如何解决从子 class 为其父调用方法？

Question

If I add a Detective as a Book, how do I call the setPrice method (because you just can't call a child method for a parent class)?

This is the code:

public class Book {
  String title;
//Contructors, get/setters, Override output methods
}

public class Detective extends Book {
  int price;
  //Contructors, get/setters, Override output methods
}

public class BookManager {
  Book[] list;
  int count = 0;
  final int MAX = 100;
  //Contructors, get/setters, Override output methods

  public void add(Book x) {
        if(count >= MAX) {
            System.out.println("Failed!");
        }
        list[count] = x;
        count++;
        System.out.println("Added!");
    }

  public void updatePrice(String title, int newPrice) {
     for(int i = 0; i < count; i++) {
        if(list[i].equals(title) && list[i] instanceof Detective) {
          //list[i].setPrice(newPrice) is wrong//
        } 
     }
  }
}

public static void main(String[] args) {
  BookManager list = new BookManager();
  Detective de = new Detective("abc", 123);
  list.add(de);
  //list.updatePrice("abc", 456); is wrong//
}

Is there another way to update the price?

Answer 1

Some options, depends how the data should be modeled.

---

1 - just use a cast to Detective to use its methods:

if (list[i].equals(title) && list[i] instanceof Detective) {
    Detective dectective = (Detective) list[i];
    detective.setPrice(newPrice);

---

2 - shouldn't every Book have a price?

public class Book {
    String title;
    //Contructors, get/setters, Override output methods

    public void setPrice(int price) {
        ...
    }
}

now it's trivial to call it:

// instanceof not need here for this to work
if (list[i].equals(title) && list[i] instanceof Detective) {
    list[i].setPrice(newPrice);

eventually the method is empty in Book but overridden in Detective

public class Book {
    ...

    public void setPrice(int price) {
        // intentionally empty, overridden in aubclasses
    }
}

public class Detective extends Book {
    ...
    @Override
    public void setPrice(int p) {
        ...
    }
}

---

3 - one step further, assuming there is no just-a-Book, that is, only subclasses of Book : make the class and the method abstract :

public abstract class Book {  // maybe make  this an interface
    ...
    public abstract void setPrince(int p);
}

and each subclass must implement that method

public class Detective extends Book {
    ...
    @Override
    public void setPrice(int p) [
        ...
    }
}

and calling as in

if (list[i].equals(title) && list[i] instanceof Detective) {
    list[i].setPrice(newPrice);

This does not allow creation of book as in new Book(...) ; to create a book, only subclasses are allowed, eg Book book = new Detective(...)

Answer 2

What i usually do is define an interface that the parent implements and the child can call.

Interface:

public interface IBookListener {
    void updatePrice (String title, int newPrice);
}

In parent:

public class BookManager implements IBookListener {

    public void add(Book x) {
        x.setListener(this);
        ...
    }
    ...
    public void updatePrice (String title, int newPrice) {
        ...
    }
}

In child:

public class Book {
    ...
    private IBookListener listener;
    public setLister(IBookListener listener) { 
        this.listener = listener;
    }

    public someMethod () {
        listener.updatePrice("title", 1);
    }
}