Java List and ArrayList declaration

Question

Can someone explain the following code to me. As in why is "List" and "ArrayList" are not the same? Does this mean that the variable neighborColumns is a List data structure that holds type Column objects but only public methods from class List can be used on variable neighborColumns? thanks in advance.

List<Column> neighborColumns = new ArrayList<Column>();

Rohit Jain · Accepted Answer · 2012-10-06 06:34:10Z

3

This code is ok (Design to interface (List): -

List<Column> neighborColumns = new ArrayList<Column>();
List<Column> neighborColumns = new LinkedList<Column>();   // OK To change..

In the above code, you are basically designing to an interface rather than implementation..List is an interface and ArrayList is a concrete class implementing it.

An advantage is that, you can switch between various implementation, without changing the design..

Below code is a problem (Design to implementation (ArrayList):-

ArrayList<Column> neighborColumns = new ArrayList<Column>();

//OOPs.. Cry (Incompatible Types)
ArrayList<Column> neighborColumns = new LinkedList<Column>();

Here since you are designing to an implementation.. So you can not change the implementation on the RHS.. The compiler will cry as above..

So, in general also, you should always design to an interface, thus not exposing the implementation, and also gaining flexibility of changing the implementation anytime..

edited Oct 6, 2012 at 6:34

answered Oct 6, 2012 at 6:19

Rohit Jain

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4 Comments

letter Q Over a year ago

when you say changing the implementation do you mean that I can add new methods to the concrete class that in this case is ArrayList or I am completely misunderstanding you?

Rohit Jain Over a year ago

@12345.. No changing the implementation means, making your list point to a different class objects.

letter Q Over a year ago

So would I be able to make changes like the following?

Rohit Jain Over a year ago

@12345.. Yeah of course.. You should write your code like this only..See my edit..

Mark Byers · Accepted Answer · 2012-10-06 06:17:31Z

1

List is an interface while ArrayList is a specific class that implements that interface.

It is a good programming practice to program against interfaces rather than specific classes where possible, because it makes it easier to change to using a different list implementation later.

answered Oct 6, 2012 at 6:17

Mark Byers

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