I studied polymorphism and understand that it can do dynamic method binding like below.
Assuming that class Animal is abstract class.
public class AnimalReference
{
public static void main(String args[])
Animal ref // set up var for an Animal
Cow aCow = new Cow("Bossy"); // makes specific objects
Dog aDog = new Dog("Rover");
// now reference each as an Animal
ref = aCow; ref.speak();
ref = aDog; ref.speak();
}
I used to create instance of ArrayList like:
ArrayList myList = new ArrayList();
But usually I figured that people write:
Collection myList = new ArrayList();
So my confusion is what is the benefit of declaring as Collection? Also I didn't know you can have "Collection" (which is an interface not abstract class) in front of "myList".
Why it is not good practice to just say:
ArrayList myList = new ArrayList();
I read Collection interface and ArrayList Java documents as well as online tutorials but still not really clear.. Could anyone give me some explanation?
Interfaces allow us to define polymorphism in a declarative way, unrelated to implementation. Two elements are polymorphic with respect to a set of behaviors if they realize the same interfaces.
The Collection interface is used to pass around collections of objects where maximum generality is desired. For example, by convention all general-purpose collection implementations have a constructor that takes a Collection argument.
Why use Polymorphism in Java? Polymorphism in Java makes it possible to write a method that can correctly process lots of different types of functionalities that have the same name. We can also gain consistency in our code by using polymorphism.
ArrayList class is used to create a dynamic array that contains objects. List interface creates a collection of elements that are stored in a sequence and they are identified and accessed using the index. ArrayList creates an array of objects where the array can grow dynamically.
If you declare myList
as ArrayList
, you fix its concrete type. Everyone using it will depend on this concrete type, and it is easy to (even inadvertently) call methods which are specific to ArrayList
. If sometime later you decide to change it to e.g. LinkedList
or CopyOnWriteArrayList
, you need to recompile - and possibly even change - client code. Programming for interfaces eliminates this risk.
Note that between Collection
and ArrayList
, there is another level of abstraction: the List
interface. Typically the usage pattern of a list is very different from that of a map, set or queue. So the type of collection you need for a job is usually decided early on, and is not going to change. Declaring your variable as a List
makes this decision clear, and gives its clients useful information regarding the contract this collection obeys. Collection
OTOH is usually not very useful for more than iterating through its elements.
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