of those classes would have. With your requirement, I would implement a solution something like this:

abstract class Building { }

final class Cottage extends Building { }

final class House extends Building { }

final class Castle extends Building { }

final class BuildingManager {
    private final static BuildingManager _instance = new BuildingManager();
    private Set<Building> buildings;

    private BuildingManager() {
        buildings = new HashSet<>();
    }

    public static BuildingManager getManager() {
        return _instance;
    }

    public Building getNewBuilding() {
        // decide which building to create, but we'll always go with
        // a castle for this example

        Building castle = new Castle();
        buildings.add(castle);
        return castle;
    }

    public int getBuildingCount() {
        return buildings.size();
    }
}

BuildingManager is solely responsible for managing creation and counting of Building's.

With that in place, you would use it like this:

BuildingManager manager = BuildingManager.getManager();

Building building = manager.getNewBuilding();
Building building2 = manager.getNewBuilding();

System.out.println(manager.getBuildingCount() + " buildings exist.");

which will output:

2 buildings exist.

To answer your specific questions:

I want totalNumber in Buildings class to always show the total number of buildings that have been made. Would I use the "final" keyword for the "totalNumber" field in this case?

No. A final value field couldn't be changed once initialized. You could use object to store this (kudos to @scottb to mentioning AtomicInteger), but that doesn't change the fact that counting how many buildings exist in your application's state is outside of the responsibilities that Building would have.

What happens if inside an object of the subclass "Cottage", cott I call cott.totalNumber = cott.totalNumber += 1? Will it update the field it inherited?

No. It would be a fair assumption that if Building kept a counter, it would be a static field or method, and thus be something like

Building.incrementCount();

An inherited field from a superclass is still unique to that specific instance.

Accessing "final" fields of a superclass in a subclass (and updating them) in Java

2 Answers2