Question

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Why should the interface for a Java class be prefered?

ArrayList<Integer> al = new ArrayList<Integer>();


List<Integer> l = new ArrayList<Integer>();

what is the difference between these 2 lines? Is there any rules that I should use former one rather than later one in any case? Or vise versa? What is advantage or disadvantage of using particular one?

Thanks.

Answer 1

The first line creates an ArrayList and stores it in a variable of type of ArrayList, the second line stores it in a variable of type List.

List is an interface of which ArrayList is an implementation. The rule of thumb for deciding which type to store the instance in (List or a specific implementation, like ArrayList) is that you should store at the most generalized level suitable for your needs. This means that if you know that the variable must conform to behavior only exhibited by ArrayList and not a List in general, then you should use ArrayList, otherwise, use List. (This holds for LinkedList or other List implementations, too)