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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: 24199

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.

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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
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