Question

I got the following problem:

I made a custom list CustomList which extends ArrayList and added a new method to it:

public class CustomList extends ArrayList<CustomObj> {

    public CustomObj get(String searchName) {
    ...
    }
}

now in my MainActivity.java I make a new object of this CustomList, but I’m using:

List<CustomObj> list = new CustomList();    (1)

and NOT:

CustomList list = new CustomList();         (2)

so far so good!
but when I try to access the function get(String searchName), there is no function I could use!

why? cause when I call it when creating the CustomList via (2) it’ll totally work

Answer 1

its is because you are making List(Interface) has no any such method defined , you are making object of CustomList but reference type is List .so it will give you compile time errer if you force to call this method on List reference.This is an example of simple Polymorphism.

You can imagine a case where Mordern car extends Old age car and mordern car has GPS navigation system where as old doesn’t in this case if you try to get the detail of navigation system by old car reference which doesn’t know about GPS , you wont get anything.