ArrayList and List Interface Polymorphism?

Question

I'm using the .readAll method of the opencsv library which returns a List<String>. I thought List was an interface and therefore cannot be instantiated, so how can it return a List<> object?

I thought that because ArrayList<> extends the List interface, I could do the following by polymorphism:

ArrayList<String[]> transactions = reader.readAll();

Where .readAll() returns a List<> object. However, it doesn't work as it expects List<> not ArrayList<>.

Why doesn't polymorphism work here, i.e. why can't I assign a List<> interface return value to the implementing class ArrayList? And how can an interface be instantiated as the method return value?

Thanks

You can't assign a general List to an ArrayList variable because there is no guarantee that the List actually is an ArrayList. Every ArrayList is a List, therefor you can do `List x = new ArrayList<>();`, but not every List is an ArrayList, therefor the opposite is not possible. — OH GOD SPIDERS, Mar 05 '21 at 17:34
Related: [What does it mean to “program to an interface”?](https://stackoverflow.com/questions/383947/what-does-it-mean-to-program-to-an-interface) — Ivar, Mar 05 '21 at 17:38

score 2 · Accepted Answer · answered Mar 05 '21 at 17:35

Polymorphism work in the reverse order of what you understood, means that any object of a parent class/interface (in this case a List) can hold value of a child class (or a class that implements the interface):

List a = new ArrayList(); //correct
ArrayList b = new List(); //incorrect

so how can it return a List<> object?

You can use interface as a return type. Read this question for clarification.

score 1 · Answer 2 · answered Mar 05 '21 at 18:49

Interesting question. Its answer is not straightforward.

We can’t create an instance(interface can’t be instantiated) of an interface but we can make reference of it that refers to the Object of its implementing class.

e.g:

public List getData(){
  ...
  ...
  ArrayList<String> sArrList = new ArrayList(); 

  ...
  return sArrList;
}

So, when you return sArrList, which is an ArrayList but your return type is List. From point 1, list reference points an ArrayList, which on is, its implementation.

But the reverse thing is not possible. But why?

Look, ArrayList is a solid implementation of the List interface. There are some solid implementations of List interface like ArrayList, LinkedList... .

So, when we try to assign a List to an ArrayList, Java compiler fails to decide which solid implementation it is actually. Is it an arraylist or linkedlist?

So, you can overcome this situation by typecasting. You confirm java complier that, ok, don't worry, it is an arraylist. you can perform all arraylist operation here without further thinking :)

ArrayList and List Interface Polymorphism?

2 Answers2