Doubt on Advantage of Generic Eg over Non-Generic Eg!!!!

[Tanya Shetty]

Hi,

I have a doubt on the advantage of Generics over the Non-Generic version.

Source : http://java.sun.com/docs/books/tutorial/java/generics/QandE/generics-answers.html

  Non-Generic Version

    import java.util.List;
    import java.util.ArrayList;

    public class Library {
        private List resources = new ArrayList();
        public void addMedia(Media x) {
            resources.add(x);
        }
        public Media retrieveLast() {
            int size = resources.size();
            if (size > 0) {
                return (Media)resources.get(size - 1);
            }
            return null;
        }
    }

    interface Media {
    }

    interface Book extends Media {
    }

    interface Video extends Media {
    }

    interface Newspaper extends Media {
    }
  

  Generic Version

    import java.util.List;
    import java.util.ArrayList;

    public class Library<E extends Media> {
        private List<E> resources = new ArrayList<E>();
        public void addMedia(E x) {
            resources.add(x);
        }
        public E retrieveLast() {
            int size = resources.size();
            if (size > 0) {
                return resources.get(size - 1);
            }
            return null;
        }
    }
  

In both Versions, Media is to be defined as an interface along with the Book, Video and Newspapers as the sub-interfaces.

In the Non-Generic Version,

- We have to check the type of the argument resource "Media" before adding any resources to the List resources,
which will throw a compile time error, if add(Media x) is called with any other incompatible argument.

In the Generic Version,

- We have the check "E extends Media" which will help to detect compile time error avoiding run-time exceptions due to incompatible types.

Also, the interface Media with the sub-interfaces will have to be defined in both versions.. then how is the non-generic version helpful!!!

I have read the tutotrial (link shown above) and I fail to understand, the advantage of using genric-version over non-generic one for this example.. can somebody please explain this one....

Tanya.

[Henry Wong]

The advantage of using generics is so that you can have the compiler do the type checking -- and not having to do any runtime check, or even casting (and casting correctly).

For simple examples, as you have shown, it is probably of little benefit, as it is unlikely that you would cast incorrectly, in such a simple example. Regardless, why not use generics anyway? It would be kinda annoying to not use generics -- then have to add generics, when the application got complex to the point where you are starting to cast incorrectly.

Henry

[riyas nedumpurath]

The advantage I see is that your Generic Library class can be created with any class extending Media. Suppose in future you need a DVD class extending Media. You can just say Library<DVD>. Compiler checks that you only DVD to List resources . And retrieveLast() method will return DVD object(you dont need to cast). Hope that is clear.