Polymorphism: Why use "List list = new ArrayList" instead of "ArrayList list = new ArrayList"?

interface, java, list, polymorphism

Solution

The main reason you'd do this is to decouple your code from a specific implementation of the interface. When you write your code like this:

List list = new ArrayList();  

the rest of your code only knows that data is of type `List`, which is preferable because it allows you to switch between different implementations of the `List` interface with ease.

For instance, say you were writing a fairly large 3rd party library, and say that you decided to implement the core of your library with a `LinkedList`. If your library relies heavily on accessing elements in these lists, then eventually you'll find that you've made a poor design decision; you'll realize that you should have used an `ArrayList` (which gives O(1) access time) instead of a `LinkedList` (which gives O(n) access time). Assuming you have been programming to an interface, making such a change is easy. You would simply change the instance of `List` from,

List list = new LinkedList();

to

List list = new ArrayList();  

and you know that this will work because you have written your code to follow the contract provided by the `List` interface.

On the other hand, if you had implemented the core of your library using `LinkedList list = new LinkedList()`, making such a change wouldn't be as easy, as there is no guarantee that the rest of your code doesn't make use of methods specific to the `LinkedList` class.

All in all, the choice is simply a matter of design... but this kind of design is very important (especially when working on large projects), as it will allow you to make implementation-specific changes later without breaking existing code.

Problem

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