从抽象类访问私有变量，访问方法为抽象

Question

Let's say I have a class called book that is abstract, has a private variable price, and also has it's getter method as abstract.

    public abstract class Book{

        private double price;

        public abstract double getPrice();

    }

Now let's say I have a used Book class that inherits all of Book's attributes, but also has an age associated with it. Also I want this class to override the getPrice method from it's parent class. This is where I get stuck since the price variable is private, and the parent class has an abstract getter method.

    public class UsedBook extends Book{

        private int age;

        //constructor
        public UsedBook(){
            age = 1;
        }

        @Override
        public double getPrice(){
            //How can I implement this method so as a user I can access the price?
        }

    }

Thank You

Answer 1

If all implementations of Book must implement getPrice() , and the implementation is always to just return the price, it should not be abstract. You should just define the getter within your abstract class:

public abstract class Book{

    private double price;

    public double getPrice() {
        return price;
    }

}//Ends book class

If this doesn't work, and you need to directly access the price variable from inheriting classes, then you should change it to be protected instead of private .

Answer 2

If you do not want the price field variable to be publicly accessible from the Abstract class then, you should change the access modifier from private to protected .

protected access modifier means that the field isn't publicly accessible via instance object. However any subclass that inherits the Abstract class directly or indirectly has the protected fields inherited as a field variable, and the same rules goes apply, it cannot be publicly accessible.

So to wrapped it up, the Book class:

public abstract class Book {
  protected double price;

  public abstract double getPrice();
}

The subclass that inherits the Book class:

public class UsedBook extends Book{

  private int age;

  public UsedBook(){
    this.age = 1;
    this.price = 0; // You should also set the field variable from the abstract "Book" class
  }

  @Override
  public double getPrice(){
    return this.price;
  }
}

Answer 3

Mark price as protected then you can access to it from sub classes :

public abstract class Book{

    protected double price;

    public abstract double getPrice();

}

public class UsedBook extends Book{

...

    @Override
    public double getPrice(){
        return price;
    }

..

}