File APIs for Java Developers
Manipulate DOC, XLS, PPT, PDF and many others from your application.
The moose likes Beginning Java and the fly likes Interface Objects Big Moose Saloon
  Search | Java FAQ | Recent Topics | Flagged Topics | Hot Topics | Zero Replies
Register / Login

Win a copy of Customer Requirements for Developers this week in the Jobs Discussion forum!
JavaRanch » Java Forums » Java » Beginning Java
Bookmark "Interface Objects" Watch "Interface Objects" New topic

Interface Objects

Phillipe Rodrigues
Ranch Hand

Joined: Oct 30, 2007
Posts: 165
I am a bit confused on the following:

Collection is an interface and ArrayList is a concrete class.Then

Collection c =new ArrayList(); ......................(1)

How above(1) is posible as interface objects cannot be created.What could be the idea behind the above instruction(1)

Ernest Friedman-Hill
author and iconoclast

Joined: Jul 08, 2003
Posts: 24189

You're creating an instance of the class ArrayList, and you're holding it in a variable of type "Collection". This is like buying a pet of type "Goldfish" and putting it in a fish tank. You can't buy a "Fish" -- you can only buy an Angelfish, a Goldfish, a Guppy, etc. But the tank can hold any kind of Fish.

A Collection variable can hold any kind of Collection -- and ArrayList, which implements List which extends Collection, is a kind of Collection.

[Jess in Action][AskingGoodQuestions]
Phillipe Rodrigues
Ranch Hand

Joined: Oct 30, 2007
Posts: 165
What about the below line numbers for the below code.When do we use them.What could be the possible cause for assiging class objects to an interface reference?

I agree. Here's the link:
subject: Interface Objects
jQuery in Action, 3rd edition