将子类传递给具有抽象类参数的方法

Question

I've been trying to understand what is happening with this example, but haven't been able to so far. Can anyone help?

Say I have these two Abstract classes:

public abstract class AbstractBook<T extends AbstractPage> implements Serializable{
    public AbstractBook() {
    }

    public abstract void addPage(T var1);
}

and

public abstract class AbstractPage implements Serializable {
    public AbstractPage() {
    }
}

And classes that are extended like these:

public class Book extends AbstractBook {

    private ArrayList<Page> allPages;
    private String title;

    Book(String title){
       allPages = new ArrayList<Page>();
    }

    @Override
    public void addPage(AbstractPage page) {
        this.allPages.add();
    }
}

and

public class Page extends AbstractPage {

  private String content;

  public Page(String Content){
    this.content = Content;
  }

}

In the Book class there is a method called addPage . I can pass a Page class object to this method, however; I can't add that object to the ArrayList<Page> allPages .

Why exactly is that? I mean I think it is because the compiler expects type AbstractPage instead of Page . But why then I am capable of calling the method like thisbook.addPage(pageObject) ?

So In other words, it lets me pass a subclass object, but it doesn't let me do anything with that passed object.

And how do you get around this so that I can add that pageObject to the ArrayList<Page> allPages structure?

Answer 1

You can make it work by modifying your Book class:

public class Book extends AbstractBook<AbstractPage> {

  private ArrayList<AbstractPage> allPages;
  private String title;

  Book(String title){
    allPages = new ArrayList<AbstractPage>();
  }

  @Override
  public void addPage(AbstractPage page) {
    this.allPages.add(page);
  }
}

You should use the type parameter of AbstractBook when defining the class Book . The ArrayList has to bee of type AbstractPage if you want to insert elements of this type.

Answer 2

You can't add AbstractPage to list of Page s, because Page is one level up in terms of inheritance from AbstractPage : Object <- AbstractPage <- Page . If you want to add Page s to your list, you have to extend AbstractBook<Page> :

public class Book extends AbstractBook<Page> {

    private ArrayList<Page> allPages;
    private String title;

    Book(String title){
       allPages = new ArrayList<Page>();
    }

    @Override
    public void addPage(Page page) {
        this.allPages.add();
    }
}