Why instantiate an ArrayList polymorphically ?

[Glen Iris]

Hey Guys,

I a m trying to figure out why the book uses:

List <String> list = new ArrayList<String>();

as opposed to:

ArrayList <String> list = new ArrayList<String>();

To quote the book from page 567:

In practice, you'll typically want to instantiate an ArrayList polymorphically

Why?

[Shaily Alex]

You should declare it as a List<String>, and initialize it as an ArrayList<String>.

List is an interface, and ArrayList is an implementing class. It's almost always preferable to code against the interface and not the implementation. This way, if you need to change the implementation later, it won't break consumers who code against the interface.

[Dan Drillich]

Please have a look at Why should ArrayList be assigned polymorphically as a List?

Regards,
Dan

[Seetharaman Venkatasamy]

most of the people want to know specification, because it is short!

[Dennis Deems]

Dan Drillich wrote:Please have a look at Why should ArrayList be assigned polymorphically as a List?

Regards,
Dan

It was an interesting discussion, but didn't really answer the question. I found the top response to a similar question posed at StackOverflow to be well stated.

In my opinion the best reason to declare List instead of ArrayList is that if you ever change your mind about using ArrayList, you will have lots of painful code cleanup to do.

[Ram Narayan.M]

The need of using Interfaces is to generalize the Structure of methods used in concrete class which provides same set of services (or) methods to the client (but internal implementation differs)...

If no concrete class follow interfaces, each class will provide the methods to the client in different format to client program. Whenever client desires to change from one service/method provider to other, many changes will be there in code...

So, Interfaces comes to rescue which imposes all concrete classes to follow the same structure of methods, so it facilitates the client program to use only Interfaces instead of confused with lot of concrete classes and client can change only the assignment part of
concrete class instance to "Interface" type reference.(That is, Generalization become effective due to Polymorphism feature)

In this case, we use "ArrayList" concrete class directly...Even, Concrete classes can be hided... We can write a separate class which provides the Concrete class instance to be assigned to reference of "Interface" type based on some key input (Key input maybe integer, character or string)...

[Glen Iris]

Thank you guys especially Dennis.

:-)