How to create ArrayList properly? [duplicate]

Question

Below are two ways how to create ArrayList:

List<String/*or other object*/> arrList = new ArrayList();//Imports List, ArrayList
ArrayList<String/*or other object*/> arrList = new ArrayList();//Imports just ArrayList

What is the difference? Which method should be used?

Answer 1

The first form is recommended for the public interface of a class (say, the declaration of the attributes in a class). It's a well-known advice that you should program for an interface, not for a concrete implementation (this is stated in Design Patterns) - the number of classes you have to import has little to do with the quality of code and good design.

The advantage of the first form is that it'll allow you to easily swap implementations later on, if the need arises - whereas the second implementation sets in stone the implementing class, making your code harder to evolve and less flexible.

There are cases when it's OK to declare and use concrete classes, though (for instance, for a little bump in performance). But the public interface of a class should use interfaces whenever possible, it'll make future changes easier to implement.

Answer 2

First ways is called coding to interface

Second one is called coding to implementation.

The advantage with the second one is that it gives you the flexibility to change the implemneatation class later without any code change. For example today you are using ArrayList but if tomorrow you want to change it to LinkedList then simply change the implmentation class in your list definition.