how to get the child class in inheritance?

[java_fun2007]

hi,
if I have a store for renting videos .. and I have Video class (parent) and two (child) classes DVD and Cassete which they are extend the video class.

And the user wants to rent a DVD which is the child, how do I do that? can I write just getDVD() as simple as that or maybe I need to use a keyword like "getinstance of DVD " or something.

class Video have attributes like title, date of production and director.
I'm not good in inheritance help please.

[tim]

Hello java_fun2007

If you create an instance of DVD or of Cassete, it is also an instance of Video, since DVD and Cassete are extended from Video and contains all the inheritable attributes and methods of Video. That means that you can have a variable, media, that can contain a DVD or Cassete. So lets a media variable with a DVD in it. The new keyword use used to create instances of a class.

Code:

Video media = new DVD();

Now, we know that media is an Video but we might want to know if it is a DVD or a Cassete. For that we use the instanceof keyword for a boolean test.

Code:

if (media instanceof DVD)
    System.out.println("I'm a DVD");
if (media instanceof Cassete)
    System.out.println("I'm a Cassete");

I use this stuff regularly, so I hope this helped you.

[java_fun2007]

Thank you Tim for making this clear to me!

so, to get the title of a DVD I write like this?

Code:

if (media instanceof DVD)
System.out.println(media.getTitle());

[wsaryada]

If the getTitle() method is defined in the Video class then you don't have to check whether the media is an instance of DVD, because when a Video object refered to an instance of DVD the getTitle() will return the DVD title.

[jimm1]

yeah. Rather just have

System.out.println(media.getTitle());

If the title field is defined in the Video super class, then the children inherit it.

[tim]

Hello java_fun2007.

I forgot to tell you about casting. Lets say that you have defined some method in the DVD class that is not inherited or overridden from its super class, but you need to use it. For this you can use casting. When you cast you must use brackets and class name of the class you need. For example:

Code:

Vedio media = new DVD();
if (media instanceof DVD){
    // now we know it's an DVD and we can cast
    DVD dvd = (DVD) media;
    // now you can use dvd with all the methods and attributes of a real DVD instance.
}

I hope this helped. This is pretty important stuff.

[java_fun2007]

thank you so much for your help .

I guess I understand now! If I have another question about inheritance I will ask here again.