Java Arraylist and inheritance calling right method. help

Question

I created 4 classes: Cd , Dvd , Item and Database . Cd and Dvd extend from Item . The Database class stores an ArrayList of Items .

I'm stuck on creating a method in the Database class, which calls the method display in either Cd or Dvd . However I managed to display the Cds .

How can display all items? Eg

Item number = 1
CD

Item number = 2
DVD

Item number = 3
Cd

Many thanks.

Edit :

The display methods are different in Cd and Dvd .

public void displayAll() {

    for (int i = 0; i < items.size(); i++) {

        Cd theCd = (Cd) items.get(i);
        // Dvd theDVD = (Dvd) items.get(i);

        System.out.println("Item Number = " + i);
        theCd.display();

        // theDvd.display

    }

}

My class diagram is:

Class Diagram http://img571.imageshack.us/img571/1460/unledsca.png

Answer 1

You probably should have public abstract void display(); in your Item class. One would imagine it makes sense for Items s to be able to display themselves. Then you would implement it in CD and DVD (sounds like you already have). Then you'd have no need for the downcast:

public void displayAll()
{        
     for (int i = 0; i < items.size(); i++) {  
         Item item = (Item) items.get(i); 
         System.out.println("Item Number = " + i);
         item.display();
     }
}

And if you're using a JDK version that supports generics, consider using ArrayList<Item> instead of ArrayList . Then you can get rid of the Item cast as well.

Answer 2

You should get Item from the list, and not a Cd, then just invoke the methods declared in the Item class. The methods in the subclasses will automatically be invoked if they are overriding methods.

Edit: Just checked your classdiagram. The method display should be added to the Item class, and overridden by the subclasses.

Answer 3

You're looking for instanceof

if (item instanceof Cd)

However, as others have said ... it may point toward a bad class design. Why do you need to know what the type is?

Answer 4

Don't explicitly cast; that defeats the purpose of polymorphism. Declare an abstract display method in your Item base class, and then override it in Cd and Dvd .

Then your loop can be:

List<Item> my_list = new ArrayList<Item>();

...

for (int i = 0; i < items.size(); i++) {
    System.out.println("Item number = " + i);
    items[i].display();
}

This will always call the correct method.

Answer 5

 for (int i = 0; i < items.size(); i++) {  


     Item theItem = (Item)items.get(i); 

     System.out.println("Item Number = " + i);
     theItem.display();


 }

Cast to Item, instead of CD or DVD. Make sure Item has a display() method. Also, if you are using Java 5 or above, have a look at generics