如何在类中创建静态变量，只能由类修改，而不能由它的实例修改？

Question

How to create a class variable which can only be modified by the class only and not it's instances

class BullDog{
    String name;
    static String breed = "BullDog";

    public BullDog(String name){
        this.name = name;
    }
}

public class MainClass{
    public static void main(String [] args){
        Dog Mydog = new BullDog("Fluffy");
        Mydog.breed = "Shepherd"; // Should not be modified

        System.out.Println(Dog.breed);
    }
}

Real Output:

Shepherd

Desired Output:

BullDog

Answer 1

Make the field final that is constant. And static final is on class level.

public class BullDog {
    public String name;
    public static final String breed = "BullDog";

    public BullDog(String name){
        this.name = name;
    }
}

The problem is that inheritance is not possible. A Poodle needs its own static breed, and you cannot have a Dog providing a breed

public abstract class Dog {
    public String name;

    public Dog(String name){
        this.name = name;
    }

    public abstract String getBreed();   
}

public class BullDog extends Dog {

    public BullDog(String name){
        super(name);
    }

    @Override
    public String getBreed() {
        return "Bulldog";
    }
}

public class Poodle extends Dog {

    public Poodle(String name){
        super(name);
    }

    @Override
    public String getBreed() {
        return "Wolf";
    }
}

List<Dog> dogs = Arrays.asList(new BullDog("fluffy"), new Poodle("Herman"));

Answer 2

Make the static variable private. Refer to this for details.